SSE Transport Has No Authentication and Wildcard CORS, Exposing All 86 GitLab Tools Including Destructive Operations

I reviewed mcp-gitlab-server at commit 80a7b4cf3fba6b55389c0ef491a48190f7c8996a and found that the SSE HTTP transport — advertised in the README and comparison table as a differentiating feature — runs with no authentication and wildcard CORS on every endpoint. The maintainers' own roadmap confirms auth is a known gap. I'm opening this to make the impact concrete and give the fix a clear path before the feature's user base grows.

When USE_SSE=true, the HTTP server in src/transport.ts sets:

res.setHeader('Access-Control-Allow-Origin', '*');
res.setHeader('Access-Control-Allow-Methods', 'GET, POST');
res.setHeader('Access-Control-Allow-Headers', 'Content-Type');

The httpServer.listen(port) call at line 97 passes no host argument — Node.js defaults to 0.0.0.0, binding on all interfaces. Two endpoints are exposed with no credential check:

• GET /sse — opens an SSE connection, returns a session endpoint URL
• POST /messages?sessionId=<id> — sends MCP messages to the server using the loaded GITLAB_PERSONAL_ACCESS_TOKEN

Any caller who can reach the port — LAN, cloud instance, or via the browser-tab vector the wildcard CORS enables — gets full access to all 86 tools the server exposes using the operator's GitLab PAT. That includes delete_repository, delete_group, push_files, create_merge_request, update_repository_settings, and any other tool the server exposes. The PAT doesn't leave the process, but every API call it backs is available to the unauthenticated caller.

The wildcard CORS makes the browser-tab vector direct: any web page the operator visits while the server is running can open an SSE connection and make tool calls via cross-origin fetch. No user interaction beyond visiting the page.

PoC — reproduces from the documented USE_SSE=true configuration:

# Step 1: connect SSE and capture the session endpoint
curl -N http://localhost:3000/sse &
# Output includes: event: endpoint
#                  data: /messages?sessionId=<UUID>

# Step 2: call any tool — no auth header needed
curl -X POST "http://localhost:3000/messages?sessionId=<UUID>" \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "tools/call",
    "params": {
      "name": "get_repository",
      "arguments": {"project_id": "target-org/private-repo"}
    }
  }'
# Returns repository data using the operator's GitLab PAT

# Same path works for delete_repository, push_files, etc.

Root cause

The HTTP transport in src/transport.ts ships with no authentication layer at all and a wildcard Access-Control-Allow-Origin: * on every response. The structural defect is that the SSE server stands up a stateful, mutation-capable RPC endpoint that is backed by the operator's GITLAB_PERSONAL_ACCESS_TOKEN without any inbound credential check, then advertises itself to every cross-origin browser context via the wildcard CORS header. The httpServer.listen(port) call at line 97 also passes no host argument, so the bind defaults to 0.0.0.0 and exposes the auth-less surface on every interface. Auth isn't fail-opening on a missing config — there is no auth check at any code path on either /sse or /messages?sessionId=....

Auth boundary violated

Trust-domain boundary — untrusted cross-origin browser context (and any unauthenticated network caller) crossing into the trusted server-state-mutating GitLab API surface that the operator's PAT backs. Respected-here: nothing. The transport carries no Authorization check, no origin allowlist, no session-binding to the originating client, and no host restriction. Ignored-there: the SSE handler at src/transport.ts accepts an arbitrary Origin (since Access-Control-Allow-Origin: *), opens a session, and the matching POST /messages?sessionId=... proxies tool calls — including delete_repository, push_files, update_repository_settings — to the GitLab API using the operator's PAT. Any web page the operator visits while the server runs can drive the full 86-tool surface via cross-origin fetch.

The roadmap in README.md at line 190 includes - [ ] SAML/OAuth3 authentication — confirming the maintainers are already tracking this gap. The issue is disclosure of impact in the interim: operators who follow the README's SSE setup instructions and don't see an auth requirement in the docs may reasonably assume the transport is safe to use on a network-accessible host.

CVSS 4.0: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:L/SC:N/SI:N/SA:N — ~6.3 (Medium). AT:P reflects the USE_SSE=true precondition. When that precondition is met, the effective severity for those deployments is High — full GitLab PAT access without authentication. The Medium CVSS is the aggregate across all deployments; for any operator who has activated SSE mode (which the README promotes as a feature), the finding is functionally High.

Fix — four concrete changes:

1. Require MCP_GITLAB_AUTH_TOKEN as a startup precondition when USE_SSE=true. If the env var is unset, the server should exit with a clear message before the HTTP server starts:

   if (process.env.USE_SSE === 'true') {
     if (!process.env.MCP_GITLAB_AUTH_TOKEN) {
       console.error(
         'ERROR: MCP_GITLAB_AUTH_TOKEN must be set when USE_SSE=true. ' +
         'SSE transport without authentication exposes all GitLab tools to unauthenticated callers.'
       );
       process.exit(1);
     }
   }

   The token check in src/transport.ts validates it on every request:

   const authToken = process.env.MCP_GITLAB_AUTH_TOKEN;
   if (authToken) {
     const provided = req.headers['authorization']?.replace(/^Bearer /, '');
     if (provided !== authToken) {
       res.writeHead(401);
       res.end(JSON.stringify({ error: 'Unauthorized' }));
       return;
     }
   }

2. Bind to 127.0.0.1 by default for the SSE transport rather than 0.0.0.0. An explicit MCP_GITLAB_HOST=0.0.0.0 flag with a startup banner warning can expose it to the network for operators who need that — but the safe default should be loopback-only.

3. Replace the wildcard Access-Control-Allow-Origin: * with a localhost-only default. When network exposure is intentional (explicit flag + auth token set), an explicit CORS_ORIGINS allowlist should be required.

4. The SAML/OAuth3 roadmap item is the right long-term direction. In the interim — before that ships — the three changes above are entirely in the existing codebase with no new dependencies.

No prior security advisories, CVEs, or public security issues exist for this package — I checked the repository issue list and npm advisory database before opening this.