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Vulnerability record · CVE-2021-3618 · published 23 March 2022

CVE-2021-3618: F5 nginx improper certificate validation vulnerability

F5 · Nginx

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.

7.4 CVSS 3.1 High EPSS 2.0% · top 19.7% CWE-295 · Improper certificate validation
7.4CVSS 3.1 base score, v2 5.8
2.0%EPSS exploitation probability, 30 days
NoNot in CISA KEV
5Affected product versions listed by NVD
6References
17 Jun 2026Last modified by NVD

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.

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

Affected products

5 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://alpaca-attack.com/ Third Party Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=1975623 Issue TrackingPatchThird Party Advisory
https://lists.debian.org/debian-lts-announce/2022/11/msg00031.html Mailing ListThird Party Advisory
https://alpaca-attack.com/ Third Party Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=1975623 Issue TrackingPatchThird Party Advisory
https://lists.debian.org/debian-lts-announce/2022/11/msg00031.html Mailing ListThird Party Advisory

Track CVE-2021-3618 inside VULONE

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Source: NIST National Vulnerability Database (record CVE-2021-3618), CISA KEV, FIRST EPSS (scores of 2026-09-29). This page is refreshed as NVD updates the record.