CVE-2021-3618: Debian Linux

High severity, CVSS 7.4. EPSS: 2% chance of exploitation in the next 30 days.

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 Debian Linux: version 10.0 only
• F5 Nginx: before 1.21.0 (fixed in 1.21.0)
• Fedoraproject Fedora: version 33 only; version 34 only; version 35 only
• Sendmail Sendmail: before 8.17 (fixed in 8.17)
• Vsftpd Project Vsftpd: before 3.0.4 (fixed in 3.0.4)

Published 2022-03-23. Last modified 2026-06-17.