Open WebUI CVE-2026-87011

The attack requires ENABLE_OAUTH_BACKCHANNEL_LOGOUT to be explicitly set to true in the Open WebUI configuration-this requires the operator to have configured an OIDC OAuth integration and enabled backchannel logout support, which is not a default setting. Given that prerequisite is met, the endpoint is fully unauthenticated (PR:N): no valid session, token, or credentials are required. The attacker does not need a valid OIDC logout token; invalid tokens are sufficient to trigger the expensive pre-validation network fetches. Exploitation is most impactful against single-worker deployments where the event-loop block cannot be absorbed by parallel workers. Deployments behind a reverse proxy with request rate limits have a meaningfully reduced attack surface.

The CVSS 7.5 score (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H) reflects a pure availability-impact denial of service with no authentication barrier on the vulnerable endpoint. The critical limiting factor-not captured by the raw CVSS score-is that the ENABLE_OAUTH_BACKCHANNEL_LOGOUT configuration flag must be explicitly enabled, meaning only deployments with an active OIDC OAuth integration and backchannel logout turned on are exposed. No EPSS data was provided in the input, and this CVE is not listed in CISA KEV; no public POC has been identified. The CWE-405 amplification characteristic means a low-volume attack generates disproportionate outbound load toward the identity provider, which could affect the broader SSO infrastructure beyond the Open WebUI instance itself. Single-worker deployments are most severely affected because the event-loop block is not distributed. Real-world priority is moderate: DoS-only impact, configuration-conditional exposure, self-hosted niche deployment-but organizations running Open WebUI in production with backchannel logout enabled should treat this as an urgent patch.

An attacker who identifies an Open WebUI instance with backchannel logout enabled sends a sustained flood of POST requests to /oauth/backchannel-logout carrying syntactically plausible but invalid logout tokens. Each request causes the server to perform multiple outbound HTTP fetches to the OIDC provider's discovery and JWKS endpoints, while the synchronous key-lookup call blocks the async event loop; the single-worker process becomes unable to serve legitimate user requests and the identity provider receives amplified spurious traffic.