CVE-2021-3618

Severity: high (CVSS 7.4 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 2.0% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2022-03-23 · updated 2026-06-17
Weakness: CWE-295

Description

ALPACA is an application layer protocol content confusion attack, exploiting TLS servers implementing different protocols but using compatible certificates, such as multi-domain or wildcard certificates. A MiTM attacker having access to victim's traffic at the TCP/IP layer 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.

Affected products

Debian Linux: 10.0
nginx: before 1.21.0 — fixed in 1.21.0
Fedoraproject Fedora: 33, 34, 35
Sendmail: before 8.17 — fixed in 8.17
Vsftpd Project Vsftpd: before 3.0.4 — fixed in 3.0.4

How to fix

Upgrade to a fixed version: nginx 1.21.0; Sendmail 8.17; Vsftpd Project Vsftpd 3.0.4. Check the vendor advisory for mitigations.

References

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