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CVE-2026-59822 – Authentication Bypass – LiteLLM prior to v1.84.0

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Summary

CVE-2026-59822 is a high-severity authentication bypass vulnerability in LiteLLM (AI Gateway/proxy server by BerriAI) affecting all versions prior to 1.84.0. The flaw resides in LiteLLM’s MCP Streamable HTTP endpoint, where a faulty OAuth2 passthrough fallback mechanism allows an unauthenticated attacker to craft a fabricated Authorization header and gain access to MCP tooling without a valid LiteLLM key. The vulnerability carries a CVSS 4.0 score of 8.8 (High).

Technical details

  • Root cause: The MCP Streamable HTTP endpoint’s authentication handler contains a fallback code path designed for OAuth2 passthrough to upstream MCP servers. When LiteLLM key validation fails, this fallback improperly substitutes an empty UserAPIKeyAuth() object in place of a rejected authentication context, rather than terminating the request.
  • Trigger conditions: An attacker supplies any fabricated Bearer token in the Authorization header to the MCP Streamable HTTP endpoint. No valid LiteLLM API key, account credentials, or prior access is required.
  • Attack vector: Fully remote, over the network, with no authentication and no user interaction required (AV:N/AC:L/AT:N/PR:N/UI:N).
  • Impact: Successful exploitation allows unauthorized requests to reach MCP tooling and all connected services. The CVSS confidentiality impact is rated High (VC:H), meaning sensitive data exposed through MCP tools — including upstream LLM API keys, model outputs, and backend service credentials — may be accessed without authorization. Integrity impact is rated Low (VI:L).

Affected software

  • BerriAI LiteLLM — all versions prior to 1.84.0

Severity

  • CVSS 4.0 Base Score: 8.8 (High)
  • Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:L/VA:N/SC:N/SI:N/SA:N
  • CWEs: CWE-287 (Improper Authentication), CWE-306 (Missing Authentication for Critical Function)

Mitigation and recommended actions

  • Immediate patch: Upgrade LiteLLM to version 1.84.0 or later, which corrects the OAuth2 fallback gating logic in the MCP authentication handler.
  • If immediate patching is not possible: Block all external access to /mcp/ endpoint paths at your reverse proxy or API gateway to prevent unauthenticated requests from reaching the vulnerable handler.
  • Additional hardening: Consider disabling MCP routes entirely on internet-exposed LiteLLM instances until the patch can be applied.

IONIX Status

The IONIX research team is tracking ongoing exploitation attempts and recommends immediate patching. Potentially affected assets are outlined in this post.

References

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