Data exfiltration may be performed only at certain times of day or at certain intervals. This could be done to blend traffic patterns with normal activity or availability.
When scheduled exfiltration is used, other exfiltration techniques likely apply as well to transfer the information out of the network, such as Exfiltration Over Command and Control Channel and Exfiltration Over Alternative Protocol.
Network intrusion detection and prevention systems that use network signatures to identify traffic for specific adversary command and control infrastructure and malware can be used to mitigate activity at the network level. Signatures are often for unique indicators within protocols and may be based on the specific obfuscation technique used by a particular adversary or tool, and will likely be different across various malware families and versions. Adversaries will likely change tool command and control signatures over time or construct protocols in such a way to avoid detection by common defensive tools. 
Monitor process file access patterns and network behavior. Unrecognized processes or scripts that appear to be traversing file systems and sending network traffic may be suspicious. Network connections to the same destination that occur at the same time of day for multiple days are suspicious.
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