Affected: rack

Rack is a modular Ruby web server interface. From versions 3.0.0.beta1 to before 3.1.21 and 3.2.0 to before 3.2.6, Rack::Utils.forwarded_values parses the RFC 7239 Forwarded header by splitting on semicolons before handling quoted-string values. Because quoted values may legally contain semicolons, a header can be interpreted by Rack as multiple Forwarded directives rather than as a single quoted for value. In deployments where an upstream proxy, WAF, or intermediary validates or preserves quoted Forwarded values differently, this discrepancy can allow an attacker to smuggle host, proto, for, or by parameters through a single header value. This issue has been patched in versions 3.1.21 and 3.2.6.

Affected: RubyGems: rack

## Summary

`Rack::Utils.forwarded_values` parses the RFC 7239 `Forwarded` header by splitting on semicolons before handling quoted-string values. Because quoted values may legally contain semicolons, a header such as:

```http Forwarded: for="127.0.0.1;host=evil.com;proto=https" ```

can be interpreted by Rack as multiple `Forwarded` directives rather than as a single quoted `for` value.

In deployments where an upstream proxy, WAF, or intermediary validates or preserves quoted `Forwarded` values differently, this discrepancy can allow an attacker to smuggle `host`, `proto`, `for`, or `by` parameters through a single header value.

## Details

`Rack::Utils.forwarded_values` processes the header using logic equivalent to:

```ruby forwarded_header.split(';').each_with_object({}) do |field, values| field.split(',').each do |pair| pair = pair.split('=').map(&:strip).join('=') return nil unless pair =~ /\A(by|for|host|proto)="?([^"]+)"?\Z/i (values[$1.downcase.to_sym] ||= []) << $2 end end ```

The method splits on `;` before it parses individual `name=value` pairs. This is inconsistent with RFC 7239, which permits quoted-string values, and quoted strings may contain semicolons as literal content.

As a result, a header value such as:

```http Forwarded: for="127.0.0.1;host=evil.com;proto=https" ```

is not treated as a single `for` value. Instead, Rack may interpret it as if the client had supplied separate `for`, `host`, and `proto` directives.

This creates a…