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Fencer Vulns
high 7.4

CVE-2021-3618 json

ALPACA is an application layer protocol content confusion attack, exploiting TLS servers implementing different protocols but using compati…

published 2022-03-23 4 y ago modified 2026-09-08 2 mo ago also BIT-nginx-2021-3618 , BIT-nginx-gateway-2021-3618 , EUVD-2021-26922

cvss
7.4
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
2.0%
EPSS 2.0% chance of exploitation in 30 days, percentile 81%
public exploits
0
none known

Summary

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.

Patch / fix

  • bitnami nginx 1.21.0
  • bitnami nginx-gateway 1.21.0
  • f5 nginx 1.21.0
  • sendmail sendmail 8.17
  • vsftpd_project vsftpd 3.0.4
  • nginx Debian 14, Debian 13, Debian 12 1.20.2-2
  • vsftpd Debian 14, Debian 13 3.0.5-0.1
  • sendmail Debian 14, Debian 13, Debian 12 8.16.1-2

debian advisories DEBIAN-CVE-2021-3618

  • nginx Ubuntu 18.04 LTS 1.14.0-0ubuntu1.10
  • nginx Ubuntu 20.04 LTS 1.18.0-0ubuntu1.3
  • nginx Ubuntu 22.04 LTS 1.18.0-6ubuntu14.1
  • vsftpd Ubuntu 20.04 LTS 3.0.5-0ubuntu0.20.04.1
  • nginx Ubuntu Pro 16.04 LTS 1.10.3-0ubuntu0.16.04.5+esm3

ubuntu advisories UBUNTU-CVE-2021-3618

Ubuntu security data © Canonical Ltd., from https://github.com/canonical/ubuntu-security-notices via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • nginx SLES for SAP 15 SP1, SLES 15 SP1-LTSS, SLES 15 SP1-BCL +3 more 1.16.1-150100.6.16.1
  • nginx SLES for SAP 15 SP2, SLES 15 SP2-LTSS, SLES 15 SP2-BCL +6 more 1.16.1-150200.3.9.1
  • nginx SLE Module for Server Applications 15 SP3, openSUSE Leap 15.4, openSUSE Leap 15.3 1.19.8-150300.3.9.1
  • nginx SLES for SAP 15, SLES 15-LTSS, SLE High Performance Computing 15-LTSS +1 more 1.16.1-150000.3.18.1
  • vsftpd SLES for SAP 12 SP5, SLES 12 SP5 3.0.5-51.1
  • vsftpd SLES for SAP 15 SP1, SLES for SAP 15, SLES 15-LTSS +7 more 3.0.5-150000.7.19.1
  • vsftpd SLES for SAP 15 SP2, SLES 15 SP2-LTSS, SLES 15 SP2-BCL +8 more 3.0.5-150200.12.9.1
  • vsftpd SLES for SAP 12 SP5, SLES 12 SP5 3.0.5-47.7.1

suse advisories SUSE-SU-2022:4266-1SUSE-SU-2022:4265-1SUSE-SU-2022:4201-1SUSE-SU-2022:4192-1SUSE-SU-2022:3888-1SUSE-SU-2022:3458-1

  • nginx Alpine 3.24, Alpine 3.23, Alpine 3.22 +8 more 1.20.1-r1

alpine advisories ALPINE-CVE-2021-3618

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • nginx CBL-Mariner 2.0 1.20.2-2
  • vsftpd CBL-Mariner 2.0 3.0.5-1

azure linux advisories AZL-9188AZL-9190

Azure Linux vulnerability data, Copyright (c) Microsoft Corporation, from https://github.com/microsoft/AzureLinuxVulnerabilityData via OSV, under the MIT licence; changed: normalised to fixed package versions per release. MIT

  • nginx-mainline Arch Linux 1.21.0-1

arch linux advisories AVG-2103

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-3618 updated cvss 7.4

ALPACA: Application Layer Protocol Confusion - Analyzing and Mitigating Cracks in TLS Authentication

  • Red Hat Ansible Automation Platform 1.2 Ansible
    affected: workaround, no fix available
  • Red Hat Ansible Tower 3 nginx Red Hat Enterprise Linux 6 vsftpd, sendmail, sendmail-cf, sendmail-milter Red Hat Enterprise Linux 7 vsftpd, sendmail, sendmail-cf, sendmail-milter, vsftpd-sysvinit, sendmail-sysvinit
    affected: workaround, no fix planned “Out of support scope”
  • Red Hat Enterprise Linux 8 nginx, vsftpd, sendmail, sendmail-cf, nginx-mod-mail, sendmail-milter, +6 more
    affected: workaround, no fix available, no fix planned “Will not fix”
  • Red Hat Enterprise Linux 9 sendmail, sendmail-cf, sendmail-milter
    affected: workaround, no fix available “Fix deferred”
  • Red Hat Software Collections rh-nginx116-nginx, rh-nginx118-nginx, rh-nginx116-nginx-mod-mail, rh-nginx118-nginx-mod-mail, rh-nginx116-nginx-mod-stream, rh-nginx118-nginx-mod-stream, +6 more
    affected: workaround, no fix planned “Will not fix”
  • Red Hat Enterprise Linux 6 httpd, dovecot, mod_ssl, postfix, httpd-tools, httpd-manual, +4 more Red Hat Enterprise Linux 7 httpd, dovecot, mod_ssl, postfix, mod_ldap, httpd-tools, +8 more Red Hat Enterprise Linux 8 httpd, mod_md, dovecot, mod_ssl, postfix, mod_ldap, +16 more Red Hat Enterprise Linux 9 httpd, nginx, vsftpd, dovecot, mod_lua, mod_ssl, +27 more Red Hat Software Collections httpd24-httpd, httpd24-mod_md, httpd24-mod_ssl, httpd24-mod_ldap, httpd24-httpd-tools, httpd24-mod_session, +2 more
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

2.03% chance of exploitation in the next 30 days, higher than 81% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 7.4 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N
NVD 2.0 5.8 AV:N/AC:M/Au:N/C:P/I:P/A:N
OSV 3.1 7.4 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N

Affected packages

ecosystem package affected fixed in
bitnami nginx
<1.21.0
1.21.0
bitnami nginx-gateway
<1.21.0
1.21.0

Affected products

  • n/a ALPACA

Classification

Weakness
  • CWE-295 Improper Certificate Validation
Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.