Bug 2454504 (CVE-2026-34831)

Summary: CVE-2026-34831 rack: Rack: HTTP response desynchronization via incorrect Content-Length calculation with UTF-8 characters
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: akostadi, amasferr, crizzo, dmayorov, eshamard, jlledo, jvasik, kaycoth, pantinor, rblanco, tsedmik
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in Rack. A remote attacker can exploit this vulnerability by requesting a non-existent path containing percent-encoded UTF-8 characters. This causes Rack::Files#fail to incorrectly calculate the Content-Length header, using String#size instead of String#bytesize for multibyte characters. This leads to incorrect HTTP response framing, which may cause response desynchronization in systems relying on the Content-Length value.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Description OSIDB Bzimport 2026-04-02 18:02:38 UTC 
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.