CVE-2026-34831

Name	CVE-2026-34831
Description	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.
Source	CVE (at NVD; CERT, ENISA, LWN, oss-sec, fulldisc, Debian ELTS, Red Hat, Ubuntu, Gentoo, SUSE bugzilla/CVE, GitHub advisories/code/issues, web search, more)

Vulnerable and fixed packages

The table below lists information on source packages.

Source Package	Release	Version	Status
ruby-rack (PTS)	bullseye	2.1.4-3+deb11u2	vulnerable
	bullseye (security)	2.1.4-3+deb11u5	vulnerable
	bookworm	2.2.20-0+deb12u1	vulnerable
	bookworm (security)	2.2.22-0+deb12u1	vulnerable
	trixie	3.1.18-1~deb13u1	vulnerable
	trixie (security)	3.1.20-0+deb13u1	vulnerable
	forky	3.1.18-1	vulnerable
	sid	3.2.5-2	vulnerable

The information below is based on the following data on fixed versions.

Package	Type	Release	Fixed Version	Urgency	Origin	Debian Bugs
ruby-rack	source	experimental	3.2.6-1
ruby-rack	source	(unstable)	(unfixed)

Notes

https://github.com/rack/rack/security/advisories/GHSA-q2ww-5357-x388
Fixed by: https://github.com/rack/rack/commit/10ecd9a8a02083297f925f0d9255a93ebde4c0da (v3.2.6)
Fixed by: https://github.com/rack/rack/commit/367a2a0ec6fbef605c9412dadfd5763b7867441f (v3.1.21)
Fixed by: https://github.com/rack/rack/commit/a75847314e8ad847a5b66e7215381c4ed51f6aa7 (v2.2.23)