CVE-2026-34835
MediumCVSS 4.8Exploitation Probability (EPSS)
Low risk9th percentile - higher than 9% of all known CVEs
Summary
Rack::Request incorrectly parses the Host header, accepting characters like /, ?, #, and @ that are not allowed in RFC-compliant hostnames. The req.host method returns the full unvalidated value, allowing bypass of host validation based on simple prefix or suffix checks.
Risk Assessment
An attacker can perform host header poisoning, leading to malicious redirects, unsafe link generation, or incorrect origin validation.
Recommendation
Update Rack to version 3.1.21 or 3.2.6, which contain the fix.
Original NVD description (English source)
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.