Open WebUI allows chat clients to use the syntax <$skillId|label> for skill mentions. The SKILL_MENTION_RE and strip_re regular expressions in the middleware contain overlapping quantifiers. While parsing an input that includes an opening '<$' without the closing '>' the patterns trigger quadratic back‑tracking, which blocks the asyncio event loop and renders the entire Open WebUI instance unresponsive. This is a classic Regular Expression Denial of Service (ReDoS) flaw classified as CWE‑1333.

The vulnerability exists in Open WebUI releases from version 0.9.2 up to, but not including, 0.10.0, where the default skill‑mention processing configuration is enabled. Any instance running one of these unpatched builds and allowing authenticated users to send chat messages is affected. Deployments built from the public release starting at 0.10.0 contain the fixed regex definitions and are not impacted.

The CVSS score of 6.5 indicates a moderate severity, and the EPSS score of < 1% suggests a low likelihood of exploitation overall, though the vulnerability is not listed in the CISA KEV catalog. The flaw requires an authenticated user to send a crafted chat message containing an incomplete skill‑mention. Because the exploit can terminate the entire service with a single message, the overall risk to availability is high for exposed deployments that permit authenticated chat traffic. Administrators should prioritize patching or applying mitigating controls to avoid service disruption.