CVE-2021-28543: Varnish varnish-modules before 0.17.1 allows remote attackers to cause a denial of service (daemon restart) in

Q: How do I fix CVE-2021-28543?

varnish-modules: upgrade to 0.17.1; varnish-modules klarlack: upgrade to 0.17.1; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Q: Is CVE-2021-28543 being exploited?

CVE-2021-28543 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-28543 affect?

CVE-2021-28543 affects varnish-modules, varnish-modules klarlack, fedora. Severity is medium (CVSS 4.0). 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 ins

Q: How urgent is CVE-2021-28543?

Priority 38/100. Remotely exploitable, no login, no user action

Routine

MEDIUM 4.0REMOTEUPGRADEUPGRADEMITIGATE

CVE-2021-28543 is a medium-severity flaw (CVSS 4.0) affecting varnish-modules, varnish-modules klarlack, fedora. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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() functi

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	4.0 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	1.5% (top 26%)
CISA exploited list	No
Weakness	CWE-476, CWE-617
Published	2021-03-16
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: fedoraproject · varnish-cache

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-03-16 15:15 UTCPublished CVE record published
2026-06-17 03:46 UTCUpdated Record last modified

varnish-cache varnish-modules UPGRADE

Affected: < 0.17.1

Fixed in: 0.17.1

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy varnish-modules || rpm -q varnish-modules || varnish-modules --version

2. UPGRADE to 0.17.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade varnish-modules

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade varnish-modules

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

varnish-cache varnish-modules klarlack UPGRADE

Affected: < 0.17.1

Fixed in: 0.17.1

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy varnish-modules-klarlack || rpm -q varnish-modules-klarlack || varnish-modules-klarlack --version

2. UPGRADE to 0.17.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade varnish-modules-klarlack

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade varnish-modules-klarlack

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

fedoraproject fedora MITIGATE

Affected: 34

Fixed in: no fixed release known

Safe to downgrade to: a release before 34

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-28543?

varnish-modules: upgrade to 0.17.1; varnish-modules klarlack: upgrade to 0.17.1; fedora: mitigate or apply the listed mitigation. Exact commands for each OS are on this page.

Is CVE-2021-28543 being exploited?

CVE-2021-28543 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-28543 affect?

CVE-2021-28543 affects varnish-modules, varnish-modules klarlack, fedora. Severity is medium (CVSS 4.0). 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 ins

How urgent is CVE-2021-28543?

Priority 38/100. Remotely exploitable, no login, no user action

Software pages

Varnish vulnerabilities

References

← Back to latest CVEs