Bug 2457920 (CVE-2026-33555)

Summary: CVE-2026-33555 haproxy: HAProxy: Request smuggling via HTTP/3 parser desynchronization
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: amctagga, aoconnor, bniver, flucifre, gmeno, groman, mbenjamin, mhackett, sostapov, vereddy
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in HAProxy. A remote attacker could exploit this vulnerability by sending a specially crafted HTTP/3 request. The HTTP/3 parser fails to verify that the received body length matches the announced content-length when a stream is closed with an empty payload. This desynchronization with the backend server can lead to request smuggling, allowing an attacker to bypass security mechanisms and potentially access unauthorized resources.
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oVirt Team: --- RHEL 7.3 requirements from Atomic Host:
Cloudforms Team: --- Target Upstream Version:
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Bug Depends On: 2457988    
Bug Blocks:    

Description OSIDB Bzimport 2026-04-13 17:01:25 UTC
An issue was discovered in HAProxy before 3.3.6. The HTTP/3 parser does not check that the received body length matches a previously announced content-length when the stream is closed via a frame with an empty payload. This can cause desynchronization issues with the backend server and could be used for request smuggling. The earliest affected version is 2.6.