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

## Summary `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: [email protected]`, 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. ## Details `Rack::Request` parses the authority component using logic equivalent to: ```ruby AUTHORITY = / \A (?<host> \[(?<address>#{ipv6})\] | (?<address>[[[:graph:]&&[^\[\]]]]*?) ) (:(?<port>\d+))? \z /x ``` The character class used for non-IPv6 hosts accepts nearly all printable characters except `[` and `]`. This includes reserved URI delimiters such as `@`, `/`, `?`, and `#`, which are not valid hostname characters under RFC 3986 host syntax. As a result, values such as the following are accepted and returned through `req.host`: ```text [email protected] evil.com/myapp.com evil.com#myapp.com ``` Applications that attempt to allowlist hosts using string prefix or suffix checks may therefore treat attacker-controlled hosts as trusted. For example: ```ruby req.host.start_with?("myapp.com") ``` accepts: ```text [email protected] ``` and: ```ruby req.host.end_with?("myapp.com") ``` accepts: ```text evil.com/myapp.com ``` When those values are later used to build absolute URLs or enforce origin restrictions, the application may produce attacker-controlled results. ## Impact Applications that rely on `req.host`, `req.url`, or `req.base_url` may be affected if they perform naive host validation or assume Rack only returns RFC-valid hostnames. In affected deployments, an attacker may be able to bypass host allowlists and poison generated links, redirects, or origin-dependent security decisions. This can enable attacks such as password reset link poisoning or other host header injection issues. The practical impact depends on application behavior. If the application or reverse proxy already enforces strict host validation, exploitability may be reduced or eliminated. ## Mitigation * Update to a patched version of Rack that rejects invalid authority characters in `Host`. * Enforce strict `Host` header validation at the reverse proxy or load balancer. * Do not rely on prefix or suffix string checks such as `start_with?` or `end_with?` for host allowlisting. * Use exact host allowlists, or exact subdomain boundary checks, after validating that the host is syntactically valid.

Quoted source text, attributed separately from HOL analysis.