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

Summary redirecturl is treated as safe when url.Parse(...).IsAbs() is false. Protocol-relative URLs like //ikotaslabs.com have an empty scheme and pass that check, allowing post-login redirects to attacker-controlled sites.

Details - url.Parse("//ikotaslabs.com") => empty Scheme, Host="ikotaslabs.com". - IsAbs() returns false for //ikotaslabs.com, so the code treats it as allowed. - Browser resolves //ikotaslabs.com to current-origin scheme (e.g. https://ikotaslabs.com), enabling phishing flows after login.

PoC 1. Send or visit: http://localhost/login?redirecturl=//ikotaslabs.com 2. Complete normal login flow. 3. After login the app redirects to https://ikotaslabs.com (or http:// depending on origin).

Acknowledgements This vulnerability was discovered using the automated vulnerability analysis tools VulScribe and PwnML. The research and tool development were conducted with support from the MITOU Advanced Program (未踏アドバンスト事業), implemented by the Information-technology Promotion Agency (IPA), Japan.