CVE-2021-45046: Apache Log4j2 Thread Context Message Pattern and Context Lookup Pattern vulnerable to a denial of service attack

[Ron Grabowski <rgrabowski () apache org>]

Severity: moderate (CVSS: 3.7 AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L)

Description:

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 a denial 
of service (DOS) attack. Log4j 2.15.0 restricts JNDI LDAP lookups to localhost by default. Note that previous 
mitigations involving configuration such as to set the system property `log4j2.noFormatMsgLookup` to `true` do NOT 
mitigate this specific vulnerability.

Log4j 2.16.0 fixes this issue by removing support for message lookup patterns and disabling JNDI functionality by 
default.  

This issue can be mitigated in prior releases (<2.16.0) by removing the JndiLookup class from the classpath (example: 
zip -q -d log4j-core-*.jar org/apache/logging/log4j/core/lookup/JndiLookup.class).

References:

https://logging.apache.org/log4j/2.x/security.html
https://www.cve.org/CVERecord?id=CVE-2021-44228