EUVD-2026-18478
GHSA-g2pf-xv49-m2h5
CVSS Vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
Lifecycle Timeline
4Description
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.
Analysis
Host header validation bypass in Rack 3.0.0.beta1-3.1.20 and 3.2.0-3.2.5 allows unauthenticated remote attackers to poison Host headers by injecting RFC-noncompliant characters (/, ?, #, @) that pass the AUTHORITY regex but are accepted by req.host, req.url, and req.base_url. Applications relying on naive prefix or suffix matching for host validation, link generation, or origin checks can be bypassed, enabling host header poisoning attacks. …
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