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.

As mitigation for CVE-2020-1945 Apache Ant 1.10.8 changed the permissions of temporary files it created so that only the current user was allowed to access them. Unfortunately the fixcrlf task deleted the temporary file and created a new one without said protection, effectively nullifying the effort. This would still allow an attacker to inject modified source files into the build process.

dom4j before 2.0.3 and 2.1.x before 2.1.3 allows external DTDs and External Entities by default, which might enable XXE attacks. However, there is popular external documentation from OWASP showing how to enable the safe, non-default behavior in any application that uses dom4j.

Improper validation of certificate with host mismatch in Apache Log4j SMTP appender. This could allow an SMTPS connection to be intercepted by a man-in-the-middle attack which could leak any log messages sent through that appender. Fixed in Apache Log4j 2.12.3 and 2.13.1