CVE-2026-59220
Open WebUI: ReDoS in skill-mention regexes causes whole-instance DoS on default config
Summary
## Summary Two regexes in `backend/open_webui/utils/middleware.py` that parse ` ` skill-mention tags backtrack in O(n²) on input that contains ` `. 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. ## Affected versions `>= 0.9.2, ]+)\|?[^>]*>') # line 2247 — used by strip_skill_mentions(), called unconditionally (line 2662) strip_re = re.compile(r' ]+\|?([^>]*)>') ``` `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. ## Root cause `[^|>]` 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. ## Proof of concept Standalone (no Open WebUI required): ```python import re, time EXTRACT = re.compile(r' ]+)\|?[^>]*>') STRIP = re.compile(r' ]+\|?([^>]*)>') for n in (8_000, 16_000, 32_000, 64_000): s = ' 6} {name:>7} = {(time.perf_counter()-t)*1000:8.1f} ms') ``` Time 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. End-to-end against a live instance (default config): 1. `docker run ghcr.io/open-webui/open-webui:v0.9.6` on defaults. 2. Log in as any user (no admin or skill setup). 3. Send a chat message containing ` `. 4. One CPU core pegs in `re`; UI and API stop responding for every user until the worker is killed. ## Patch Rewrite 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 ` `, ` `, and bare ` ` mentions. ```python SKILL_MENTION_RE = re.compile(r' ]+)(?:\|[^>]*)?>') strip_re = re.compile(r' ]+(?:\|([^>]*))?>') ``` After the patch the same hostile input returns in under 1 ms. Shipped in v0.10.0. ## Credit Reported by @Vlad-WKG, including a correct root-cause analysis and patch.
Affected packages
| Ecosystem | Package | Affected versions | Fixed versions |
|---|---|---|---|
| PyPI | open-webui | — | 0.10.0 |
Remediation: Upgrade to 0.10.0 or later.
References
Includes data from the GitHub Advisory Database, licensed under CC-BY 4.0.
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EPSS scores provided by the FIRST.org Exploit Prediction Scoring System.