Because antivirus software relies heavily on constantly updated "signatures" (cloud-based definitions), having the underlying engine code didn't automatically grant the ability to bypass new detections. A Lesson in Insider Threats
The primary concern with any source code leak is that it provides "blueprints" for attackers. By studying how an antivirus engine detects threats, malware authors can: KASPERSKY.AV.2008.SRCS.ELCRABE.RAR
The code responsible for handling virus definitions. Security Implications and Response KASPERSKY.AV.2008.SRCS.ELCRABE.RAR