CVE-2026-34835
low-risk
Published 2026-04-02
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.
Do I need to act?
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0.10% chance of exploitation
EPSS score — low exploit probability
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Not on CISA KEV list
No confirmed active exploitation reported to CISA
?
Patch status unknown
Check vendor advisories for fix availability and mitigation guidance
4
CVSS 4.8/10
Medium
NETWORK
/ HIGH complexity
Affected Products (1)
Affected Vendors
References (1)
20
/ 100
low-risk
Severity
15/34 · Moderate
Exploitability
0/34 · Minimal
Exposure
5/34 · Minimal