CVE-2026-34835: Rack::Request accepts invalid Host characters, enabling host allowlist bypass

Rack::Request parses the Host header using an AUTHORITY regular expression that accepts characters not permitted in RFC-compliant hostnames, including /, ?, #, and @. Because req.host returns the full parsed value, applications that validate hosts using naive prefix or suffix checks can be bypassed.

For example, a check such as req.host.start_with?("myapp.com") can be bypassed with Host: myapp.com@evil.com, and a check such as req.host.end_with?("myapp.com") can be bypassed with Host: evil.com/myapp.com.

This can lead to host header poisoning in applications that use req.host, req.url, or req.base_url for link generation, redirects, or origin validation.