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Public story · 2026-09-09 · high
The flaw lets an attacker fake an Authorization header and skip key validation to reach MCP tools behind LiteLLM.
Why now: CISA added the flaw to its Known Exploited Vulnerabilities catalog on September 2, with the remediation deadline landing September 16.
CISA added CVE-2026-59822 to its Known Exploited Vulnerabilities catalog on September 2, setting a September 16 deadline for federal agencies to patch it. It's the first MCP-specific flaw CISA has confirmed exploited in the wild.
The bug sits in LiteLLM's MCP Streamable HTTP endpoint. An attacker sends a fabricated Authorization header, and when LiteLLM's key validation fails, an OAuth2 passthrough fallback kicks in.
That fallback swaps in an empty UserAPIKeyAuth() instead of rejecting the request. The empty auth object is enough to reach MCP tool listing and invocation, per The Hacker News. No valid key needed, just a header shaped like one.
LiteLLM 1.84.0 has the fix. The Hacker News piece covers this alongside other flaws CISA added to the catalog the same day. It doesn't say how many self-hosted LiteLLM instances are still running the vulnerable fallback path. It also doesn't say how attackers are finding them.
If you're running LiteLLM as an MCP gateway for agents, this bug is the reminder that the gateway's auth path decides what an unauthenticated request can touch. Internal or customer-facing, that gateway now needs the same scrutiny as the tools sitting behind it. The September 16 date only binds federal agencies. The vulnerable code doesn't check who's running it.
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