The security bug relates to the fact that the AVG antivirus creates a memory space with full RWX (read-write-execute) privileges where it normally runs. For that particular version of the AVG antivirus, this memory space was not randomized and was often shared with other applications, like, for example, Acrobat Reader or the enSilo product that collided with the antivirus.

If an attacker knew about the antivirus’ predictable behavior and where this address space was, they could force their malicious code to execute inside that memory address and have the same privileges as the antivirus process (which is system-level).

Source: AVG, McAfee, Kaspersky Fix Common Vulnerability in Their Antivirus Products