Encrypted Channel: Asymmetric Cryptography

Other sub-techniques of Encrypted Channel (2)

ID	Name
T1573.001	Symmetric Cryptography
T1573.002	Asymmetric Cryptography

Adversaries may employ a known asymmetric encryption algorithm to conceal command and control traffic rather than relying on any inherent protections provided by a communication protocol. Asymmetric cryptography, also known as public key cryptography, uses a keypair per party: one public that can be freely distributed, and one private. Due to how the keys are generated, the sender encrypts data with the receiver’s public key and the receiver decrypts the data with their private key. This ensures that only the intended recipient can read the encrypted data. Common public key encryption algorithms include RSA and ElGamal.

For efficiency, may protocols (including SSL/TLS) use symmetric cryptography once a connection is established, but use asymmetric cryptography to establish or transmit a key. As such, these protocols are classified as Asymmetric Cryptography.

ID: T1573.002

Sub-technique of: T1573

Tactic: Command And Control

Platforms: Linux, Windows, macOS

Data Sources: Malware reverse engineering, Netflow/Enclave netflow, Packet capture, Process monitoring, Process use of network

Version: 1.0

Created: 16 March 2020

Last Modified: 30 March 2020

Version Permalink

Live Version

Procedure Examples

Name	Description
adbupd	adbupd contains a copy of the OpenSSL library to encrypt C2 traffic.^[10]
ADVSTORESHELL	A variant of ADVSTORESHELL encrypts some C2 with RSA.^[19]
Attor	Attor's Blowfish key is encrypted with a public RSA key.^[25]
BISCUIT	BISCUIT uses SSL for encrypting C2 communications.^[13]
CHOPSTICK	CHOPSTICK encrypts C2 communications with TLS.^[7]
Cobalt Group	Cobalt Group has used the Plink utility to create SSH tunnels.^[29]
ComRAT	ComRAT can use SSL/TLS encryption for its HTTP-based C2 channel. ComRAT has used public key cryptography with RSA and AES encrypted email attachments for its Gmail C2 channel.^[27]
Dridex	Dridex has encrypted traffic with RSA.^[20]
Emotet	Emotet is known to use RSA keys for encrypting C2 traffic. ^[17]
Empire	Empire can use TLS to encrypt its C2 channel.^[4]
FIN6	FIN6 used the Plink command-line utility to create SSH tunnels to C2 servers.^[30]
FIN8	FIN8 has used the Plink utility to tunnel RDP back to C2 infrastructure.^[28]
Gazer	Gazer uses custom encryption for C2 that uses RSA.^[22]^[23]
GreyEnergy	GreyEnergy encrypts communications using RSA-2048.^[21]
Hi-Zor	Hi-Zor encrypts C2 traffic with TLS.^[8]
Koadic	Koadic can use SSL and TLS for communications.^[1]
Metamorfo	Metamorfo's C&C communication has been encrypted using OpenSSL.^[26]
OilRig	OilRig used the Plink utility and other tools to create tunnels to C2 servers.^[31]
PoetRAT	PoetRAT used TLS to encrypt command and control (C2) communications.^[24]
POSHSPY	POSHSPY encrypts C2 traffic with AES and RSA.^[11]
POWERSTATS	POWERSTATS has encrypted C2 traffic with RSA.^[12]
Pupy	Pupy's default encryption for its C2 communication channel is SSL, but it also has transport options for RSA and AES.^[3]
ServHelper	ServHelper may set up a reverse SSH tunnel to give the attacker access to services running on the victim, such as RDP.^[18]
Sykipot	Sykipot uses SSL for encrypting C2 communications.^[9]
Tor	Tor encapsulates traffic in multiple layers of encryption, using TLS by default.^[2]
Tropic Trooper	Tropic Trooper has used SSL to connect to C2 servers.^[32]^[33]
Volgmer	Some Volgmer variants use SSL to encrypt C2 communications.^[5]
WannaCry	WannaCry uses Tor for command and control traffic and routes a custom cryptographic protocol over the Tor circuit.^[16]
XTunnel	XTunnel uses SSL/TLS and RC4 to encrypt traffic.^[6]^[7]
Zebrocy	Zebrocy uses SSL and AES ECB for encrypting C2 communications.^[14]^[15]

Mitigations

Mitigation	Description
Network Intrusion Prevention	Network intrusion detection and prevention systems that use network signatures to identify traffic for specific adversary malware can be used to mitigate activity at the network level.
SSL/TLS Inspection	SSL/TLS inspection can be used to see the contents of encrypted sessions to look for network-based indicators of malware communication protocols.

Detection

SSL/TLS inspection is one way of detecting command and control traffic within some encrypted communication channels.^[34] SSL/TLS inspection does come with certain risks that should be considered before implementing to avoid potential security issues such as incomplete certificate validation.^[35]

In general, analyze network data for uncommon data flows (e.g., a client sending significantly more data than it receives from a server). Processes utilizing the network that do not normally have network communication or have never been seen before are suspicious. Analyze packet contents to detect communications that do not follow the expected protocol behavior for the port that is being used.^[36]

Encrypted Channel: Asymmetric Cryptography

Other sub-techniques of Encrypted Channel (2)

Procedure Examples

Mitigations

Detection

References