LiteLLM: MCP Authentication Bypass via OAuth2 Passthrough Fallback
Prioritize patching. It under exploitation confirmed by CISA and has a public proof of concept.
Official vendor statements in CSAF/VEX format: whether their product is affected, already fixed, or ruled out — and why. These are the vendor's assertions, not Vexday's judgment.
Apply mitigations in accordance with vendor instructions, ensuring compliance with CISA’s BOD 26-04 Prioritizing Security Updates Based on Risk (see URL in Notes) guidance and CISA’s “Forensics Triage Requirements” (see URL in Notes). Follow applicable BOD 26-04 guidance for cloud services or discontinue use of the product if mitigations are unavailable. Stakeholders are responsible for evaluating each asset's internet exposure and ensuring adherence to BOD 26-04 patching guidelines.
LiteLLM, a proxy server for AI APIs, had a flaw where attackers could use fake authentication headers to bypass security checks and access MCP tools without a valid key. This allowed unauthorized users to make requests they shouldn't be able to make.
The vulnerability exists in the MCP Streamable HTTP endpoint where a malformed Authorization header triggers an OAuth2 passthrough fallback mechanism that replaces proper LiteLLM key validation with an empty UserAPIKeyAuth() object. An unauthenticated attacker can exploit this authentication bypass to reach protected MCP tooling without providing valid credentials, affecting systems running versions prior to 1.84.0.