Apache Log4cxx’s XMLLayout fails to escape XML 1.0 forbidden characters in logged data, charting, NDC and MDC keys and values in versions prior to 1.7.0. The resulting invalid XML documents are rejected by conforming parsers, causing log entries to be silently dropped or processed systems to fail to index them. An attacker who can inject or influence logged messages can use this flaw to erase evidence of activity, degrading audit trails and hindering detection of malicious actions.

Affects Apache Log4cxx, the Brew distribution of Log4cxx and the Conan package of Log4cxx when used in any version earlier than 1.7.0. Users of these libraries should verify the version in use and consider upgrading.

The vulnerability carries a CVSS score of 6.3, indicating medium severity, and an EPSS score below 1 %, suggesting a low probability of widespread exploitation. It is not listed in the CISA Known Exploited Vulnerabilities catalog. Exploitation requires an attacker’s ability to supply log data that the application records, a condition that may arise from local code execution or from remote inputs that are directly logged. The likely attack vector is via application input that ends up in log messages or contextual data libraries.