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

Is it exploited?

Yes. CISA added it to the Known Exploited Vulnerabilities catalog on 1 May 2023, 503 days after the CVE record was published.

How likely is exploitation in the next 30 days?

100%

EPSS score, percentile 100 of all scored records

How severe?

9.0 Critical

CVSS v3.1 base score, from CISA's enrichment of the record

What should I do?

CISA deadline 22 May 2023. Apply updates per vendor instructions.

Advisories and patches (4)

Which versions are affected?

Affected

• from Apache Log4j2 to before 2.16.0

Versions by product

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

What to do

CISA due date for this action: 22 May 2023.

Advisories and patches

• 20211210 Vulnerabilities in Apache Log4j Library Affecting Cisco Products: December 2021 (Vendor advisory)
• DSA-5022 (Vendor advisory)
• FEDORA-2021-5c9d12a93e (Vendor advisory)
• FEDORA-2021-abbe24e41c (Vendor advisory)

Linked from CISA's catalog entry

• https://logging.apache.org/log4j/2.x/security.html
• https://nvd.nist.gov/vuln/detail/CVE-2021-45046

Affected products and versions

As listed in the CVE record.

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

About this 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.

Summary: CISA's KEV catalog, as of 28 Oct 2026.

• CISA added CVE-2021-45046 to its Known Exploited Vulnerabilities (KEV) catalog on 1 May 2023.
• CISA marks it as known to be used in ransomware campaigns.
• The due date CISA sets for the required action is 22 May 2023.
• The CVE record was published on 14 Dec 2021, 503 days before it was added to KEV.
• The CVE record carries a CVSS v3.1 base score of 9.0 (Critical), taken from CISA's enrichment of the record.
• FIRST's EPSS model puts the probability of exploitation in the next 30 days at 100% (percentile 100), as of 28 Oct 2026.
• CISA's SSVC assessment in the CVE record: exploitation active, automatable no, technical impact total.

Source: CISA Known Exploited Vulnerabilities (KEV) catalog, 2026.10.28, as of 28 Oct 2026; CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026; FIRST Exploit Prediction Scoring System (EPSS), v2026.06.15, as of 28 Oct 2026. About the data.

Description

Apache Log4j2 Thread Context Message Pattern and Context Lookup Pattern vulnerable to a denial of service attack

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.

Source: CVE List V5 (CVE Program), cve_2026-10-28_0000Z, as of 28 Oct 2026. About the data.

Weakness

• CWE-917: Improper Neutralization of Special Elements used in an Expression Language Statement ('Expression Language Injection')

CWE and CAPEC are maintained by MITRE; the entries are those named in the CVE record and the KEV entry.

Timeline

References

• VU#930724 (Third-party advisory)
• psirt.global.sonicwall.com
• oracle.com
• cve.org
• [oss-security] 20211214 CVE-2021-45046: Apache Log4j2 Thread Context Message Pattern and Context Lookup Pattern vulnerable to a denial of service attack
• logging.apache.org
• cert-portal.siemens.com
• intel.com

9 more in the CVE record.

More exploited Apache vulnerabilities

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• CVE-2026-34486: Apache Tomcat Missing Encryption of Sensitive Data Vulnerability (added 4 Aug 2026)
• CVE-2026-34197: Apache ActiveMQ Improper Input Validation Vulnerability (added 16 Apr 2026)
• CVE-2024-38475: Apache HTTP Server Improper Escaping of Output Vulnerability (added 1 May 2025)
• CVE-2025-24813: Apache Tomcat Path Equivalence Vulnerability (added 1 Apr 2025)
• CVE-2024-45195: Apache OFBiz Forced Browsing Vulnerability (added 4 Feb 2025)