An adversary may compress data (e.g., sensitive documents) that is collected prior to exfiltration in order to make it portable and minimize the amount of data sent over the network. The compression is done separately from the exfiltration channel and is performed using a custom program or algorithm, or a more common compression library or utility such as 7zip, RAR, ZIP, or zlib.
|Exaramel for Windows|
Lazarus Group malware IndiaIndia saves information gathered about the victim to a file that is compressed with Zlib, encrypted, and uploaded to a C2 server. Lazarus Group malware RomeoDelta archives specified directories in .zip format, encrypts the .zip file, and uploads it to its C2 server.
|Network Intrusion Prevention||
Network intrusion prevention or data loss prevention tools may be set to block specific file types from leaving the network over unencrypted channels. An adversary may move to an encrypted channel or use other mechanisms of encapsulating the traffic in these situations.
Compression software and compressed files can be detected in many ways. Common utilities that may be present on the system or brought in by an adversary may be detectable through process monitoring and monitoring for command-line arguments for known compression utilities. This may yield a significant amount of benign events, depending on how systems in the environment are typically used.
If the communications channel is unencrypted, compressed files can be detected in transit during exfiltration with a network intrusion detection or data loss prevention system analyzing file headers. 
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