Vulnerability identifier: #VU59319

Vulnerability risk: Medium

CVSSv3.1: 6.4 [CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N/E:U/RL:O/RC:C]

CVE-ID: CVE-2021-3618

CWE-ID: CWE-346

Exploitation vector: Network

Exploit availability: No

Vulnerable software:
nginx
Server applications / Web servers

Vendor: NGINX

Description

The vulnerability allows a remote attacker to bypass implemented security restrictions.

The vulnerability exists due to a logic error in TLS implementation when handling different protocols but using compatible certificates, such as multi-domain or wildcard certificates. A remote attacker with ability to perform TCP/IP layer MitM attack can redirect traffic from one subdomain to another, resulting in a valid TLS session. This breaks the authentication of TLS and cross-protocol attacks may be possible where the behavior of one protocol service may compromise the other at the application layer.

This attack technique was dubbed ALPACA (application layer protocol content confusion attack).

Mitigation
Install updates from vendor's website.

Vulnerable software versions

nginx: 1.0.0 - 1.0.15, 1.1.0 - 1.19.10, 1.2.0 - 1.20.0, 1.3.0 - 1.3.16, 1.4.0 - 1.4.7, 1.5.0 - 1.5.13, 1.6.0 - 1.6.3, 1.7.0 - 1.7.12, 1.8.0 - 1.8.1, 1.9.0 - 1.9.15

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External links
http://alpaca-attack.com/
http://bugzilla.redhat.com/show_bug.cgi?id=1975623
http://hg.nginx.org/nginx/rev/ec1071830799

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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.