What happened
CVE-2026-87011 (CVSS 7.5, high) affects Open WebUI, a widely-used self-hosted AI platform, from version 0.9.0 until 0.11.1. The unauthenticated POST /oauth/backchannel-logout handler fetched the OIDC discovery document and signing keys before validating a submitted logout token, creating a flaw in the logout-token validation order that could be abused in the OIDC backchannel logout flow.
Why it matters
Open WebUI is a popular self-hosted front-end for LLM deployments (Ollama, OpenAI-compatible APIs, etc.) used broadly by individuals and organizations running their own AI chat interfaces; a flaw in its authentication/session-invalidation logic directly threatens the confidentiality of user conversations and any connected credentials, and because it's unauthenticated and network-reachable it has a meaningful blast radius across all self-hosted instances that have OIDC configured.
Attack vector
Unauthenticated attacker sends a crafted request to POST /oauth/backchannel-logout; the handler fetches OIDC discovery/signing-key material before validating the submitted logout token, creating a logic flaw exploitable in the OIDC backchannel-logout flow.
Affected systems
Open WebUI versions 0.9.0 until 0.11.1 with OIDC configured
Mitigation
Upgrade to Open WebUI 0.11.1 or later per the linked fix commit.