CVE-2021-3618

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.

Conditions required for the attack to work:

1. As per mentioned above you need a MiTM atack who can divert victim's traffic at the TCP/IP layer and therefore the only plausible attack model is on the local network.
2. If you are hosting several TLS-enabled application servers on the same hostname, or if you use multi-domain certificates, or if you use wild-card certificates, you may be vulnerable to the general confusion attack.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

1. Avoid wildcard certificates or multi-domain certificates if you are hosting several TLS-enabled application servers on the same hostname.
2. Two extensions in TLS can provide some protection to the application layer protocol: SNI and ALPN.