Log forging vulnerabilities occur when:
1. Data enters an application from an untrusted source.
2. The data is written to an application or system log file.

An attacker could take advantage of this behavior to forge log entries or inject malicious content into the log.
Such as logger.warn to log some values that come from user input from a web interface, Fortify reports it as a log forging issue.

To fix it by removing these values from the logging if they don't have to be included in the logging.