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.

Published: 2022-03-23 Last update: 2024-11-21 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-3618 is rated Moderate Risk (54.2/100): CVSS High severity, with medium exploitation likelihood (EPSS 0.61%). Review affected assets and schedule remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2021-3618

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-03-04 0.32% 0.61% +0.29%
2 2026-03-01 0.61% 0.32% -0.29%
3 2026-02-04 — 0.61% —

Full EPSS history (42 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-3618

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.4 3.1 HIGH
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:H)
Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:N)
Service keeps running; no real outage angle.
 2.2 5.2 [email protected]
5.8 2.0 MEDIUM
AV:N/AC:M/Au:N/C:P/I:P/A:N Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:M)
Exploitation needs some favorable conditions, but not exceptional ones.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:P)
Partial confidentiality impact.
Integrity impact (I:P)
Partial integrity impact.
Availability impact (A:N)
No availability impact.
 8.6 4.9 [email protected]

Weakness enumeration for CVE-2021-3618

OS Trackers for CVE-2021-3618

vendor priority summary link
alpine high CVE-2021-3618: 1 source package rows (nginx); 39 state rows across 8 repos (3.12-main, 3.17-main, 3.18-main, 3.19-main, 3.20-main, 3.21-main, 3.22-main, edge-main); fixed 13, open 26. https://security.alpinelinux.org/vuln/CVE-2021-3618
debian not yet assigned CVE-2021-3618 not yet assigned priority: Debian including 3 source packages (nginx, sendmail, vsftpd), 15 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 12, open 3. https://security-tracker.debian.org/tracker/CVE-2021-3618
redhat medium — https://access.redhat.com/security/cve/CVE-2021-3618
suse high CVE-2021-3618 severity important: SUSE including 24 source package names (go1.17, go1.17-doc, …), 154 product×package rows across 57 product lines (SUSE CaaS Platform 4.0, SUSE Enterprise Storage 6, … (57 product lines)): Fixed 73, Will Not Fix 42, Known Not Affected 39. https://www.suse.com/security/cve/CVE-2021-3618/
ubuntu low CVE-2021-3618 low priority: Ubuntu including 3 source packages (nginx, sendmail, vsftpd), 45 status rows across 15 suites (bionic, focal, hirsute, impish, jammy, kinetic, lunar, mantic, noble, oracular, plucky, questing, trusty, upstream, xenial): not-affected 20, released 9, needed 8, ignored 7, needs-triage 1. https://ubuntu.com/security/CVE-2021-3618

Affected software / configurations for CVE-2021-3618

Vendor Product Version Raw CPE
f5 nginx < 1.21.0 cpe:2.3:a:f5:nginx:*:*:*:*:*:*:*:*
sendmail sendmail < 8.17 cpe:2.3:a:sendmail:sendmail:*:*:*:*:*:*:*:*
vsftpd_project vsftpd < 3.0.4 cpe:2.3:a:vsftpd_project:vsftpd:*:*:*:*:*:*:*:*
fedoraproject fedora 33 cpe:2.3:o:fedoraproject:fedora:33:*:*:*:*:*:*:*
fedoraproject fedora 34 cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*
fedoraproject fedora 35 cpe:2.3:o:fedoraproject:fedora:35:*:*:*:*:*:*:*
debian debian_linux 10.0 cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*

References for CVE-2021-3618

URL Tags
https://alpaca-attack.com/ Third Party Advisory
https://bugzilla.redhat.com/show_bug.cgi?id=1975623 Issue Tracking Patch Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/11/msg00031.html Mailing List Third Party Advisory
cvelogic Threat Intelligence