What’s Vulnerable › Vulnerabilities
CVE-2021-36740
- Severity: medium (CVSS 6.5 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 1.6% probability of exploitation in the next 30 days
- Public exploit or detection template: none known
- Published: 2021-07-14 · updated 2026-06-17
- Weakness: CWE-444
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.
Affected products
- Debian Linux: 10.0, 11.0
- Fedoraproject Fedora: 33, 34
- Varnish-Cache Varnish Cache: from 6.0.0 before 6.0.8; 6.0.8-r1, 6.0.8-r2 — fixed in 6.0.8
- varnish-software Varnish Cache: from 6.0.0 through 6.0.5; from 6.0.0 through 6.0.7
- Varnish Cache Project Varnish Cache: from 5.0.0 through 5.2.1; from 6.1.0 through 6.6.0
How to fix
Upgrade to a fixed version: Varnish-Cache Varnish Cache 6.0.8. Check the vendor advisory for mitigations.
References
- https://docs.varnish-software.com/security/VSV00007/
- https://github.com/varnishcache/varnish-cache/commit/82b0a629f60136e76112c6f2c6372cce77b683be
- https://github.com/varnishcache/varnish-cache/commit/9be22198e258d0e7a5c41f4291792214a29405cf
- 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.html
- https://www.debian.org/security/2022/dsa-5088
- https://docs.varnish-software.com/security/VSV00007/
- https://github.com/varnishcache/varnish-cache/commit/82b0a629f60136e76112c6f2c6372cce77b683be
- https://github.com/varnishcache/varnish-cache/commit/9be22198e258d0e7a5c41f4291792214a29405cf
- 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/
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