Windows Privilege Escalation Methods for Pentesters

27 Dec

Windows Privilege Escalation Methods for Pentesters

• Pentester
• Privilege Escalation,Skills
• Tags: AlwaysInstallElevated, getsystem, icacls, Insecure Registry Permissions, Meterpreter, msfvenom, Unquoted Service Paths, wmic
• no comments

Imagine that you have gotten a low-priv Meterpreter session on a Windows machine. Probably you’ll run getsystem to escalate your privileges. But what if it fails?

Don’t panic. There are still some techniques you can try.

Unquoted Service Paths

Basically, it is a vulnerability that occurs if a service executable path is not enclosed with quotation marks and contains space.

To identify these unquoted services you can run this command on Windows Command Shell:

All services with unquoted executable paths will be listed:

If you look at the registry entry for this service with Regedit you can see the ImagePath value is:
C:\Program Files (x86)\Program Folder\A Subfolder\Executable.exe

It should be like this:
“C:\Program Files (x86)\Program Folder\A Subfolder\Executable.exe”

When Windows attempts to run this service, it will look at the following paths in order and will run the first EXE that it will find:

C:\Program.exe
C:\Program Files.exe
C:\Program Files (x86)\Program.exe
C:\Program Files (x86)\Program Folder\A.exe
C:\Program Files (x86)\Program Folder\A Subfolder\Executable.exe

This vulnerability is caused by the CreateProcess function in Windows operating systems. For more information click read this article.

If we can drop our malicious exe successfully on one of these paths, upon a restart of the service, Windows will run our exe as SYSTEM. But we should have necessary privileges on one of these folders.

In order to check the permissions of a folder, we can use built-in Windows tool, icals. Let’s check permissions for C:\Program Files (x86)\Program Folder folder:

What a luck! As you can see, “Everyone” has full control on this folder.

F = Full Control
CI = Container Inherit – This flag indicates that subordinate containers will inherit this ACE.
OI = Object Inherit – This flag indicates that subordinate files will inherit the ACE.

This means we are free to put any file to this folder!

From now on, what you’re going to do depends on your imagination. I simply preferred to generate a reverse shell payload to run as SYSTEM.

MSFvenom can be used for this job:

Let’s place our payload to C:\Program Files (x86)\Program Folder folder:

At the next start of the service, A.exe will run as SYSTEM. Let’s try to stop and restart the service:

Access is denied because we don’t have permission to stop or start the service. However, it’s not a big deal, we can wait for someone to restart the machine, or we can do it ourselves with shutdown command:

As you can see, our session has died. We’ll never forget you low-priv shell. RIP.

Our target machine is restarting now. Soon, our payload will work as SYSTEM. We should start a handler right away.

Now we have gotten a Meterpreter shell with SYSTEM privileges. High five!

But wait, why did our session die so quickly? We just started!

No need to worry. It’s because, when a service starts in Windows operating systems, it must communicate with the Service Control Manager. If it’s not, Service Control Manager thinks that something is not going well and terminates the process.

All we need to do is migrating to another process before the SCM terminates our payload, or you can consider using auto-migration. 😉

BTW there is a Metasploit module for checking and exploiting this vulnerability: exploit/windows/local/trusted_service_path

This module only requires that you link it to an existing Meterpreter session before running:

However, it’s always good to know the internals. 😉

If you want to demonstrate this vulnerability yourself, you can add a vulnerable service to your test environment:

Services with Vulnerable Privileges

You know, Windows services run as SYSTEM. So, their folders, files, and registry keys must be protected with strong access controls. In some cases, we encounter services that are not sufficiently protected.

Insecure Registry Permissions

In Windows, information related to services is stored in HKLM\SYSTEM\CurrentControlSet\Services registry key. If we want to see information about our “Vulnerable Service” we should check HKLM\SYSTEM\ControlSet001\Services\Vulnerable Servicekey.

Of course, our Vulnerable Service has some weaknesses. 🙂

But the point is, how can we check these permissions from the command line? Let’s start the scenario from the beginning.

You have gotten a low-priv Meterpreter session and you want to check permissions of a service.

You can use SubInACL tool to check registry keys permissions. You can download it here but the point you need to be aware of it deployed as an msi file. If AlwaysInstallElevated policy setting is not enabled on target machine you can’t install msi files with low-priv user.(We will discuss AlwaysInstallElevated policy later in this post) And of course, you may do not want to install a new software to the target machine.

I recommend you to install it a virtual machine and find subinacl.exe file in C:\Program Files (x86)\Windows Resource Kits\Tools\. It will work smoothly without having to install msi package.

Let’s upload SubInACL tool to our target:

Now SubInACL tool ready to use. Let’s check permissions for HKEY_LOCAL_MACHINE\SYSTEM\ControlSet001\Services\Vulnerable Service.

Focus on 20th to 23rd lines. It says Everyone has Full Control on this registry key. It means we can change the executable path of this service by editing the ImagePath value. It’s a huge security weakness.

If we generate a simple reverse shell payload and drop it to our target, all that remains is changing the ImagePath value for our vulnerable service with our payload’s path.

Let’s generate a simple reverse shell payload:

Drop it to target machine:

Now let’s change the ImagePath value with our payload’s path.

At the next start of the service, Payload.exe will run as SYSTEM. But remember, we had to restart the computer to do this.

Our target machine is restarting now. Prepare your handler! Soon, our payload will work as SYSTEM.

But don’t forget! We are working with services just as in the previous method our hi-priv meterpreter session will die quickly.

Insecure Service Permissions

It is very similar to previous Insecure Registry Permissions example. Instead of changing service’s “ImagePath” registry value directly we will do it with modifying service properties.

To check which Services have vulnerable privileges we can use AccessChk tool from SysInternals Suite.

Upload AccessChk tool to target machine:

To check vulnerable services simply run this command:

All services that “testuser” can modify will be listed. SERVICE_ALL_ACCESS means we have full control over modifying the properties of Vulnerable Service.

Let’s view the properties of the Vulnerable Service:

BINARY_PATH_NAME points to Executable.exe which is executable file for this service. If we change this value with any command means this command will run as SYSTEM at the next start of the service. We can add a local admin if we want.

The first thing to do is adding a user:

After changing binpath, restart service with “sc stop” and “sc start” commands:

When you try to start service it will return an error. As we talked earlier it’s because, when a service starts in Windows operating systems, it must communicate with the Service Control Manager. “net user” cannot communicate with the SCM. No worries, our command will run as SYSTEM and the new user will be added successfully.

Now we should add new “eviladmin” user to local admins by changing “binpath” and starting service again.(We don’t need to stop it again, it is already not running because of it didn’t communicate with the SCM, you know)

Enjoy your new local admin account!

As we did before, you can prefer dropping a reverse shell payload to target machine and replacing binpath with the payload’s path.

Instead of manually applying this method you can use this metasploit module: exploit/windows/local/service_permissions

You have to link it to an existing Meterpreter session:

Insecure File/Folder Permissions

It is very similar to what we did with Unquoted Service Paths. Unquoted Service Paths takes advantage of “CreateProcess” function’s weakness in combination with folder permissions along the executable file path of a service. But here we will try to replace the executable directly.

For example, if we check permissions for our Vulnerable Service’s executable path, we can see it is not protected well:

Simply replacing “Executable.exe” file with a reverse shell payload and restarting the service will give us a meterpreter session with SYSTEM privileges.

AlwaysInstallElevated

AlwaysInstallElevated is a policy setting that directs Windows Installer to use elevated permissions when it installs any package on the system. If this policy setting is enabled, privileges are extended to all programs.

Actually enabling that is equivalent to granting administrative rights to non-privileged users. But in a way that I cannot understand, sometimes system administrators enable this setting:

You should check this registry values to understand if this policy is enabled:

[HKEY_CURRENT_USER\SOFTWARE\Policies\Microsoft\Windows\Installer]
“AlwaysInstallElevated”=dword:00000001

[HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\Installer]
“AlwaysInstallElevated”=dword:00000001

If you have gotten a low-priv Meterpreter session, the built-in command line tool, reg query will help you to check these values:

If you got an error like “ERROR: The system was unable to find the specified registry key or value.” this means this registry values never created. So, the policy is not enabled.

But if you see the following output, it means the policy setting is enabled and you can exploit it. 😉

As I said before, in this situation, Windows Installer will use elevated permissions when it installs any package. So we should generate a malicious .msi package and run it. MSFvenom can handle this.

If you want you can generate a .msi package that adds a local admin to our target machine. You should use windows/adduser as a payload:

But in this scenario, I’ll generate an executable reverse shell payload(Payload.exe) and an msi package(malicious.msi) that executes this payload. Let’s do it!

Generating Payload.exe:

Generating malicious.msi by using windows/exec as a payload. Make sure you enter the correct path for Payload.exe:

Now we can upload these two to our target machine.

Before executing the .msi file, start a new handler on another terminal window for brand new hi-priv shell:

Now we’re ready to execute!

/quiet = Suppress any messages to the user during installation
/qn = No GUI
/i = Regular (vs. administrative) installation

Enjoy your shell with SYSTEM privileges!

Instead of manually applying this technique you can use this Metasploit module: exploit/windows/local/always_install_elevated

This module only requires that you link it to an existing Meterpreter session before running:

Privilege Escalation with Task Scheduler

This method only works on a Windows 2000, XP, or 2003 machine. You must have local administrator privileges to manage scheduled tasks. If you have a meterpreter session with limited user privileges this method will not work.

On Windows 2000, XP, and 2003 machines, scheduled tasks run as SYSTEM privileges. That means if we create a scheduled task that executes our malicious executable, it will run as SYSTEM. 😉

Again, I’ll generate an executable reverse shell payload for this job. Let’s demonstrate!

Generating an executable reverse shell payload:

You can drop your payload anywhere you want. I prefer temp folder:

We should ensure that Task Scheduler service works. Attempt to start service:

It seems to be already running. Let’s check machine’s current time:

We will create a task that will run our executable about 1 minute after the current time:

Start a new handler in another terminal window for the new hi-priv shell. 1 minute later our executable will run as SYSTEM and will get a session with SYSTEM privileges:

DLL Hijacking

Suppose that none of above methods worked. But of course, we did not give up. You may want to check running processes for DLL hijacking vulnerability.

Microsoft’s this article explains DLL hijacking well:

When an application dynamically loads a dynamic-link library without specifying a fully qualified path name, Windows attempts to locate the DLL by searching a well-defined set of directories in a particular order, as described in Dynamic-Link Library Search Order. If an attacker gains control of one of the directories on the DLL search path, it can place a malicious copy of the DLL in that directory. This is sometimes called a DLL preloading attack or a binary planting attack. If the system does not find a legitimate copy of the DLL before it searches the compromised directory, it loads the malicious DLL. If the application is running with administrator privileges, the attacker may succeed in local privilege elevation.

When a process attempts to load a DLL, the system searches directories in the following order:

1. The directory from which the application loaded.
2. The system directory.
3. The 16-bit system directory.
4. The Windows directory.
5. The current directory.
6. The directories that are listed in the PATH environment variable.

So, to exploit this vulnerability we will follow this path:

• Check whether the DLL that process looking for exists in any directory on the disk.
• If it does not exist, place the malicious copy of DLL to one of the directories that I mentioned above. When process executed, it will find and load malicious DLL.
• If the DLL file already exists in any of these paths, try to place malicious DLL to a directory with a higher priority than the directory where the original DLL file exists. For example, if the original DLL exists in the C:\Windows directory and if we gain control of the directory which the application loaded and place a malicious copy of the DLL in that directory, when the application tries to load the DLL file, it will look at the directory which the application loaded. And it will find the malicious copy of DLL, and load it. So, our malicious code will be executed with higher privileges.

Okay then. Let’s start to investigate running processes:

As you can see, if we are using low-priv shell we cannot see the details about processes which running with higher privileges, such as user, path, architecture. But we can understand which processes running with higher privileges than ours. If one of these processes have some weaknesses we can exploit it to escalate our privileges.

While investigating processes, Vulnerable.exe caught my attention. Let’s find it’s location and download it:

When we examine it a little bit, we will realize that it tries to load a DLL named hijackable.dll.

The easiest way to detect DLL hijacking vulnerability is using Procmon tool.

To see the results more easily, you should add these 3 filters:

 

After adding filters, when you execute Vulnerable.exe, failed DLL loads will be listed:

As shown above, Windows attempts to locate the hijackable.dll by searching a well-defined set of directories.

In this scenario, Vulnerable.exe has DLL hijacking vulnerability. Ok I confess, actually, this executable is a simple code that loads a DLL without doing some checks:

Let’s check if hijackable.dll exists on the target machine:

It seems that DLL does not exist on the machine. But we cannot be sure at this point, maybe it exists in a directory that we don’t have permission to view. Don’t forget we still have low privileges. 🙁

The next step is checking possible weak folder permissions. I usually check if a software gets installed in the root directory such as Python. Because if a folder created in the root directory, it is writable for all authenticated users by default. And softwares like Python, Ruby, Perl etc. usually added to PATH variable.

Remember, Windows checks the directories that are listed in the PATH environment variable!

Just as I thought, Python was installed. Let’s check permissions:

BINGO! Authenticated users have modification permissions!

One last check left. We should ensure if C:\Python27 directory added in the PATH environment variable. The easiest way to do this, typing “python -h” in the shell. If the help page is displayed successfully it means the directory is added to the PATH:

Nice! Let’s create a simple reverse shell payload as a DLL:

Then place it in the C:\Python27 directory:

Now, we should restart the Vulnerable.exe process, so that the process can load malicious DLL. We can try to kill the process. If we are lucky it will be started automatically:

We are unlucky today, not even killed. Anyway, we can try restarting the machine. If the “Vulnerable.exe” is a startup application, a service, or a scheduled task it will be launched again. At worst, we will wait for someone to run it.

The machine is restarting. Let’s start a new handler and hope it starts again:

We got it! 😉

Stored Credentials

If none of that methods work, you may need to try finding some stored credentials to escalate your privileges. You may want to check these directories:

• C:\unattend.xml
• C:\sysprep.inf
• C:\sysprep\sysprep.xml

And you may want to search files using queries like this:

• dir c:\*vnc.ini /s /b /c
• dir c:\*ultravnc.ini /s /b /c
• dir c:\ /s /b /c | findstr /si *vnc.ini
• findstr /si password *.txt | *.xml | *.ini
• findstr /si pass *.txt | *.xml | *.ini

Kernel Exploits

In this blog post, I intentionally tried to explain escalation methods that do not rely upon kernel exploits. But if you are about to use an exploit to escalate your privileges, maybe this command will help you to choose which one you should use:

wmic qfe get Caption,Description,HotFixID,InstalledOn

It will list the updates that are installed on the machine.

A Note About Payloads

In this blog post, my payloads generated by MSFvenom. However today, these payloads are flagged by almost all Anti-Viruses. Because it is a popular tool and well known by AV vendors. Creating your own executables using AV bypassing techniques will give you the best results. You may consider reading these articles:

• Art of Anti Detection 1 – Introduction to AV & Detection Techniques
• Art of Anti Detection 2 – PE Backdoor Manufacturing

References

[1] – https://www.trustwave.com/Resources/SpiderLabs-Blog/My-5-Top-Ways-to-Escalate-Privileges/
[2] – https://msdn.microsoft.com/en-us/library/windows/desktop/ms682425(v=vs.85).aspx
[3] – https://msdn.microsoft.com/en-us/library/windows/desktop/ms682586(v=vs.85).aspx
[4] – https://www.synack.com/2014/12/10/dll-hijacking-in-2014/
[5] – http://www.fuzzysecurity.com/tutorials/16.html
[6] – https://www.exploit-db.com/docs/39732.pdf
[7] – https://www.tecklyfe.com/remediation-microsoft-windows-unquoted-service-path-enumeration-vulnerability/
[8] – https://toshellandback.com/2015/11/24/ms-priv-esc/
[9] – https://it-ovid.blogspot.de/2012/02/windows-privilege-escalation.html
[10] – https://blog.netspi.com/windows-privilege-escalation-part-1-local-administrator-privileges/
[11] – https://msitpros.com/?p=2012

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