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CVE-2026-45018 – Unauthenticated RCE via MCP Command Injection – Chainlit 2.4.0rc0–2.11.x

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Summary

CVE-2026-45018 is a critical OS command injection vulnerability (CWE-78) in Chainlit, a Python framework for building conversational AI applications. When the Model Context Protocol (MCP) feature is enabled, an unauthenticated attacker can send a crafted request to the POST /mcp endpoint to execute arbitrary shell commands with the privileges of the Chainlit process. The flaw carries a CVSS v3.1 base score of 9.8 (Critical) and requires no authentication or user interaction.

Technical details

  • Root cause: the validate_mcp_command() function (in backend/chainlit/mcp.py) checks only the executable name in a user-supplied fullCommand string against config.features.mcp.stdio.allowed_executables, then passes the remaining arguments unchecked to StdioServerParameters.
  • Additional flaw: if allowed_executables is left unset, its default value of None is treated as permitting every executable rather than none.
  • Trigger conditions: the deployment must have features.mcp.enabled = true in .chainlit/config.toml, and the attacker uses the stdio transport by submitting a command such as an allowed executable (e.g., npx) combined with argument flags (e.g., -c) that execute arbitrary shell commands.
  • Attack vector: network, unauthenticated, no user interaction — the /mcp endpoint does not require prior authentication.
  • Impact: full remote code execution with the privileges of the Chainlit server process, resulting in complete loss of confidentiality, integrity, and availability of the host.

Affected software

  • Chainlit (PyPI package chainlit), versions >= 2.4.0rc0 and < 2.12.0.
  • Note: MCP support has been disabled by default since Chainlit 2.7.0, which limits exposure to deployments that explicitly enabled features.mcp.enabled.

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: upgrade Chainlit to version 2.12.0 or later, which fixes the argument validation logic.
  • If immediate upgrade is not possible:
    • Set features.mcp.enabled = false in .chainlit/config.toml to disable the vulnerable MCP stdio endpoint entirely.
    • Explicitly restrict config.features.mcp.stdio.allowed_executables to a minimal, vetted list rather than leaving it unset.
    • Restrict or sandbox the host’s ability to spawn outbound processes from the Chainlit service.
    • Place an authentication requirement (e.g., an auth callback) in front of the application to reduce unauthenticated exposure.
      [{"name": "CVE-2026-45018 Record (CVE List)", "url": "https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/45xxx/CVE-2026-45018.json"}, {"name": "Chainlit Security Advisory GHSA-w3fx-mc44-mf6j", "url": "https://github.com/Chainlit/chainlit/security/advisories/GHSA-w3fx-mc44-mf6j"}, {"name": "Chainlit 2.12.0 Release Notes", "url": "https://github.com/Chainlit/chainlit/releases/tag/2.12.0"}]

References

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