Runas.exe: Executing Programs with Different Permissions

runas.exe is a built-in command-line utility in Windows operating systems that allows a user to run programs, scripts, or MMC snap-ins with different permissions than the user's current logon provides. It's essentially a way to execute something "as a different user" without needing to log off and log back on.

Origin and Purpose

runas.exe has been a part of Windows since Windows 2000 and continues to be a valuable tool for system administrators and power users. Its primary purpose is to provide a mechanism for performing tasks that require elevated privileges (e.g., installing software, modifying system settings) without constantly operating under an administrator account. This principle of least privilege helps improve system security by limiting the potential damage from malware or accidental user errors.

Functionality

runas.exe allows you to specify:

• The user account: You can run a program under a different user account (local or domain). This could be an administrator account, a service account, or even another standard user account.
• The program to execute: This can be any executable file (.exe), batch script (.bat, .cmd), or MMC console file (.msc).
• Optional parameters: You can pass command-line arguments to the program you are running.

Usage

The basic syntax of runas.exe is:

runas /user:<username> "<command>"

Or

runas /user:<username>@<domain> "<command>"

Let's break down the components:

• /user:<username> or /user:<username>@<domain>: Specifies the user account you want to use. Use the username format for local accounts and username@domain or domain\username for domain accounts. For example:
  • /user:Administrator (local administrator)
  • /user:mydomain\administrator (domain administrator)
  • /user:[email protected]
• "<command>": The command you want to execute, enclosed in double quotes. This includes the full path to the executable and any necessary command-line arguments.

Common Options:

• /profile: Loads the user's profile. This is usually necessary for interactive applications. It's the default if the user is a member of the local Administrators group.
• /noprofile: Does not load the user's profile. This can improve performance and is often used for non-interactive tasks.
• /env: Uses the current environment instead of the user's.
• /netonly: Specifies that the user information is for remote access only. This is useful for accessing network resources as a different user without affecting the local session.
• /showtrustlevels: Lists the trust levels that can be used with the /trustlevel option.
• /trustlevel: Specifies the level of authorization for the application. This is less commonly used. See the output of /showtrustlevels for available options.
• /savecred: Uses credentials previously saved by the user. This option is not supported on Windows Home editions and is considered a security risk. Avoid using this option.

Examples:

1. Running Notepad as an administrator (prompting for password):

   runas /user:administrator "notepad.exe"

   This command will open a new command prompt window, ask for the administrator password, and then launch Notepad with administrator privileges.

2. Running a batch script as a different user without loading the profile:

   runas /noprofile /user:mydomain\jsmith "C:\scripts\myscript.cmd"

   This will execute myscript.cmd as the user jsmith in the mydomain domain, without loading jsmith's profile. The command prompt will ask for jsmith's password.

3. Running the Computer Management console as a domain administrator:

   runas /user:mydomain\adminuser "mmc.exe compmgmt.msc"

   This opens the Computer Management console (compmgmt.msc) under the context of the adminuser in the mydomain domain.

4. Accessing a network share using different credentials, and launching a file from the network share: runas /netonly /user:otherdomain\otheruser "explorer.exe \\server\share\document.txt"

Security Implications

runas.exe is a powerful tool, and like all powerful tools, it has security implications:

• Password Exposure: When using runas.exe without /savecred, you will be prompted to enter the password for the specified user account. This password is typed in plain text in the command prompt. Be extremely careful when using this in shared environments or when someone might be looking over your shoulder. Anyone who sees the password can then use that account.
• /savecred (Deprecated and Dangerous): The /savecred option is highly discouraged. It saves user credentials, making them easily accessible to malware or anyone with access to the system. Microsoft strongly recommends against its use.
• Elevation of Privilege: The core function of runas.exe is to elevate privileges. If a malicious actor gains access to an account that can use runas.exe to run programs as an administrator, they effectively gain administrative control of the system. This emphasizes the importance of strong passwords and limiting access to administrative accounts.
• Principle of Least Privilege: While runas.exe helps implement the principle of least privilege (by allowing you to run specific tasks with elevated privileges without constantly logging in as an administrator), it's still crucial to use it responsibly. Only run programs as an administrator when absolutely necessary.

Is runas.exe a Virus?

No, runas.exe itself is a legitimate, built-in Windows system file. It is not a virus.

Can runas.exe Be Used by Viruses?

Yes, runas.exe can be used by malicious software, although it is not commonly the primary vector of infection. Malware might use runas.exe in several ways:

• Persistence: Malware could use runas.exe to start itself with elevated privileges every time the system boots. This is more likely to be done through scheduled tasks or registry entries, but runas.exe could be part of the command chain.
• Privilege Escalation: If malware is running with limited user privileges, it might try to exploit vulnerabilities or use stolen credentials with runas.exe to gain higher-level access (e.g., administrator privileges).
• Lateral Movement: In a network environment, malware might use runas.exe with stolen domain credentials to execute commands on other computers within the network.

It's important to reiterate that the mere presence of runas.exe on your system does not indicate a virus. However, if you see runas.exe being used in unexpected or suspicious ways (e.g., running from an unusual location, executing unknown programs), it's worth investigating further with anti-malware software and system monitoring tools. Checking the command line arguments passed to runas.exe is crucial for determining if it's being used maliciously.

Conclusion

runas.exe is a valuable and essential command-line utility for managing user permissions and executing programs with different privileges in Windows. Understanding its functionality, usage, and security implications is critical for system administrators and power users. While runas.exe itself is not a virus, it can be used by malicious software, so vigilance and responsible use are essential.