Bug 2454501 (CVE-2026-34827)

Summary: CVE-2026-34827 rack: Rack: Denial of Service via crafted multipart/form-data requests
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: high Docs Contact:
Priority: high    
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 modular Ruby web server interface. An unauthenticated attacker can exploit this vulnerability by sending a specially crafted multipart/form-data request. This request, containing numerous parts with lengthy backslash-escaped parameter values, causes the system to consume excessive CPU resources during parsing. This algorithmic complexity issue ultimately leads to a denial of service (DoS) condition in Rack applications that process multipart form data.
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Description OSIDB Bzimport 2026-04-02 18:02:29 UTC 
Rack is a modular Ruby web server interface. From versions 3.0.0.beta1 to before 3.1.21, and 3.2.0 to before 3.2.6, Rack::Multipart::Parser#handle_mime_head parses quoted multipart parameters such as Content-Disposition: form-data; name="..." using repeated String#index searches combined with String#slice! prefix deletion. For escape-heavy quoted values, this causes super-linear processing. An unauthenticated attacker can send a crafted multipart/form-data request containing many parts with long backslash-escaped parameter values to trigger excessive CPU usage during multipart parsing. This results in a denial of service condition in Rack applications that accept multipart form data. This issue has been patched in versions 3.1.21 and 3.2.6.