CVE-2026-59220: Open WebUI Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-ffpj-xv5c-p3gw",
  "modified" : "2026-07-24T16:55:55Z",
  "published" : "2026-07-24T16:55:55Z",
  "aliases" : [ "CVE-2026-59220" ],
  "summary" : "Open WebUI: ReDoS in skill-mention regexes causes whole-instance DoS on default config",
  "details" : "## Summary\nTwo regexes in `backend/open_webui/utils/middleware.py` that parse `<$skillId|label>` skill-mention tags backtrack in O(n²) on input that contains `<$` followed by a long run with no closing `>`. Both run synchronously, on the asyncio event loop, on **every** chat completion with no feature gate. Because the default deployment is a single uvicorn worker, one such input pins a CPU core inside `re` and freezes the entire instance for all users until the worker is killed. Any authenticated user can trigger it with one chat message; it also fires accidentally on benign retrieved content (a RAG chunk or tool output) containing the pattern.\n\n## Affected versions\n`>= 0.9.2, < 0.10.0`. Fixed in **v0.10.0** (there is no 0.9.7 release).\n- `SKILL_MENTION_RE` (the extract pattern) has been O(n²) since **v0.9.2**; exploitable on 0.9.2–0.9.5 with a large input (hundreds of KB).\n- **v0.9.6** added a second, far more aggressive O(n²) in the strip pattern (introduced by the \"keep label as readable text\" change), so on 0.9.6 a small input is enough to hang the instance.\n\nBoth are fixed by the same patch.\n\n## Affected component\n`backend/open_webui/utils/middleware.py` (line numbers as of v0.9.6):\n\n```python\n# line 2223 — used by extract_skill_ids_from_messages(), called unconditionally (~line 2625)\nSKILL_MENTION_RE = re.compile(r'<\\$([^|>]+)\\|?[^>]*>')\n\n# line 2247 — used by strip_skill_mentions(), called unconditionally (line 2662)\nstrip_re = re.compile(r'<\\$[^|>]+\\|?([^>]*)>')\n```\n\n`extract_skill_ids_from_messages()` runs before the `if all_skill_ids:` block (that guard gates only skill *injection*, not the regex), and `strip_skill_mentions()` runs with no guard at all. Neither requires a skill to exist or any setting to be enabled. Both functions are plain synchronous calls inside the async `process_chat_payload` coroutine, so they block the event loop; with the default `UVICORN_WORKERS=1` (`backend/start.sh`) the whole instance stalls.\n\n## Root cause\n`[^|>]` is a subset of `[^>]`, so the quantifier pair `[^|>]+ \\|? [^>]*` is ambiguous: on input that never closes with `>`, `[^|>]+` greedily consumes the tail, `>` fails, and the engine backtracks through every split point between `[^|>]+` and `[^>]*` — O(n) positions each doing O(n) work. Polynomial, not exponential, but more than enough to hang a single worker on a ~100 KB input.\n\n## Proof of concept\nStandalone (no Open WebUI required):\n\n```python\nimport re, time\nEXTRACT = re.compile(r'<\\$([^|>]+)\\|?[^>]*>')\nSTRIP   = re.compile(r'<\\$[^|>]+\\|?([^>]*)>')\nfor n in (8_000, 16_000, 32_000, 64_000):\n    s = '<$' + ('a' * n)\n    for name, rx in (('extract', EXTRACT), ('strip', STRIP)):\n        t = time.perf_counter(); rx.search(s)\n        print(f'n={n:>6} {name:>7} = {(time.perf_counter()-t)*1000:8.1f} ms')\n```\n\nTime quadruples per doubling of `n` (textbook O(n²)); the strip pattern runs for ~6 seconds on a 64k blob and for minutes on a ~96 KB one.\n\nEnd-to-end against a live instance (default config):\n1. `docker run ghcr.io/open-webui/open-webui:v0.9.6` on defaults.\n2. Log in as any user (no admin or skill setup).\n3. Send a chat message containing `<$` followed by 50k+ characters with no `>`.\n4. One CPU core pegs in `re`; UI and API stop responding for every user until the worker is killed.\n\n## Patch\nRewrite the optional `|label` as a non-capturing optional group so the two quantifiers no longer overlap. Both patterns become linear; captures and substituted output are unchanged on well-formed `<$id|label>`, `<$id|>`, and bare `<$id>` mentions.\n\n```python\nSKILL_MENTION_RE = re.compile(r'<\\$([^|>]+)(?:\\|[^>]*)?>')\nstrip_re         = re.compile(r'<\\$[^|>]+(?:\\|([^>]*))?>')\n```\n\nAfter the patch the same hostile input returns in under 1 ms. Shipped in v0.10.0.\n\n## Credit\nReported by @Vlad-WKG, including a correct root-cause analysis and patch.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "open-webui"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.9.2"
      }, {
        "fixed" : "0.10.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/open-webui/open-webui/security/advisories/GHSA-ffpj-xv5c-p3gw"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-59220"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/open-webui/open-webui/commit/61a26722155ec6ee1b629cf8dfcf975098c18331"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/open-webui/open-webui"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/open-webui/open-webui/releases/tag/v0.10.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1333" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-24T16:55:55Z",
    "nvd_published_at" : "2026-07-09T17:17:03Z"
  }
}