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.
|ADVSTORESHELL||ADVSTORESHELL compresses output data generated by command execution with a custom implementation of the Lempel–Ziv–Welch (LZW) algorithm. |
|APT1||APT1 has used RAR to compress files before moving them outside of the victim network. |
|APT28||APT28 used a publicly available tool to gather and compress multiple documents on the DCCC and DNC networks. |
|APT3||APT3 has used tools to compress data before exfilling it. |
|APT32||APT32's backdoor has used LZMA compression before exfiltration. |
|APT33||APT33 has used WinRAR to compress data prior to exfil. |
|APT39||APT39 has used WinRAR and 7-Zip to compress an archive stolen data. |
|BBSRAT||BBSRAT can compress data with ZLIB prior to sending it back to the C2 server. |
|BRONZE BUTLER||BRONZE BUTLER has compressed data into password-protected RAR archives prior to exfiltration. |
Calisto uses the
|Cardinal RAT||Cardinal RAT applies compression to C2 traffic using the ZLIB library. |
|CopyKittens||CopyKittens uses ZPP, a .NET console program, to compress files with ZIP. |
|CORALDECK||CORALDECK has created password-protected RAR, WinImage, and zip archives to be exfiltrated. |
|Daserf||Daserf hides collected data in password-protected .rar archives. |
|Denis||Denis compressed collected data using zlib. |
|Dragonfly 2.0||Dragonfly 2.0 compressed data into .zip files prior to exfiltrating it. |
|Duqu||Modules can be pushed to and executed by Duqu that copy data to a staging area, compress it, and XOR encrypt it. |
|Empire||Empire can ZIP directories on the target system. |
|Epic||Epic compresses the collected data with bzip2 before sending it to the C2 server. |
|FIN6||Following data collection, FIN6 has compressed log files into a ZIP archive prior to staging and exfiltration. |
|FIN8||FIN8 has used RAR to compress collected data before. |
|Gallmaker||Gallmaker has used WinZip, likely to archive data prior to exfiltration. |
|Honeybee||Honeybee adds collected files to a temp.zip file saved in the %temp% folder, then base64 encodes it and uploads it to control server. |
|iKitten||iKitten will zip up the /Library/Keychains directory before exfiltrating it. |
|InvisiMole||InvisiMole uses WinRAR to compress data that is intended to be exfiltrated. |
|Ke3chang||The Ke3chang group has been known to compress data before exfiltration. |
|Lazarus Group||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.   |
|Lurid||Lurid can compress data before sending it. |
|Magic Hound||Magic Hound has used RAR to stage and compress local folders. |
|menuPass||menuPass has compressed files before exfiltration using TAR and RAR.  |
|Micropsia||Micropsia creates a RAR archive based on collected files on the victim's machine. |
|MuddyWater||MuddyWater has used the native Windows cabinet creation tool, makecab.exe, likely to compress stolen data to be uploaded. |
|OopsIE||OopsIE compresses collected files with both the GZipStream class and a simple character replacement scheme before sending them to its C2 server. |
|PoshC2||PoshC2 contains a module for compressing data using ZIP. |
|Prikormka||After collecting documents from removable media, Prikormka compresses the collected files. |
|Proton||Proton zips up files before exfiltrating them. |
|PUNCHBUGGY||PUNCHBUGGY has Gzipped information and saved it to a random temp file before exfil. |
|Pupy||Pupy can compress data with Zip before sending it over C2. |
|RunningRAT||RunningRAT contains code to compress files. |
|SeaDuke||SeaDuke compressed data with zlib prior to sending it over C2. |
|Soft Cell||Soft Cell used winrar to compress and encrypt stolen data prior to exfiltration. |
|Sowbug||Sowbug extracted documents and bundled them into a RAR archive. |
|Threat Group-3390||Threat Group-3390 has used RAR to compress, encrypt, and password-protect files prior to exfiltration. |
|ZLib||The ZLib backdoor compresses communications using the standard Zlib compression library. |
|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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