Execution Guardrails: Environmental Keying
Adversaries may environmentally key payloads or other features of malware to evade defenses and constraint execution to a specific target environment. Environmental keying uses cryptography to constrain execution or actions based on adversary supplied environment specific conditions that are expected to be present on the target. Environmental keying is an implementation of Execution Guardrails that utilizes cryptographic techniques for deriving encryption/decryption keys from specific types of values in a given computing environment.
Values can be derived from target-specific elements and used to generate a decryption key for an encrypted payload. Target-specific values can be derived from specific network shares, physical devices, software/software versions, files, joined AD domains, system time, and local/external IP addresses. By generating the decryption keys from target-specific environmental values, environmental keying can make sandbox detection, anti-virus detection, crowdsourcing of information, and reverse engineering difficult. These difficulties can slow down the incident response process and help adversaries hide their tactics, techniques, and procedures (TTPs).
Similar to Obfuscated Files or Information, adversaries may use environmental keying to help protect their TTPs and evade detection. Environmental keying may be used to deliver an encrypted payload to the target that will use target-specific values to decrypt the payload before execution. By utilizing target-specific values to decrypt the payload the adversary can avoid packaging the decryption key with the payload or sending it over a potentially monitored network connection. Depending on the technique for gathering target-specific values, reverse engineering of the encrypted payload can be exceptionally difficult. This can be used to prevent exposure of capabilities in environments that are not intended to be compromised or operated within.
Like other Execution Guardrails, environmental keying can be used to prevent exposure of capabilities in environments that are not intended to be compromised or operated within. This activity is distinct from typical Virtualization/Sandbox Evasion. While use of Virtualization/Sandbox Evasion may involve checking for known sandbox values and continuing with execution only if there is no match, the use of environmental keying will involve checking for an expected target-specific value that must match for decryption and subsequent execution to be successful.
APT41 has encrypted payloads using the Data Protection API (DPAPI), which relies on keys tied to specific user accounts on specific machines. APT41 has also environmentally keyed second stage malware with an RC5 key derived in part from the infected system's volume serial number.
InvisiMole can use Data Protection API to encrypt its components on the victim’s computer, to evade detection, and to make sure the payload can only be decrypted and loaded on one specific compromised computer.
|Do Not Mitigate||
Environmental Keying likely should not be mitigated with preventative controls because it may protect unintended targets from being compromised. If targeted, efforts should be focused on preventing adversary tools from running earlier in the chain of activity and on identifying subsequent malicious behavior if compromised.
Detecting the use of environmental keying may be difficult depending on the implementation. Monitoring for suspicious processes being spawned that gather a variety of system information or perform other forms of Discovery, especially in a short period of time, may aid in detection.
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