CVE-2026-34831: Rack

Medium severity, CVSS 6.5. EPSS: 0.2% chance of exploitation in the next 30 days.

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.

Affected products

• Rack Rack: before 2.2.23 (fixed in 2.2.23); from 3.0.0, before 3.1.21 (fixed in 3.1.21); from 3.2.0, before 3.2.6 (fixed in 3.2.6)

Published 2026-04-02. Last modified 2026-07-24.