Vulnerability in Rack
CVE-2026-34835
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.
EPSS: 0.002 (9.2th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 4.8 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N.
Affected products
- Rack — versions >= 3.0.0.beta1, < 3.1.21, >= 3.2.0, < 3.2.6
Weakness classification (CWE)
Public proof-of-concept exploits
References
- security-advisories@github.com (x_refsource_CONFIRM, Exploit, Mitigation, Vendor Advisory)
Frequently asked questions
- What is CVE-2026-34835?
- CVE-2026-34835 is a medium-severity vulnerability in Rack, classified under CWE-1286. CVSS score: 4.8/10. Published 2026-04-02.
- How severe is CVE-2026-34835?
- Medium severity. CVSS v3 base score is 4.8 out of 10.
- Is CVE-2026-34835 known to be exploited?
- 1 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.