Allowing only corporate IM solutions is the best control to mitigate the malware risk associated with an IM system, because it can prevent unauthorized or malicious IM applications from accessing the network and infecting the system with malware. Corporate IM solutions can also enforce security policies and standards, such as encryption, authentication, and logging, to protect the IM system from malware attacks. Blocking attachments in IM, blocking external IM traffic, and encrypting IM traffic are also possible controls to mitigate the malware risk, but they are not as effective as allowing only corporate IM solutions. References: CISA Review Manual (Digital Version), Chapter 5, Section 5.4.4