CVE-2026-21885

Summary

Miniflux's media proxy endpoint (GET /proxy/{encodedDigest}/{encodedURL}) can be abused to perform Server-Side Request Forgery (SSRF). An authenticated user can cause Miniflux to generate a signed proxy URL for attacker-chosen media URLs embedded in feed entry content, including internal addresses (e.g., localhost, private RFC1918 ranges, or link-local metadata endpoints). Requesting the resulting /proxy/... URL makes Miniflux fetch and return the internal response.

Details

• Vulnerable route: GET /proxy/{encodedDigest}/{encodedURL} (accessible without authentication, but requires a server-generated HMAC-signed URL)
• Handler: internal/ui/proxy.go ((*handler).mediaProxy)
• Trigger: entry content is rewritten to proxy media URLs (e.g., mediaproxy.RewriteDocumentWithAbsoluteProxyURL(...)), producing signed /proxy/... URLs.
• Root cause: the proxy validates the URL scheme and HMAC signature, but does not restrict target hosts/IPs. As a result, requests to loopback/private/link-local addresses are allowed and fetched by the server.

PoC

1. Run Miniflux 2.2.15 with default configuration (media proxy enabled by default: MEDIA_PROXY_MODE=http-only).

2. Log in with any normal user account.

3. Subscribe to a feed you control that contains an entry with an image URL pointing to an internal address reachable from the Miniflux server, e.g.:

   • <img src="http://<internal-target>/secret"> (Note: <internal-target> must be reachable from the Miniflux process/network; in containerized setups, 127.0.0.1 may not refer to the host.)

4. Open the entry and locate the rewritten media proxy URL (/proxy/<encodedDigest>/<encodedURL>) in the rendered HTML/page source.

5. Request the /proxy/... URL. Expected (vulnerable): Miniflux fetches the internal URL and returns the internal response (SSRF).