CVE-2021-3618: ALPACA is an application layer protocol content confusion attack, exploiting TLS servers implementing differen

Q: How do I fix CVE-2021-3618?

nginx: upgrade to 1.21.0; sendmail: upgrade to 8.17; vsftpd: upgrade to 3.0.4. Exact commands for each OS are on this page.

Q: Is CVE-2021-3618 being exploited?

CVE-2021-3618 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-3618 affect?

CVE-2021-3618 affects nginx, sendmail, vsftpd. Severity is high (CVSS 7.4). 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 vic

Q: How urgent is CVE-2021-3618?

Priority 56/100. High severity; Remotely exploitable, no login, no user action; On your watchlist

This month

HIGH 7.4REMOTEWATCHLISTUPGRADEUPGRADEUPGRADE

CVE-2021-3618 is a high-severity flaw (CVSS 7.4) affecting nginx, sendmail, vsftpd. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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

Why this priority

High severity
Remotely exploitable, no login, no user action
On your watchlist

A fixed release or distro patch exists.

Risk facts

CVSS score	7.4 (High)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	2.0% (top 19%)
CISA exploited list	No
Weakness	CWE-295
Published	2022-03-23
Last modified	2026-06-17

Publisher (CNA): Red Hat, Inc. · Vendor: debian · f5 · fedoraproject · sendmail · vsftpd project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2022-03-23 20:15 UTCPublished CVE record published
2026-06-17 04:05 UTCUpdated Record last modified

f5 nginx UPGRADE

Affected: < 1.21.0

Fixed in: 1.21.0

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy nginx || rpm -q nginx || nginx --version

2. UPGRADE to 1.21.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade nginx

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade nginx

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

sendmail sendmail UPGRADE

Affected: < 8.17

Fixed in: 8.17

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy sendmail || rpm -q sendmail || sendmail --version

2. UPGRADE to 8.17 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade sendmail

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade sendmail

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

vsftpd project vsftpd UPGRADE

Affected: < 3.0.4

Fixed in: 3.0.4

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy vsftpd || rpm -q vsftpd || vsftpd --version

2. UPGRADE to 3.0.4 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade vsftpd

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade vsftpd

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

fedoraproject fedora MITIGATE

Affected: 33 · 34 · 35

Fixed in: no fixed release known

Safe to downgrade to: a release before 33

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

debian debian linux MITIGATE

Affected: 10.0

Fixed in: no fixed release known

Safe to downgrade to: a release before 10.0

Check your OS release

cat /etc/os-release; uname -r

Debian / Ubuntu

sudo apt update && sudo apt full-upgrade

RHEL / Amazon Linux / Fedora

sudo dnf upgrade --refresh

Windows

Install the latest cumulative update from Windows Update

If no patch exists yet

OS downgrades are not safe. Apply the vendor mitigation from the advisory and restrict exposure.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-3618?

nginx: upgrade to 1.21.0; sendmail: upgrade to 8.17; vsftpd: upgrade to 3.0.4. Exact commands for each OS are on this page.

Is CVE-2021-3618 being exploited?

CVE-2021-3618 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-3618 affect?

CVE-2021-3618 affects nginx, sendmail, vsftpd. Severity is high (CVSS 7.4). 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 vic

How urgent is CVE-2021-3618?

Priority 56/100. High severity; Remotely exploitable, no login, no user action; On your watchlist

Software pages

nginx vulnerabilities · Debian vulnerabilities · F5 BIG-IP vulnerabilities · vsftpd vulnerabilities

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References

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