CVE-2026-34835 — Rack: `Rack::Request` accepts invalid Host characters, enabling host allowlist bypass.

Description (CNA)

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 (CNA)

Vendor / product	Versions
rack / rack	affected >= 3.0.0.beta1, < 3.1.21; affected >= 3.2.0, < 3.2.6

CWE

CWE-1286

References

https://github.com/rack/rack/security/advisories/GHSA-g2pf-xv49-m2h5

ADP enrichment

CISA-ADP

No English description supplied.

No references supplied.