Migration URL validation bypass via HTTP redirect to blocked internal endpoints

Summary

A Server-Side Request Forgery (SSRF) vulnerability exists in the repository migration functionality. The application validates only the initially submitted URL hostname, but git clone --mirror follows HTTP redirects. An authenticated user can submit a public URL that redirects to a blocked internal endpoint (e.g., 127.0.0.1), importing the internal repository's contents into an attacker-controlled repository.

Vulnerability Details

The vulnerability is located in internal/form/repo.go. ParseRemoteAddr() validates the clone address hostname against a blocklist of local and private-network addresses. However, the actual migration is performed by git clone --mirror in internal/database/repo.go, which follows HTTP redirects without revalidation:

1. Attacker submits http://attacker.example/redirect.git — passes validation (public hostname).
2. Attacker's server responds with 302 redirect to http://127.0.0.1:18081/victim/private.git.
3. Git follows the redirect and clones the internal repository.
4. Gogs imports the cloned contents into the attacker's new repository.

The root cause is that Gogs validates only the initial URL and does not revalidate the final redirect target.

Impact

This vulnerability bypasses the intended localhost/private-network migration restriction. Any authenticated user who can migrate repositories can import contents from internal Git endpoints reachable from the Gogs server. This allows attackers to:

• Steal source code and secrets from internal repositories served over HTTP.
• Scan internal network services via the migration endpoint.

Reproduction Steps

Prerequisites: a Gogs instance, an attacker account that can create repositories.

1. Start a local HTTP Git server with a test repository on 127.0.0.1:18081.
2. Start a redirect server that responds to any request with a 302 redirect to http://127.0.0.1:18081/victim/private.git.
3. Verify the direct localhost URL is blocked:

curl -sS -X POST -H "Authorization: token ${TOKEN}" \
  -H "Content-Type: application/json" \
  --data '{"clone_addr":"http://127.0.0.1:18081/victim/private.git","uid":2,"repo_name":"blocked"}' \
  "${GOGS_URL}/api/v1/repos/migrate"

Result: rejected as blocked local address.