Data is encrypted before being exfiltrated in order to hide the information that is being exfiltrated from detection or to make the exfiltration less conspicuous upon inspection by a defender. The encryption is performed by a utility, programming library, or custom algorithm on the data itself and is considered separate from any encryption performed by the command and control or file transfer protocol. Common file archive formats that can encrypt files are RAR and zip.
|ADVSTORESHELL||ADVSTORESHELL encrypts with the 3DES algorithm and a hardcoded key prior to exfiltration. |
|Agent Tesla||Agent Tesla encrypts the data with 3DES before sending it over the C2 server. |
|Agent.btz||Agent.btz saves system information into an XML file that is then XOR-encoded. |
|APT32||APT32 backdoors have encrypted data before exfiltration, including by using RC4 encryption.  |
|Backdoor.Oldrea||Backdoor.Oldrea writes collected data to a temporary file in an encrypted form before exfiltration to a C2 server. |
|BRONZE BUTLER||BRONZE BUTLER has compressed and encrypted data into password-protected RAR archives prior to exfiltration. |
|CopyKittens||CopyKittens encrypts data with a substitute cipher prior to exfiltration. |
|CORALDECK||CORALDECK has created password-protected RAR, WinImage, and zip archives to be exfiltrated. |
|Daserf||Daserf hides collected data in password-protected .rar archives. |
|Duqu||Modules can be pushed to and executed by Duqu that copy data to a staging area, compress it, and XOR encrypt it. |
|Emotet||Emotet has been observed encrypting the data it collects before sending it to the C2 server. |
|Epic||Epic encrypts collected data using a public key framework before sending it over the C2 channel. Some variants encrypt the collected data with AES and encode it with base64 before transmitting it to the C2 server.  |
|FELIXROOT||FELIXROOT encrypts collected data with AES and Base64 and then sends it to the C2 server. |
|FIN6||TRINITY malware used by FIN6 encodes data gathered from the victim with a simple substitution cipher and single-byte XOR using the 0xAA key. |
|FLASHFLOOD||FLASHFLOOD employs the same encoding scheme as SPACESHIP for data it stages. Data is compressed with zlib, and bytes are rotated four times before being XOR'ed with 0x23. |
|Gold Dragon||Gold Dragon encrypts data using Base64 before being sent to the command and control server. |
|HAWKBALL||HAWKBALL has encrypted data with XOR before sending it over the C2 channel. |
|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. |
|InvisiMole||InvisiMole uses a variation of the XOR cipher to encrypt files before exfiltration. |
|Ke3chang||Ke3chang is known to use RAR with passwords to encrypt data prior to 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. A Lazarus Group malware sample encrypts data using a simple byte based XOR operation prior to exfiltration.    |
|LightNeuron||LightNeuron contains a function to encrypt and store emails that it collects. |
|menuPass||menuPass has encrypted files and information before exfiltration. |
|OSX_OCEANLOTUS.D||OSX_OCEANLOTUS.D scrambles and encrypts data using AES256 before sending it to the C2 server. |
|OwaAuth||OwaAuth DES-encrypts captured credentials using the key 12345678 before writing the credentials to a log file. |
|Patchwork||Patchwork encrypted the collected files' path with AES and then encoded them with base64. |
|Prikormka||After collecting files and logs from the victim, Prikormka encrypts some collected data with Blowfish. |
|RawPOS||RawPOS encodes credit card data it collected from the victim with XOR.   |
|Reaver||Reaver encrypts collected data with an incremental XOR key prior to exfiltration. |
|Remexi||Remexi encrypts and adds all gathered browser data into files for upload to C2. |
|RGDoor||RGDoor encrypts files with XOR before sending them back to the C2 server. |
|Soft Cell||Soft Cell used winrar to compress and encrypt stolen data prior to exfiltration. |
|SPACESHIP||Data SPACESHIP copies to the staging area is compressed with zlib. Bytes are rotated by four positions and XOR'ed with 0x23. |
|T9000||T9000 encrypts collected data using a single byte XOR key. |
|Threat Group-3390||Threat Group-3390 has used RAR to compress, encrypt, and password-protect files prior to exfiltration. |
|Turla||Turla has encrypted files stolen from connected USB drives into a RAR file before exfiltration. |
|VERMIN||VERMIN encrypts the collected files using 3-DES. |
|Zebrocy||Zebrocy uses an encryption method similar to RC4 as well as AES to encrypt data before exfiltration.  |
This type of attack technique cannot be easily mitigated with preventive controls since it is based on the abuse of system features.
Encryption software and encrypted 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 encryption utilities. This may yield a significant amount of benign events, depending on how systems in the environment are typically used. Often the encryption key is stated within command-line invocation of the software.
A process that loads the Windows DLL crypt32.dll may be used to perform encryption, decryption, or verification of file signatures.
Network traffic may also be analyzed for entropy to determine if encrypted data is being transmitted.  If the communications channel is unencrypted, encrypted files of known file types can be detected in transit during exfiltration with a network intrusion detection or data loss prevention system analyzing file headers. 
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