CVE-2026-34831

Rack: Content-Length mismatch in Rack::Files error responses

Published Apr 2, 2026 CVSS 3.1 · 4.8 Medium EPSS < 1% KEV · Not listed
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.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Debian DLA Debian DLA DLA-4706-1 ruby-rack security update
Debian DSA Debian DSA DSA-6492-1 ruby-rack security update
Github GHSA Github GHSA GHSA-q2ww-5357-x388 Rack has Content-Length mismatch in Rack::Files error responses
Ubuntu USN Ubuntu USN USN-8182-1 Rack vulnerabilities
Ubuntu USN Ubuntu USN USN-8989-1 Rack vulnerabilities
History
Type Values Removed Values Added
CPEs cpe:2.3:a:rack:rack:*:*:*:*:*:ruby:*:*
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Type Values Removed Values Added
First Time appeared Rack
Rack rack
Vendors & Products Rack
Rack rack
Type Values Removed Values Added
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.
Title Rack: Content-Length mismatch in Rack::Files error responses
Weaknesses CWE-130
CWE-135
References
Metrics cvssV3_1

{'score': 4.8, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
GitHub_M
Published
Updated
Apr 3, 2026, 5:48 PM
Reserved
Mar 30, 2026, 8:52 PM
Status
Analyzed
Published
Apr 2, 2026, 5:16 PM
Modified
Jul 24, 2026, 9:10 PM
Severity
Moderate
Public date
Apr 2, 2026, 4:43 PM