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{"schema_version":"1.9.0","id":"GHSA-ffpj-xv5c-p3gw","published":"2026-07-24T16:55:55Z","modified":"2026-08-04T14:41:01.473313729Z","aliases":["CVE-2026-59220","PYSEC-2026-3596"],"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.","affected":[{"package":{"name":"open-webui","ecosystem":"PyPI","purl":"pkg:pypi/open-webui"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0.9.2"},{"fixed":"0.10.0"}]}],"versions":["0.9.2","0.9.3","0.9.4","0.9.5","0.9.6"],"database_specific":{"source":"https://github.com/github/advisory-database/blob/main/advisories/github-reviewed/2026/07/GHSA-ffpj-xv5c-p3gw/GHSA-ffpj-xv5c-p3gw.json"}}],"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"],"github_reviewed":true,"github_reviewed_at":"2026-07-24T16:55:55Z","nvd_published_at":"2026-07-09T17:17:03Z","severity":"MODERATE"},"severity":[{"type":"CVSS_V3","score":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H"}]} |