CVE-2026-21885: Miniflux Media Proxy SSRF via /proxy endpoint allows access to internal network resources

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: MEDIAPROXYMODE=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).

Impact Type: SSRF (Server-Side Request Forgery) via media proxy Who is impacted: Miniflux instances with media proxy enabled (default configuration typically enables it for HTTP/mixed content handling). Impact: attackers with a valid Miniflux account can fetch internal resources reachable from the Miniflux server (e.g., localhost services, private network services, and link-local endpoints such as 169.254.169.254), potentially exposing sensitive data.

Suggested CVSS (v3.1) CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N (Base 6.5)

If there any questions or issues reproducing this, please contact: jeongwoolee340@gmail.com