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CVE-2021-3618

No known exploitation. EPSS puts it in the 81st percentile. Fixed versions are out.

CVSS 3.17.4HighEPSS percentile81st2.0% chance in 30 daysCISA KEVNot listedNo confirmed exploitation
No public exploit knownFix5 products3 fixed builds listed below
Check it yourself

OpenVAS · 36 checks

e.g. OID 1.3.6.1.4.1.25623.1.0.170828

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Published Updated Sources: CVE.org, NVD, BDU

NVD CVE.org

What to do

Find your product and version, then upgrade to the build on the right.

  • n/a ALPACAAffected: vsftpd 3.0.4, nginx 1.21.0, sendmail 8.17Upgrade to: not statedStated by: CVE record
  • F5 NginxAffected: < 1.21.0Upgrade to: 1.21.0Stated by: NVD
  • SendmailAffected: < 8.17Upgrade to: 8.17Stated by: NVD
  • Vsftpd Project VsftpdAffected: < 3.0.4Upgrade to: 3.0.4Stated by: NVD
  • Red Hat Inc. Red Hat Enterprise LinuxAffected: 6Upgrade to: not statedStated by: BDU

Versions as the sources state them. 8 more products have version data. Full version matrix after sign-in

Check it yourselfA free OpenVAS check exists for this CVE. Run it against your hosts.Is your organisation exposed?Request an assessment to establish which checks fit your systems.

What it is

From the CVE record

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.

In plain language

No plain-language summary for this CVE yet.

Exploitation

Where each signal puts this CVE on the scale from published to confirmed exploited.

EPSS81st
CISA KEV

Not in the catalog. CISA has not confirmed exploitation.

Public exploits

No public exploit or proof of concept found in the sources we track.

EPSS

2.0% chance of exploitation activity in the next 30 days, which ranks it in the 81st percentile of scored CVEs.

Exploit Prediction Scoring System, FIRST.org. A probability, not a confirmation.

Lifecycle

6 events over 1555 days, from the signal feeds we watch.

  1. OpenVAS check added
  2. Patch availablerecord updated
  3. Publishedweakness classified, att&ck mapped

Affected products

And 11 more affected products. See all after sign-in

Scale

How widely the affected products are used. This is not a count of vulnerable hosts.

Estimate Public sources, each figure dated. How much of it is already patched is not known.

nginx

Web

  • 30%of websites

    Among websites whose web server we know.

    W3Techs ↗ · as of 2 Nov 2026 · Medium confidence

Version split · W3Techs · as of 2 Nov 2026

  • 1.2416%
  • 1.2915%
  • 1.1812%
  • 1.3011%
  • 1.2110%
  • 1.316.2%
  • Other listed29%

Technical detail

CVSS 3.1 vector

Open in the CVSS calculator

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

Scored 7.4 by NVD.

How it is reached

  • Attack Vector NetworkExploitable remotely over the network without any special conditions
  • Attack Complexity HighRequires specific conditions like a race condition or non-default configuration
  • Privileges Required NoneNo authentication required — anyone can exploit this
  • User Interaction NoneNo user interaction needed — fully automated exploitation

Scope

  • Scope UnchangedImpact is limited to the vulnerable component itself

Impact if exploited

  • Confidentiality HighTotal information disclosure — all data in the component is compromised
  • Integrity HighTotal loss of integrity — attacker can modify any data in the component
  • Availability NoneNo availability impact

Weaknesses

ATT&CK techniques

Mapped from the weaknesses above (CWE to ATT&CK), not observed in attacks.

Sources

References in the record

And 13 more references. See all after sign-in

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We'll flag the next CVE, public exploit or patch for Red Hat Enterprise Linux, not every advisory.