Bug 2454483 (CVE-2026-26961)

Summary: CVE-2026-26961 github.com/rack/rack: Rack: Content smuggling via multipart boundary parsing mismatch
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: low Docs Contact:
Priority: low    
Version: unspecifiedCC: anthomas, ehelms, ggainey, juwatts, mhulan, nmoumoul, osousa, pcreech, rchan, smallamp, tmalecek
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. A remote attacker can exploit a vulnerability in Rack::Multipart::Parser by crafting a Content-Type header with multiple boundary parameters. This allows the attacker to bypass security inspections performed by upstream proxies or Web Application Firewalls (WAFs), leading to the Rack application processing different multipart content than what was validated by the intermediary. This content smuggling can enable unauthorized actions or data manipulation.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
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Description OSIDB Bzimport 2026-04-02 18:01:25 UTC 
Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Multipart::Parser extracts the boundary parameter from multipart/form-data using a greedy regular expression. When a Content-Type header contains multiple boundary parameters, Rack selects the last one rather than the first. In deployments where an upstream proxy, WAF, or intermediary interprets the first boundary parameter, this mismatch can allow an attacker to smuggle multipart content past upstream inspection and have Rack parse a different body structure than the intermediary validated. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.