Path Interception by PATH Environment Variable

Adversaries may execute their own malicious payloads by hijacking environment variables used to load libraries. The PATH environment variable contains a list of directories (User and System) that the OS searches sequentially through in search of the binary that was called from a script or the command line. 

Adversaries can place a malicious program in an earlier entry in the list of directories stored in the PATH environment variable, resulting in the operating system executing the malicious binary rather than the legitimate binary when it searches sequentially through that PATH listing.

For example, on Windows if an adversary places a malicious program named "net.exe" in `C:\example path`, which by default precedes `C:\Windows\system32\net.exe` in the PATH environment variable, when "net" is executed from the command-line the `C:\example path` will be called instead of the system's legitimate executable at `C:\Windows\system32\net.exe`. Some methods of executing a program rely on the PATH environment variable to determine the locations that are searched when the path for the program is not given, such as executing programs from a [Command and Scripting Interpreter](https://attack.mitre.org/techniques/T1059).(Citation: ExpressVPN PATH env Windows 2021)

Adversaries may also directly modify the $PATH variable specifying the directories to be searched.  An adversary can modify the `$PATH` variable to point to a directory they have write access. When a program using the $PATH variable is called, the OS searches the specified directory and executes the malicious binary. On macOS, this can also be performed through modifying the $HOME variable. These variables can be modified using the command-line, launchctl, [Unix Shell Configuration Modification](https://attack.mitre.org/techniques/T1546/004), or modifying the `/etc/paths.d` folder contents.(Citation: uptycs Fake POC linux malware 2023)(Citation: nixCraft macOS PATH variables)(Citation: Elastic Rules macOS launchctl 2022)

Platforms: Windows
macOS
Linux
Tactics:
phase_name: persistence
phase_name: privilege-escalation
phase_name: defense-evasion

Detection:
Monitor file creation for files named after partial directories and in locations that may be searched for common processes through the environment variable, or otherwise should not be user writable. Monitor the executing process for process executable paths that are named for partial directories. Monitor file creation for programs that are named after Windows system programs or programs commonly executed without a path (such as "findstr," "net," and "python"). If this activity occurs outside of known administration activity, upgrades, installations, or patches, then it may be suspicious.

Data and events should not be viewed in isolation, but as part of a chain of behavior that could lead to other activities, such as network connections made for Command and Control, learning details about the environment through Discovery, and Lateral Movement.

References:
source_name: mitre-attack
description: Elastic Security 7.17. (2022, February 1). Modification of Environment Variable via Launchctl. Retrieved September 28, 2023.
source_name: Elastic Rules macOS launchctl 2022
description: ExpressVPN Security Team. (2021, November 16). Cybersecurity lessons: A PATH vulnerability in Windows. Retrieved September 28, 2023.
source_name: ExpressVPN PATH env Windows 2021
description: Nischay Hegde and Siddartha Malladi. (2023, July 12). PoC Exploit: Fake Proof of Concept with Backdoor Malware. Retrieved September 28, 2023.
source_name: uptycs Fake POC linux malware 2023
description: Vivek Gite. (2023, August 22). MacOS – Set / Change $PATH Variable Command. Retrieved September 28, 2023.
source_name: nixCraft macOS PATH variables