CVE-2021-28543: Fedoraproject Fedora

High severity, CVSS 7.5. EPSS: 1.5% chance of exploitation in the next 30 days.

Varnish varnish-modules before 0.17.1 allows remote attackers to cause a denial of service (daemon restart) in some configurations. This does not affect organizations that only install the Varnish Cache product; however, it is common to install both Varnish Cache and varnish-modules. Specifically, an assertion failure or NULL pointer dereference can be triggered in Varnish Cache through the varnish-modules header.append() and header.copy() functions. For some Varnish Configuration Language (VCL) files, this gives remote clients an opportunity to cause a Varnish Cache restart. A restart reduces overall availability and performance due to an increased number of cache misses, and may cause higher load on backend servers.

Affected products

• Fedoraproject Fedora: version 34 only
• Varnish-Cache Varnish-Modules: before 0.17.1 (fixed in 0.17.1)
• Varnish-Cache Varnish-Modules Klarlack: before 0.17.1 (fixed in 0.17.1)

Published 2021-03-16. Last modified 2026-06-17.