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

Description

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.

Scores

EPSS (exploitation in 30 days)

0.20% · top 90% of all CVEs (5 Nov 2026)

CISA SSVC

Exploitation: none · Automatable: no · Technical impact: partial

The list uses the assigner's score when there is one, then CISA's, then NVD's: Medium 4.8.

Weakness

• CWE-1286: Improper Validation of Syntactic Correctness of Input (opens in a new tab)All about CWE-1286 and its CVEs

Affected products

• rack · rack

  • affected >= 3.0.0.beta1, < 3.1.21
  • affected >= 3.2.0, < 3.2.6

Affected packages

From reviewed advisories of the GitHub Advisory Database.

• RubyGemsrackGHSA-g2pf-xv49-m2h5 (opens in a new tab)

References (1)

• https://github.com/rack/rack/security/advisories/GHSA-g2pf-xv49-m2h5 (opens in a new tab)x_refsource_CONFIRM

Sources

The same CVE in the primary databases and aggregators, to check against.

• CVE.org record (opens in a new tab)
• CVE List JSON (GitHub) (opens in a new tab)
• NVD (opens in a new tab)
• FIRST EPSS (opens in a new tab)
• OSV.dev (opens in a new tab)
• GitHub Advisory GHSA-g2pf-xv49-m2h5 (opens in a new tab)
• CIRCL Vulnerability-Lookup (opens in a new tab)

Badge for a README or a ticket

[![CVE-2026-34835](https://trackcve.app/badge/CVE-2026-34835.svg)](https://trackcve.app/cve/CVE-2026-34835)

Record last updated 2 Apr 2026, 17:44 UTC.