#VU55501 Inconsistent interpretation of HTTP requests in Varnish Cache - CVE-2021-36740

Vulnerability identifier: #VU55501

Vulnerability risk: Medium

CVSSv4.0: 2.7 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N/E:U/U:Green]

CVE-ID: CVE-2021-36740

CWE-ID: CWE-444

Exploitation vector: Network

Exploit availability: No

Vulnerable software:
Varnish Cache
Server applications / IDS/IPS systems, Firewalls and proxy servers

Vendor: Varnish Software

Description

The vulnerability allows a remote attacker to preform HTTP request smuggling attacks.

The vulnerability exists due to improper validation of HTTP/2 requests. A remote attacker can send a specially crafted HTTP/2 POST request with a large Content-Length header to the server and smuggle arbitrary HTTP headers.

Successful exploitation of vulnerability may allow an attacker to poison HTTP cache or bypass VCL authorization.

Mitigation
Install updates from vendor's website.

Vulnerable software versions

Varnish Cache: 6.0.0 - 6.0.7, 6.1.0 - 6.1.1, 6.2.0 - 6.2.3, 6.3.0 - 6.3.2, 6.4.0, 6.5.0 - 6.5.1, 6.6.0

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External links
https://github.com/varnishcache/varnish-cache/commit/82b0a629f60136e76112c6f2c6372cce77b683be
https://github.com/varnishcache/varnish-cache/commit/9be22198e258d0e7a5c41f4291792214a29405cf
https://docs.varnish-software.com/security/VSV00007/
https://varnish-cache.org/security/VSV00007.html

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Q & A

Can this vulnerability be exploited remotely?

Yes. This vulnerability can be exploited by a remote non-authenticated attacker via the Internet.

Is there known malware, which exploits this vulnerability?

No. We are not aware of malware exploiting this vulnerability.