CVE-2025-59476: 
Java vulnerability analysis and mitigation

CVE-2025-59476 is a medium-severity log message injection vulnerability discovered in Jenkins. The vulnerability affects Jenkins versions up to 2.527 and LTS versions up to 2.516.2. It was discovered by Manuel Fernandez from Stackhopper Security and publicly disclosed in the Jenkins Security Advisory on September 17, 2025 (Jenkins Advisory).

The vulnerability exists in the log formatter component that prepares messages for console output, including jenkins.log and equivalent files. The core issue is that the formatter does not properly restrict or transform characters that can be inserted from user-specified content in log messages. The vulnerability has been assigned a CVSS score of 4.4, indicating medium severity (Cybersecurity News).

This vulnerability allows attackers with the ability to control log message contents to insert line break characters, followed by forged log messages. This can potentially mislead administrators reviewing log output. Additionally, characters like ASCII BS (backspace) may result in log viewers hiding content, while Unicode-enabled log viewers may be misled by 'Trojan Source' characters (Jenkins Advisory).

The vulnerability has been fixed in Jenkins weekly version 2.528 and LTS version 2.516.3. The fix adds an indicator at the beginning of injected lines, marking them with [CR], [LF], or [CRLF] (representing the kind of line break), followed by >. Administrators are advised to upgrade to these versions immediately. Additionally, it is recommended to use a log/text viewer that highlights unusual characters in its output and to restrict log access to trusted personnel (Jenkins Advisory).