PYSEC-2026-2527

Vulnerability from pysec - Published: 2026-07-13 14:36 - Updated: 2026-07-13 16:04
VLAI
Details

Impact

This is a stored Cross-Site Scripting (XSS) vulnerability affecting the website and author comment fields. The website field was HTML-escaped using quote=False, which left single and double quotes unescaped. Since the frontend inserts the website value directly into a single-quoted href attribute via string concatenation, a single quote in the URL breaks out of the attribute context, allowing injection of arbitrary event handlers (e.g. onmouseover, onclick).

The same escaping was missing entirely from the user-facing comment edit endpoint (PUT /id/) and the moderation edit endpoint (POST /id//edit/).

Any visitor to a page embedding isso comments is impacted. No authentication or interaction beyond mouse movement is required to trigger a payload — an attacker can post a comment anonymously (moderation is off by default) with a crafted website URL, and the payload persists in the database and fires on every page load. With the full-page invisible overlay technique described in the report, the victim only needs to move their mouse.

Patches

The issue is fixed in commit 3cf27c2. Users should upgrade to a version containing that commit once released. The fix applies html.escape(..., quote=True) to the website field across all three write paths (POST /new, PUT /id/, POST /id//edit/), and adds input validation and escaping to the moderation edit endpoint which previously had neither.

Workarounds

Enabling comment moderation (moderation = enabled = true in isso.cfg) prevents unauthenticated users from publishing comments, raising the bar for exploitation. However, it does not fully mitigate the issue since a moderator activating a malicious comment would still expose visitors. There is no configuration-only workaround that fully prevents the vulnerability.

Resources

  • https://docs.python.org/3/library/html.html#html.escape — note the quote parameter
Severity
6.1 (Medium)
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Impacted products
Name purl
isso pkg:pypi/isso
Aliases
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "isso",
        "purl": "pkg:pypi/isso"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.13.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "0.10",
        "0.10.1",
        "0.10.2",
        "0.10.3",
        "0.10.4",
        "0.10.5",
        "0.10.6",
        "0.11.0",
        "0.11.1",
        "0.12.0",
        "0.12.1",
        "0.12.2",
        "0.12.4",
        "0.12.5",
        "0.12.6",
        "0.12.6.1",
        "0.12.6.2",
        "0.13.0",
        "0.13.1.dev0",
        "0.8",
        "0.8.1",
        "0.8.2",
        "0.8.3",
        "0.9",
        "0.9.1",
        "0.9.10",
        "0.9.2",
        "0.9.3",
        "0.9.4",
        "0.9.5",
        "0.9.6",
        "0.9.7",
        "0.9.8",
        "0.9.9"
      ]
    }
  ],
  "aliases": [
    "CVE-2026-27469",
    "GHSA-9fww-8cpr-q66r"
  ],
  "details": "## Impact\n\nThis is a stored Cross-Site Scripting (XSS) vulnerability affecting the website and author comment fields. The website field was HTML-escaped using quote=False, which left single and double quotes unescaped. Since the frontend inserts the website value directly into a single-quoted href attribute via string concatenation, a single quote in the URL breaks out of the attribute context, allowing injection of arbitrary event handlers (e.g. onmouseover, onclick).\n\nThe same escaping was missing entirely from the user-facing comment edit endpoint (PUT /id/\u003cid\u003e) and the moderation edit endpoint (POST /id/\u003cid\u003e/edit/\u003ckey\u003e).\n\nAny visitor to a page embedding isso comments is impacted. No authentication or interaction beyond mouse movement is required to trigger a payload \u2014 an attacker can post a comment anonymously (moderation is off by default) with a crafted website URL, and the payload persists in the database and fires on every page load. With the full-page invisible overlay technique described in the report, the victim only needs to move their mouse.\n\n## Patches\n\nThe issue is fixed in commit 3cf27c2. Users should upgrade to a version containing that commit once released. The fix applies html.escape(..., quote=True) to the website field across all three write paths (POST /new, PUT /id/\u003cid\u003e, POST /id/\u003cid\u003e/edit/\u003ckey\u003e), and adds input validation and escaping to the moderation edit endpoint which previously had neither.\n\n## Workarounds\n\n  Enabling comment moderation (moderation = enabled = true in isso.cfg) prevents unauthenticated users from publishing comments, raising the bar for exploitation. However, it\n  does not fully mitigate the issue since a moderator activating a malicious comment would still expose visitors. There is no configuration-only workaround that fully prevents\n   the vulnerability.\n\n## Resources\n\n  - https://docs.python.org/3/library/html.html#html.escape \u2014 note the quote parameter",
  "id": "PYSEC-2026-2527",
  "modified": "2026-07-13T16:04:21.211428Z",
  "published": "2026-07-13T14:36:38.311279Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/isso-comments/isso/security/advisories/GHSA-9fww-8cpr-q66r"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-27469"
    },
    {
      "type": "WEB",
      "url": "https://github.com/isso-comments/isso/commit/0afbfe0691ee237963e8fb0b2ee01c9e55ca2144"
    },
    {
      "type": "WEB",
      "url": "https://docs.python.org/3/library/html.html#html.escape"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/isso-comments/isso"
    },
    {
      "type": "PACKAGE",
      "url": "https://pypi.org/project/isso"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/advisories/GHSA-9fww-8cpr-q66r"
    }
  ],
  "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": "Isso affected by Stored XSS via comment website field"
}



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