CVE-2026-59822: LiteLLM: MCP Authentication Bypass via OAuth2 Passthrough Fallback
Impact
LiteLLM's MCP Streamable HTTP endpoint could allow an unauthenticated attacker to establish an authenticated MCP session using an arbitrary Bearer token.
The MCP auth handler supported OAuth2 passthrough for upstream MCP servers, but the fallback path could replace failed LiteLLM key validation with an empty UserAPIKeyAuth() object. This allowed requests with a fabricated Authorization header to reach MCP tooling without a valid LiteLLM key.
An attacker could use this to list and call configured MCP tools and access connected services exposed through MCP.
Patches
The issue is fixed in 1.84.0.
We recommend upgrading to 1.84.0 or later.
Workarounds
If upgrading is not immediately possible, disable MCP routes or block access to /mcp/ and related MCP endpoints at your reverse proxy or API gateway.
References
v1.84.0
Other sources
LiteLLM is a proxy server (AI Gateway) to call LLM APIs in OpenAI (or native) format. Prior to 1.84.0, LiteLLM's MCP Streamable HTTP endpoint allowed an unauthenticated attacker to use a fabricated Authorization header to trigger an OAuth2 passthrough fallback path that replaced failed LiteLLM key validation with an empty UserAPIKeyAuth() object, allowing requests to reach MCP tooling without a valid LiteLLM key. This issue is fixed in version 1.84.0.
— MITRE
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
pip/litellmto a version that resolves this vulnerability.Fixed in 1.84.0 - Upgrade
Upgrade
litellmto a version that resolves this vulnerability.Fixed in 1.84.0 - Compensating control
If upgrading is not immediately possible, disable MCP routes or block access to /mcp/ and related MCP endpoints at your reverse proxy or API gateway.
Event History
Frequently Asked Questions
What is the severity of CVE-2026-59822?
The severity of CVE-2026-59822 is rated as high with a score of 8.8.
How do I fix CVE-2026-59822?
To fix CVE-2026-59822, upgrade LiteLLM to version 1.84.0 or later.
What type of vulnerability is CVE-2026-59822?
CVE-2026-59822 is an authentication bypass vulnerability via OAuth2 Passthrough Fallback.
Who is impacted by CVE-2026-59822?
Users of LiteLLM versions prior to 1.84.0 are impacted by CVE-2026-59822.
What can an attacker do with CVE-2026-59822?
An attacker can exploit CVE-2026-59822 to trigger unauthorized access to the LiteLLM API using a fabricated Authorization header.