How severe is CVE-2021-28543?

CVE-2021-28543 has a CVSS 3.x base score of 4.0, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability low.

Is CVE-2021-28543 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (71st percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-28543?

CVE-2021-28543 primarily affects Varnish-cache Varnish-modules. In total, 3 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.

How do I fix CVE-2021-28543?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

When was CVE-2021-28543 published?

CVE-2021-28543 was published on 2021-03-16 and last updated on 2026-06-17.

CVE-2021-28543

CVE-2021-28543 is a medium-severity vulnerability in Varnish-cache Varnish-modules with a CVSS 3.x base score of 4.0. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-476.

Key facts

Severity: Medium (CVSS 3.x base score 4.0)
CVSS v2: 5.0
EPSS exploit prediction: 1% (71st percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-476
Affected product: Varnish-cache Varnish-modules
Published: 2021-03-16
Last modified: 2026-06-17

Description

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.

Frequently asked questions

What is CVE-2021-28543?: 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.
How severe is CVE-2021-28543?: CVE-2021-28543 has a CVSS 3.x base score of 4.0, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity none, and availability low.
Is CVE-2021-28543 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (71st percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-28543?: CVE-2021-28543 primarily affects Varnish-cache Varnish-modules. In total, 3 product configurations (CPEs) are listed as vulnerable; see the affected-products list for the exact versions.
How do I fix CVE-2021-28543?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
When was CVE-2021-28543 published?: CVE-2021-28543 was published on 2021-03-16 and last updated on 2026-06-17.

References

Affected products (3)

cpe:2.3:a:varnish-cache:varnish-modules:*:*:*:*:*:*:*:*
cpe:2.3:a:varnish-cache:varnish-modules_klarlack:*:*:*:*:*:*:*:*
cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*

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