EUVD-2026-21740
CVSS Vector
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:L
Lifecycle Timeline
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Description
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.
Analysis
Varnish Enterprise before version 6.0.16r12 is vulnerable to a workspace overflow denial of service attack that crashes the daemon through the headerplus.write_req0() VCL function. Unauthenticated remote attackers can craft HTTP requests with excessive header fields to trigger a panic and terminate the Varnish server, particularly impacting deployments using shared VCL through Varnish Controller. …
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