Bug 2457698 (CVE-2026-40395)

Summary: CVE-2026-40395 Varnish Enterprise: Varnish Enterprise: Denial of Service via workspace overflow
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerability-draftAssignee: Product Security DevOps Team <prodsec-dev>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedKeywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in Varnish Enterprise. A remote attacker can exploit this vulnerability by sending a request with an excessive number of header fields. This can cause a "workspace overflow" within the vmod_headerplus module, leading to a daemon panic and crashing the Varnish Enterprise server. This results in a Denial of Service (DoS), making the server unavailable to legitimate users.
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Description OSIDB Bzimport 2026-04-12 20:01:37 UTC 
Varnish Enterprise before 6.0.16r12 allows a "workspace overflow" denial of service (daemon panic) for shared VCL. The headerplus.write_req0() function from vmod_headerplus updates the underlying req0, which is normally the original read-only request from which req is derived (readable and writable from VCL). This is useful in the active VCL, after amending req, to prepare a refined req0 before switching to a different VCL with the return (vcl(<label>)) action. This is for example how the Varnish Controller operates shared VCL deployments. If the amended req contained too many header fields for req0, this would have resulted in a workspace overflow that would in turn trigger a panic and crash the Varnish Enterprise server. This could be used as a Denial of Service attack vector by malicious clients.