Affected: Homebrew: travis

## 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: 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.

## 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 myapp.com@evil.com 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:…

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::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. 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. This issue has been patched in versions 3.1.21 and 3.2.6.

Affected: RubyGems: rack

## 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: 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.

## 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 myapp.com@evil.com 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:…

Affected: Ubuntu:25.10: ruby-rack, Ubuntu:26.04:LTS: ruby-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::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. 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. This issue has been patched in versions 3.1.21 and 3.2.6.