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Actively exploitedKnown ransomware use

CVE-2021-45046 Apache Log4j2 Deserialization of Untrusted Data Vulnerability

Apache Log4j2 contains a deserialization of untrusted data vulnerability due to the incomplete fix of CVE-2021-44228, where the Thread Context Lookup Pattern is vulnerable to remote code execution in certain non-default configurations.

From the CISA KEV catalogue version 2026.11.06, with Cybergen analysis. Updated 6 Nov 2026.

Our Assessment

Should You Worry?

Patch nowCybergen assessment
  • CISA confirmed exploitation in the wild and added it to the Known Exploited Vulnerabilities catalogue on 1 May 2023.
  • CISA lists it as known to be used in ransomware campaigns.
  • Rated Critical severity (CVSS 9.0, scored by CISA).
  • It has been in the catalogue for 1,285 days. Systems still unpatched now have almost certainly been scanned for it.
The Vulnerability

What It Is

It was found that the fix to address CVE-2021-44228 in Apache Log4j 2.15.0 was incomplete in certain non-default configurations. This could allows attackers with control over Thread Context Map (MDC) input data when the logging configuration uses a non-default Pattern Layout with either a Context Lookup (for example, $${ctx:loginId}) or a Thread Context Map pattern (%X, %mdc, or %MDC) to craft malicious input data using a JNDI Lookup pattern resulting in an information leak and remote code execution in some environments and local code execution in all environments. Log4j 2.16.0 (Java 8) and 2.12.2 (Java 7) fix this issue by removing support for message lookup patterns and disabling JNDI functionality by default.

Affected Versions

ProductVersions
Apache Software Foundation Apache Log4jApache Log4j2 to before 2.16.0

From the public CVE record, as supplied by the vendor. Always confirm against the vendor advisory.

From Watch to Action

What To Do Now

  1. 1

    Find it

    Search your asset inventory and external attack surface for Apache Log4j2. Include cloud images, appliances and anything a supplier runs for you.

    Attack Surface Management
  2. 2

    Fix it

    Apply updates per vendor instructions.

    Vulnerability Management
  3. 3

    Hunt for compromise

    If an affected system was reachable before you patched, assume it may have been hit. Check logs, new accounts, web shells and outbound connections from the date of exposure.

    Incident Response
  4. 4

    Prove it is fixed

    Rescan or test the system after patching. Upgrades that fail silently and forgotten secondary instances are common.

    Penetration Testing

Vendor Advisories

Regulation

The Compliance Clock

Cyber EssentialsRated critical, so if it affects devices in your Cyber Essentials scope, the update must be installed within 14 days of the vendor releasing it.NIS2If exploitation causes a significant incident, essential and important entities owe their CSIRT an early warning within 24 hours and a notification within 72 hours.DORAFinancial entities must classify ICT incidents and send an initial notification for a major incident within strict deadlines, with exploited flaws a common cause.US federal (BOD 22-01)Federal civilian agencies had until 22 May 2023 to remediate. A sensible benchmark for everyone else.

General guidance, not legal advice. Your obligations depend on your sector, scope and jurisdiction.

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More Exploited Apache Flaws

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Evidence

Sources

  1. CISA Known Exploited Vulnerabilities catalogue
  2. NVD: CVE-2021-45046
  3. CVE record: CVE-2021-45046
  4. FIRST EPSS
  5. logging.apache.org/log4j/2.x/security.html
  6. psirt.global.sonicwall.com/vuln-detail/SNWLID-2021-0032
  7. oracle.com/security-alerts/alert-cve-2021-44228.html
  8. cve.org/CVERecord?id=CVE-2021-44228
  9. openwall.com/lists/oss-security/2021/12/14/4
  10. kb.cert.org/vuls/id/930724
  11. cert-portal.siemens.com/productcert/pdf/ssa-661247.pdf
  12. intel.com/content/www/us/en/security-center/advisory/intel-sa-00646.html