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Date: Sat, 18 Dec 2021 18:02:02 -0600
From: Matt Sicker <>
Subject: CVE-2021-45105: Apache Log4j2 does not always protect from infinite
 recursion in lookup evaluation

Severity: high


Apache Log4j2 versions 2.0-alpha1 through 2.16.0 (excluding 2.12.3)
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 and 2.12.3.

This issue is being tracked as LOG4J2-3230


Implement one of the following mitigation techniques:

* Java 8 (or later) users should upgrade to release 2.17.0.

Alternatively, this can be mitigated in configuration:

* In PatternLayout in the logging configuration, replace Context
Lookups like `${ctx:loginId}` or `$${ctx:loginId}` with Thread Context
Map patterns (%X, %mdc, or %MDC).
* Otherwise, in the configuration, remove references to Context
Lookups like `${ctx:loginId}` or `$${ctx:loginId}` where they
from sources external to the application such as HTTP headers or user input.


Independently discovered by Hideki Okamoto of Akamai Technologies, Guy
Lederfein of Trend Micro Research working with Trend Micro’s Zero Day
Initiative, and another anonymous vulnerability researcher


Matt Sicker
PMC Member, Logging Services, Apache Software Foundation

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