CVE-2026-87011 in Open WebUI
Summary
by MITRE • 09/09/2026
Open WebUI is an extensible, feature-rich, and user-friendly self-hosted AI platform. From 0.9.0 until 0.11.1, the unauthenticated POST /oauth/backchannel-logout handler in backend/open_webui/utils/oauth.py fetched the OIDC discovery document and signing keys before validating a submitted logout token. Each request repeated uncached network fetches, and the signing-key lookup blocked the async event loop, so requests carrying invalid tokens could stall the single-worker instance and amplify traffic to the identity provider when ENABLE_OAUTH_BACKCHANNEL_LOGOUT was enabled. This issue is fixed in version 0.11.1.
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Analysis
by VulDB Data Team • 09/09/2026
The vulnerability identified within Open WebUI versions ranging from 0.9.0 through 0.11.1 represents a critical flaw in the authentication and session management logic, specifically affecting the OAuth backchannel logout handler located at the POST /oauth/backchannel-logout endpoint. This component is responsible for processing logout requests initiated by an identity provider to terminate user sessions on the Open WebUI platform. The core technical deficiency lies in the order of operations performed during request validation. Instead of first verifying the integrity and authenticity of the submitted OIDC logout token, the system proceeds immediately to fetch the OpenID Connect discovery document and retrieve the corresponding signing keys from the configured identity provider. This sequence error means that any incoming request, regardless of whether it contains a valid or maliciously crafted token, triggers these resource-intensive operations before security checks are applied.
This architectural flaw leads directly to two distinct operational impacts: denial of service through resource exhaustion and potential amplification attacks against external infrastructure. Because the system does not validate the token prior to network interaction, an unauthenticated attacker can send a high volume of requests with invalid or malformed logout tokens. Each such request forces the backend server to perform uncached network fetches to retrieve discovery documents and signing keys. These operations are computationally expensive and introduce significant latency. In environments where ENABLE_OAUTH_BACKCHANNEL_LOGOUT is enabled, this behavior allows an attacker to stall the single-worker instance of the application by consuming CPU cycles and blocking the async event loop with I/O-bound tasks that ultimately fail validation later in the process chain.
Furthermore, the vulnerability facilitates traffic amplification against the identity provider. Since every request triggers a fetch for signing keys even if the token is invalid, an attacker can generate substantial external network traffic directed at the OIDC issuer. This not only degrades the performance of the Open WebUI instance but also places undue load on the authentication service, potentially causing disruptions for legitimate users relying on that identity provider. The lack of caching for these discovery documents and keys exacerbates the issue, as repeated requests result in redundant network round-trips rather than utilizing locally stored valid metadata.
From a classification perspective, this vulnerability aligns with CWE-20: Improper Input Validation, specifically regarding the failure to validate input before processing it. It also relates to CWE-755: Improper Handling of Unusual or Complex Inputs, as the system fails to reject malformed tokens early in the request lifecycle. In terms of offensive security frameworks, this behavior is consistent with ATT&CK technique T1498: Network Denial of Service, where an attacker leverages inefficient resource handling to degrade service availability. The specific mechanism involves exhausting server resources through repetitive network calls triggered by unvalidated inputs.
The recommended mitigation for organizations running affected versions is to upgrade Open WebUI immediately to version 0.11.1 or later, where the logic has been corrected to validate tokens before initiating external network requests. For environments that cannot patch instantly due to operational constraints, implementing a web application firewall rule to rate-limit POST requests to the /oauth/backchannel-logout endpoint can help mitigate the impact by throttling the volume of malicious traffic reaching the vulnerable handler. Additionally, ensuring that caching mechanisms for OIDC discovery documents are properly configured and utilized within the application environment may reduce the severity of performance degradation during an attack, although upgrading remains the only definitive solution to address the root cause of the improper validation sequence.