CVE-2026-45019

Vulnerability Description

Chainlit is a Python framework for building production-ready conversational AI applications. From 2.4.0rc0 until 2.12.0, Chainlit deployments with features.mcp.enabled set to true in .chainlit/config.toml expose the POST /mcp endpoint without requiring authentication. For sse and streamable-http transports, ConnectSseMCPRequest and ConnectStreamableHttpMCPRequest in backend/chainlit/types.py accept a user-controlled url and optional headers dictionary without scheme validation, private-address filtering, or an allowlist. The connect_mcp handler in backend/chainlit/server.py passes these values to sse_client() or streamablehttp_client(), allowing the Chainlit server to make blind outbound requests to arbitrary internal or external services, including cloud metadata endpoints, with attacker-controlled Authorization and Cookie headers. The SSE URL sink has existed since 2.4.0rc0, while attacker-controlled header forwarding and streamable-http support were added in 2.6.4. The response is consumed internally and not returned, but the attacker can issue state-changing authenticated requests, discover internal services, scan ports, and probe metadata endpoints. This issue is fixed in version 2.12.0.

CVSS Score

Related Weaknesses (CWE)

References

• https://github.com/Chainlit/chainlit/blob/2.12.0/docs/security-advisory-2026-mcp
• https://github.com/Chainlit/chainlit/commit/0565fd0eccb915fce159929598b053ed79f6
• https://github.com/Chainlit/chainlit/releases/tag/2.12.0
• https://github.com/Chainlit/chainlit/security/advisories/GHSA-hvfh-5mj3-5f3j