Rack::Request accepts invalid Host characters, enabling host allowlist bypass

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:

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:

[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:

req.host.start_with?("myapp.com")

accepts:

[email protected]

and:

req.host.end_with?("myapp.com")

accepts:

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.