CVE-2026-34835

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 products

Linuxmint, Rack, Ubuntu

Rack

< 3.1.21, 3.2.6

Fix

Available

CVSS 3.1

6.5 MEDIUM

EPSS

0.2% (9th percentile)

Weakness

CWE-1286

NVD status

Analyzed

Published

2026-04-02