CVE-2026-34831
MediumCVSS 4.8Exploitation Probability (EPSS)
Low risk4th percentile - higher than 4% of all known CVEs
Summary
In the Rack library (modular Ruby web server interface) versions prior to 2.2.23, 3.1.21, and 3.2.6, the Rack::Files#fail method sets the Content-Length header using String#size instead of String#bytesize. When the response body contains multibyte UTF-8 characters, the declared length is smaller than the actual bytes sent. An attacker can trigger this mismatch by requesting a non-existent path with percent-encoded UTF-8 characters, leading to incorrect HTTP response framing and potential response desynchronization in deployments relying on the wrong Content-Length value.
Risk Assessment
This vulnerability could allow an attacker to desynchronize HTTP responses, potentially leading to session hijacking or other request smuggling attacks.
Recommendation
Immediately upgrade the Rack library to version 2.2.23, 3.1.21, or 3.2.6 (depending on the branch in use).
Original NVD description (English source)
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.