This post was authored by Paul Rascagneres.
In the CCleaner 64bit stage 2 previously described in our blog, we explained that the attacker modified a legitimate executable that is part of “Symantec Endpoint”. This file is named EFACli64.dll. The modification is performed in the runtime code included by the compiler, more precisely in the __security_init_cookie() function. The attacker modified the last instruction to jump to the malicious code. The well-known IDA Pro disassembler has trouble displaying the modification as we will show later in this post. Finally, we will present a way to identify this kind of modification and the limitation in this approach.