CVE-2021-28543Network attack vector

Daemon restart via Header VMOD NULL dereference

Published Mar 16, 2021 · Updated Aug 3, 2024

Denial of service in Varnish varnish-modules 0.17.0 allows remote attackers to restart Varnish Cache through requests processed by affected VCL. The Header VMOD's header.append() and header.copy() paths permit an assertion failure or NULL-pointer dereference during header processing. Exposure requires Varnish Cache 6.5 and VCL that invokes either affected function; a restart increases cache misses and backend load.

CVSS severity7.5
High
EPSS probability1.49%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial of service in Varnish varnish-modules 0.17.0 allows remote attackers to restart Varnish Cache through requests processed by affected VCL. The Header VMOD's header.append() and header.copy() paths permit an assertion failure or NULL-pointer dereference during header processing. Exposure requires Varnish Cache 6.5 and VCL that invokes either affected function; a restart increases cache misses and backend load.

The record

CVE
CVE-2021-28543
Published
Mar 16, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-617, CWE-476, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 16, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
7.8NIST NVDAV:N/AC:L/Au:N/C:N/I:N/A:P

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-617Reachable Assertion
CWE-476NULL Pointer Dereference
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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