CVE-2021-36740

Name: CVE-2021-36740
Creator: NVD / NIST
Published: 2021-07-14T17:15:08.253+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.5/10EPSS 1.60%

Last modified Jun 17, 2026

CVE-2021-36740 is a medium-severity vulnerability rated 6.5/10 on the CVSS scale. Varnish Cache, with HTTP/2 enabled, allows request smuggling and VCL authorization bypass via a large Content-Length header for a POST request. This affects Varnish Enterprise 6.0.x before 6.0.8r3, and Varnish Cache 5.x and 6.x before 6.5.2, 6.6.x before 6.6.1, and 6.0 LTS before 6.0.8.. EPSS estimates a 1.60% chance of exploitation in the next 30 days.

Description

Varnish Cache, with HTTP/2 enabled, allows request smuggling and VCL authorization bypass via a large Content-Length header for a POST request. This affects Varnish Enterprise 6.0.x before 6.0.8r3, and Varnish Cache 5.x and 6.x before 6.5.2, 6.6.x before 6.6.1, and 6.0 LTS before 6.0.8.

Metrics

CVSS 3.1

6.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N

EPSS Probability

1.60%

72.7th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-444

Affected Software

Vendor	Product	Versions	Update
Varnish-Cache	Varnish Cache	>= 6.0.0, < 6.0.8	—
Varnish-Cache	Varnish Cache	6.0.8	R1
Varnish-Software	Varnish Cache	>= 6.0.0, <= 6.0.5	—
Varnish-Software	Varnish Cache	>= 6.0.0, <= 6.0.7	—
Varnish Cache Project	Varnish Cache	>= 5.0.0, <= 5.2.1	—
Varnish Cache Project	Varnish Cache	>= 6.1.0, <= 6.6.0	—
Fedoraproject	Fedora	33	—
Fedoraproject	Fedora	34	—
Debian	Debian Linux	10.0	—
Debian	Debian Linux	11.0	—

References

https://docs.varnish-software.com/security/VSV00007/Mitigation, Vendor Advisory
https://github.com/varnishcache/varnish-cache/commit/82b0a629f60136e76112c6f2c6372cce77b683bePatch, Third Party Advisory
https://github.com/varnishcache/varnish-cache/commit/9be22198e258d0e7a5c41f4291792214a29405cfPatch, Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/THV2DQA2GS65HUCKK4KSD2XLN3AAQ2V5/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZHBNLDEOTGYRIEQZBWV7F6VPYS4O2AAK/
https://varnish-cache.org/security/VSV00007.htmlMitigation, Vendor Advisory
https://www.debian.org/security/2022/dsa-5088Third Party Advisory
https://docs.varnish-software.com/security/VSV00007/Mitigation, Vendor Advisory
https://github.com/varnishcache/varnish-cache/commit/82b0a629f60136e76112c6f2c6372cce77b683bePatch, Third Party Advisory
https://github.com/varnishcache/varnish-cache/commit/9be22198e258d0e7a5c41f4291792214a29405cfPatch, Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/THV2DQA2GS65HUCKK4KSD2XLN3AAQ2V5/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/ZHBNLDEOTGYRIEQZBWV7F6VPYS4O2AAK/
https://varnish-cache.org/security/VSV00007.htmlMitigation, Vendor Advisory
https://www.debian.org/security/2022/dsa-5088Third Party Advisory

Timeline

Published: Jul 14, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-36740?

How severe is CVE-2021-36740?

CVE-2021-36740 has a CVSS score of 6.5/10 (MEDIUM severity). The EPSS model estimates a 1.60% probability of exploitation in the next 30 days.

How do I fix CVE-2021-36740?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-36740?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST