PyPI · open-webui
Open WebUI: Any authenticated user can stall a worker via a knowledge-search pattern that backtracks catastrophically
The built-in knowledge search tools let a chat participant choose the pattern used to grep knowledge files. Patterns containing regex metacharacters were compiled with Python's backtracking re engine and run against every line of every reachable file, with no time limit anywhere on that path. A single crafted pattern and a single short line of matching text pin one CPU core for as long as the attacker wants, and because the search runs synchronously inside the event loop, that worker serves nobody else while it spins.
ENABLE_KB_EXEC at its default of False the model is handed grep_knowledge_files, which is the affected path.USER_PERMISSIONS_CHAT_FILE_UPLOAD defaults to true and a user always has read access to their own upload, so both halves of the input are attacker-supplied.UVICORN_WORKERS at its default of 1 lose the whole instance; multi-worker deployments lose one worker per request.Availability, against every other user of the affected worker. Cost scales exponentially with the length of the matching text: measured on the vulnerable code, a 24 character subject takes 1.2s, 28 takes 19s and 30 takes 74s, and a 40 character subject extrapolates to roughly a day of CPU. The same subject against a literal pattern takes under a microsecond. There is no confidentiality or integrity effect, and no data is read or altered.
Fixed in 0.11.0 by https://github.com/open-webui/open-webui/pull/27471. Pattern matching moved from re to the regex engine, which supports a per-search timeout, and every tool call now runs its searches under a single 2 second matching budget, after which the tool returns an error instead of continuing to match. Upgrading is sufficient; there is nothing an operator has to configure.
backend/open_webui/tools/knowledge_fs.py, build_matcher: compiled the caller's pattern and returned an unbounded match function.backend/open_webui/tools/builtin.py, grep_knowledge_files: the default-configuration caller, which ran that matcher over every line of every reachable file.build_matcher treated any pattern containing regex metacharacters as a regex, so no explicit flag was needed to reach the compiler. From there the only limits in place were on results, not on work: a cap on matches returned and a cap on files scanned, neither of which bounds the time a single line can consume. Backtracking cost is exponential in the length of the matched text rather than in the pattern, so capping pattern length or line length would not have bounded it either. The engine had no timeout available and none was imposed elsewhere.
Against a real instance as an ordinary user:
x characters, and no y.grep_knowledge_files with the pattern (x|x)*y and that file's id.Growth measured directly against build_matcher on the vulnerable code:
| subject length | time |
|---|---|
| 16 | 4.7ms |
| 20 | 73ms |
| 24 | 1.21s |
| 28 | 19.3s |
| 30 | 73.9s |
@Classic298, for the finding and the fix.
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