CVE-2026-44895

CVSS v4.0

8.8

High

Vector

AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:H/SC:N/SI:N/SA:N

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

A review of mcp-gitlab-server at commit 80a7b4cf3fba6b55389c0ef491a48190f7c8996a uncovered 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.

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:

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;
 }
}

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.
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.
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 — a search of the repository issue list and npm advisory database did not yield any duplicate issues.

Fix

Missing Authentication

Found an issue in the description? Have something to add? Feel free to write us 👾

dbugs@ptsecurity.com

Weakness Enumeration

CWE-306CWE-942

Related Identifiers

CVE-2026-44895

GHSA-8JR5-6GVJ-RFPF

Affected Products

@Yoda.Digital/Gitlab-Mcp-Server

CVE-2026-44895

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

Root cause

Auth boundary violated

Weakness Enumeration

Related Identifiers

Affected Products

References · 3