CVE-2021-28543 [Medium] | Denial of Service in Varnish-Modules

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.

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2025-11-21	0.72%	0.92%	+0.20%
2	2025-11-18	0.92%	0.72%	-0.20%
3	2025-03-30	—	0.92%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2025-11-21

0.72%

0.92%

+0.20%

2025-11-18

0.92%

0.72%

-0.20%

2025-03-30

—

0.92%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
4.0	3.1	MEDIUM	`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:L` Click to expand Attack vector (AV:N) Could be attacked over the internet or any normal routed network—not just someone sitting at the machine. Attack complexity (AC:H) Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work. Privileges required (PR:N) No account or special rights needed—anonymous or random user is enough. User interaction (UI:N) Nobody has to click “OK” or open a trap file; it can work without a victim helping. Scope (S:C) Breaking this can reach past the original component and bite other resources—bigger blast radius. Confidentiality (C:N) Doesn’t really leak secrets in a meaningful way. Integrity (I:N) Data isn’t meaningfully altered or forged. Availability (A:L) Might cause slowdowns, glitches, or partial disruption—not a full brick.	2.2	1.4	[email protected]
7.5	3.1	HIGH	`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H` Click to expand Attack vector (AV:N) Could be attacked over the internet or any normal routed network—not just someone sitting at the machine. Attack complexity (AC:L) Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup. Privileges required (PR:N) No account or special rights needed—anonymous or random user is enough. User interaction (UI:N) Nobody has to click “OK” or open a trap file; it can work without a victim helping. Scope (S:U) Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems. Confidentiality (C:N) Doesn’t really leak secrets in a meaningful way. Integrity (I:N) Data isn’t meaningfully altered or forged. Availability (A:H) Could take the service down hard or make it unusable for people who depend on it.	3.9	3.6	[email protected]
5.0	2.0	MEDIUM	`AV:N/AC:L/Au:N/C:N/I:N/A:P` Click to expand Access vector (AV:N) Can be exploited remotely over network reachability. Access complexity (AC:L) Exploitation conditions are straightforward and predictable. Authentication (AU:N) No authentication is required. Confidentiality impact (C:N) No confidentiality impact. Integrity impact (I:N) No integrity impact. Availability impact (A:P) Partial availability impact.	10.0	2.9	[email protected]

Base score

Version

Severity

Vector

Exploitability

Impact

Score source

4.0

3.1

MEDIUM

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:N/I:N/A:L Click to expand

Attack vector (AV:N): Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:H): Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
Privileges required (PR:N): No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N): Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:C): Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:N): Doesn’t really leak secrets in a meaningful way.
Integrity (I:N): Data isn’t meaningfully altered or forged.
Availability (A:L): Might cause slowdowns, glitches, or partial disruption—not a full brick.

2.2

1.4

[email protected]

7.5

3.1

HIGH

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand

Attack vector (AV:N): Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L): Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N): No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N): Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U): Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N): Doesn’t really leak secrets in a meaningful way.
Integrity (I:N): Data isn’t meaningfully altered or forged.
Availability (A:H): Could take the service down hard or make it unusable for people who depend on it.

3.9

3.6

[email protected]

5.0

2.0

MEDIUM

AV:N/AC:L/Au:N/C:N/I:N/A:P Click to expand

Access vector (AV:N): Can be exploited remotely over network reachability.
Access complexity (AC:L): Exploitation conditions are straightforward and predictable.
Authentication (AU:N): No authentication is required.
Confidentiality impact (C:N): No confidentiality impact.
Integrity impact (I:N): No integrity impact.
Availability impact (A:P): Partial availability impact.

10.0

2.9

[email protected]

OS Trackers for CVE-2021-28543

vendor	priority	summary	link
`debian`	unimportant	CVE-2021-28543 unimportant priority: Debian including 1 source packages (varnish-modules), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5.	https://security-tracker.debian.org/tracker/CVE-2021-28543
`redhat`	low	—	https://access.redhat.com/security/cve/CVE-2021-28543
`ubuntu`	medium	CVE-2021-28543 medium priority: Ubuntu including 1 source packages (varnish-modules), 6 status rows across 6 suites (bionic, focal, groovy, trusty, upstream, xenial): DNE 3, not-affected 2, released 1.	https://ubuntu.com/security/CVE-2021-28543

Affected software / configurations for CVE-2021-28543

Vendor	Product	Version	Raw CPE
varnish-cache	varnish-modules	< 0.17.1	cpe:2.3:a:varnish-cache:varnish-modules::::::::
varnish-cache	varnish-modules_klarlack	< 0.17.1	cpe:2.3:a:varnish-cache:varnish-modules_klarlack::::::::
fedoraproject	fedora	34	cpe:2.3:o:fedoraproject:fedora:34:::::::*

References for CVE-2021-28543

URL	Tags
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/OPFBRQUJNWHCB3GQHSSAPRLQU6Q6PY43/
https://varnish-cache.org/security/VSV00006.html	Vendor Advisory

URL

CVE-2021-28543

Exploit prediction scoring system (EPSS) score for CVE-2021-28543

Common vulnerability scoring system (CVSS) metrics for CVE-2021-28543

Weakness enumeration for CVE-2021-28543

OS Trackers for CVE-2021-28543

Affected software / configurations for CVE-2021-28543

References for CVE-2021-28543