The TrailingSlashMiddleware in internal/api/server.go is vulnerable to an open redirect attack. An attacker can craft a URL with a protocol-relative path (e.g., //evil.com/) that, after trailing slash removal, results in a Location header of //evil.com — which browsers interpret as an absolute URL to an external domain.
Details
The TrailingSlashMiddleware strips trailing slashes from request paths and issues a 308 Permanent Redirect to the cleaned path. However, it does not validate or sanitize the resulting path before using it as the redirect target.
When a request is made with a path like //evil.com/, the middleware processes it as follows:
PoC
Start the registry server locally or identify a deployed instance
Send a request with a double-slash path followed by an external domain:
curl -v https://<registry-host>//evil.com/
<img width="3066" height="969" alt="image" src="https://github.com/user-attachments/assets/a5305f00-29bf-4708-952a-478d608f2074" />
Observe the 308 Permanent Redirect response with Location: //evil.com:
When accessed in a browser, the user is redirected to https://evil.com
Impact
Phishing: Attackers can abuse the trusted registry domain to redirect users to credential-harvesting pages
Malware distribution: Redirect users to sites serving malicious downloads
Trust abuse: Links originating from the official MCP Registry domain carry implicit trust