Vulnerability · Published Apr 2, 2026 · Assigned by GitHub_M
CVE-2026-34831Rack: Content-Length mismatch in Rack::Files error responses
Medium 4.8
In plain English
A medium-severity security flaw in rack from rack. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 4% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | ruby-rack | 2.2.22-0+deb12u2 | DEBIAN-CVE-2026-34831 |
| Debian:13 | ruby-rack | 3.1.20-0+deb13u2 | DEBIAN-CVE-2026-34831 |
| Debian:14 | ruby-rack | 3.2.6-2 | DEBIAN-CVE-2026-34831 |
| RubyGems | rack | 2.2.23 | GHSA-q2ww-5357-x388 |
| RubyGems | rack | 3.1.21 | GHSA-q2ww-5357-x388 |
| RubyGems | rack | 3.2.6 | GHSA-q2ww-5357-x388 |
| Ubuntu:24.04:LTS | ruby-rack | 2.2.7-1ubuntu0.7 | UBUNTU-CVE-2026-34831 |
| Ubuntu:25.10 | ruby-rack | 3.1.16-0.1ubuntu0.3 | UBUNTU-CVE-2026-34831 |
| Ubuntu:26.04:LTS | ruby-rack | 3.2.4-1ubuntu1.1 | UBUNTU-CVE-2026-34831 |
| Ubuntu:26.04:LTS | ruby-rack | 3.2.4-1ubuntu1.1 | USN-8989-1 |
Affected products (as reported)
Weakness: CWE-130, CWE-135 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
References
- https://github.com/rack/rack/security/advisories/GHSA-q2ww-5357-x388 x_refsource_CONFIRM