The 'APT User Agent' detection rule focuses on identifying suspicious user agent strings commonly associated with Advanced Persistent Threats (APTs) by analyzing proxy logs. A user agent string is a line of text that a browser or application sends to web servers, containing information about the operating system, browser version, and other details. This rule targets various known APT user agents that may indicate malicious activity, including impersonation tactics used during exploitation phases and command-and-control communications. The detection is implemented by matching user agent strings found in proxy logs against a predefined list of suspicious entries. Additionally, it logs relevant fields such as ClientIP, c-uri, and c-useragent to help correlate network activities with the identified threats. Given the increasing sophistication of cyber threats, particularly those utilizing APT techniques, the high-level alert produced by this rule serves as a crucial component in a comprehensive security posture, helping organizations to proactively respond to potentially malicious activities originating from client applications on the network.