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CVE-20 21-45105 Stack Overflow Error due to Recursive Lookups

by | Dec 18, 2021

Reading Time: 2 minutes

Another important vulnerability CVE-20 21-45105 for Log4J was discovered This
vulnerability involves Apache Log4j2  not being able to protect from
infinite recursion in lookup evaluation.

This vulnerability has been upgraded in Log4J versioion 2.17.0 for Java 8 and
up. The easiest way to mitigate this vulnerability is to upgrade log4j

The following details have been published from

Deep dive: What is this vulnerability and what kind of attack does this lead

This vulnerability is a Denial of Service attack that happens through the
following steps:

  1. Attackers craft malicious input
  2. If the logging configuration uses a non-default Pattern Layout with a
    Context Lookup (for example, $${ctx:loginId}),
  3. and if attackers craft malicious input
  4. The malicious input can cause recursive lookups
  5. Recursive lookups cause Denial of Service (DOS) attack to due
    StackOverflowError that will cause a termination of the process attacked.

Mitigation as suggested by the Log4J team:

  • 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 originate from sources external
    to the application such as HTTP headers or user input.

MDC or Mapped Diagnostic Context allows applications to log in scenarios with multiple
threads and multiple simultaneous clients where mapped information can be
stored for individual clients in an MDC which is a ThreadLocal. A ThreadLocal class provides thread-local variables.

The Context lookups like ${ctx:loginId}or $${ctx:loginId}  lead to a
recursive call with the right kind of malicious input. Details at

Mitigating with Accuknox Open source Repo:

Apply mitigations with Accuknox’s open sourced policy repo for Log4J
vulnerabilities at the following URL

The primary approach to mitigation for this is to block any malicious input
for older versions, and follow mitigations suggested by the Log4J team by
disabling recursive lookups or the best solution upgrade the Log4J  +
Apply Accuknox policy templates on the repo at

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