Vulnerability in Rack
CVE-2026-34831
Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Files#fail sets the Content-Length response header using String#size instead of String#bytesize. When the response body contains multibyte UTF-8 characters, the declared Content-Length is smaller than the number of bytes actually sent on the wire. Because Rack::Files reflects the requested path in 404 responses, an attacker can trigger this mismatch by requesting a non-existent path containing percent-encoded UTF-8 characters. This results in incorrect HTTP response framing and may cause response desynchronization in deployments that rely on the incorrect Content-Length value. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.
Published · last modified .
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.
EPSS exploit prediction
EPSS: 0.002 (14.8th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 15th percentile — 14.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.001 · newest: 0.002 · change: +0.001
Affected products
- Rack — versions < 2.2.23, >= 3.0.0.beta1, < 3.1.21, >= 3.2.0, < 3.2.6
Weakness classification (CWE)
References
- github.com/rack/rack/security/advisories/GHSA-q2ww-5357-x388 (Vendor Advisory)
Frequently asked questions
- What is CVE-2026-34831?
- CVE-2026-34831 is a medium-severity vulnerability in Rack, classified under Improper Handling of Length Parameter Inconsistency. CVSS score: 4.8/10. Published 2026-04-02.
- How severe is CVE-2026-34831?
- Medium severity. CVSS v3 base score is 4.8 out of 10.