Hijack Execution Flow: Path Interception by PATH Environment Variable

Adversaries may execute their own malicious payloads by hijacking environment variables used to load libraries. Adversaries may place a program in an earlier entry in the list of directories stored in the PATH environment variable, which Windows will then execute when it searches sequentially through that PATH listing in search of the binary that was called from a script or the command line.

The PATH environment variable contains a list of directories. Certain methods of executing a program (namely using cmd.exe or the command-line) rely solely on the PATH environment variable to determine the locations that are searched for a program when the path for the program is not given. If any directories are listed in the PATH environment variable before the Windows directory, %SystemRoot%\system32 (e.g., C:\Windows\system32), a program may be placed in the preceding directory that is named the same as a Windows program (such as cmd, PowerShell, or Python), which will be executed when that command is executed from a script or command-line.

For example, if C:\example path precedes C:\Windows\system32 is in the PATH environment variable, a program that is named net.exe and placed in C:\example path will be called instead of the Windows system "net" when "net" is executed from the command-line.

ID: T1574.007
Sub-technique of:  T1574
Tactics: Persistence, Privilege Escalation, Defense Evasion
Platforms: Windows
Data Sources: File monitoring, Process monitoring
Defense Bypassed: Application control
CAPEC ID: CAPEC-capec
Contributors: Stefan Kanthak
Version: 1.0
Created: 13 March 2020
Last Modified: 20 June 2020

Procedure Examples

Name Description
Empire

Empire contains modules that can discover and exploit path interception opportunities in the PATH environment variable.[12]

PowerSploit

PowerSploit contains a collection of Privesc-PowerUp modules that can discover and exploit path interception opportunities in the PATH environment variable.[10][11]

Mitigations

Mitigation Description
Audit

Find and eliminate path interception weaknesses in program configuration files, scripts, the PATH environment variable, services, and in shortcuts by surrounding PATH variables with quotation marks when functions allow for them. Be aware of the search order Windows uses for executing or loading binaries and use fully qualified paths wherever appropriate.

Clean up old Windows Registry keys when software is uninstalled to avoid keys with no associated legitimate binaries. Periodically search for and correct or report path interception weaknesses on systems that may have been introduced using custom or available tools that report software using insecure path configurations.[7][8][9]

Execution Prevention

Adversaries will likely need to place new binaries in locations to be executed through this weakness. Identify and block potentially malicious software executed path interception by using application control tools, like Windows Defender Application Control, AppLocker, or Software Restriction Policies where appropriate.[1][2][3][4][5][6]

Restrict File and Directory Permissions

Ensure that proper permissions and directory access control are set to deny users the ability to write files to the top-level directory C: and system directories, such as C:\Windows\, to reduce places where malicious files could be placed for execution. Require that all executables be placed in write-protected directories.

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