What’s Vulnerable › Vulnerabilities
CVE-2026-34835: Rack: `Rack::Request` accepts invalid Host characters, enabling host allowlist bypass.
- Severity: medium (CVSS 6.5 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 0.2% probability of exploitation in the next 30 days
- Public exploit or detection template: yes
- Published: 2026-04-02 · updated 2026-07-24
- Weakness: CWE-1286
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.
Affected products
- rack: from 3.0.0 before 3.1.21; from 3.2.0 before 3.2.6 — fixed in 3.1.21, 3.2.6
How to fix
Upgrade to a fixed version: rack 3.1.21, 3.2.6. Check the vendor advisory for mitigations.
References
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