Privilege escalation is one of the key components of any attack that involves penetrating a system. If threat actors have limited access due to a current user’s privilege levels, they will naturally aim to escalate their privileges before expanding the scope of the attack. How can security professionals detect malicious escalation techniques before adversaries get a chance to compromise critical systems and sensitive data?
There are several ways to do that, but let’s focus specifically on one of the tricks most commonly employed by malware developers and attackers: the named pipe impersonation technique.
Named Pipe Impersonation
One of the most well-known penetration testing frameworks is called Metasploit. Although designed for testing purposes, attackers can — and often do — use concepts from this framework maliciously to escalate their privileges on a compromised host to a system account, which is a high-privilege account type on Windows-based endpoints. Named pipe impersonation is a technique used in the Metasploit platform to escalate these privileges.
The legitimate named pipe technique is built into the Windows OS to facilitate communications between processes. The pipe technique uses a file to exchange messages between the two processes. For example, if one process wants to contact another, it can send a message over the network or by using a file, where one process writes the message to that file and the other one reads it.
Pipes are also used by various malware codes for covert communications. In the NotPetya ransomware attacks, for example, the malware reportedly spread through organizational networks. To move from one user to the next, it needed credentials, and it needed to obtain them stealthily. To do that, NotPetya typically started a new process to dump the victim’s credentials, and then used a named pipe to communicate between the NotPetya process and the credential-dumping process. This enabled it to covertly collect the dumped credentials.
But what do we mean by impersonating a named pipe? How do attackers use it to escalate their privileges to a system account? This malicious action goes back to the concept of privilege levels.
Let’s reflect by means of the following example: Let’s say I’m a service provider, you are my client and you ask me to execute a database query. As a provider, I may have full access to that database, but you, as a client, may have limited access that corresponds to the rights attributed to you as a user.
Since you are a user/client, I can use your credentials to execute the query on your behalf — if your access rights allow you to perform that database query. You will then receive the results, also according to the rights level. If the results cannot be provided to you as a user, it could mean that you don’t have sufficient access.
But what if you, as the client, somehow have higher privileges than the service provider? You can pass those escalated rights to the service provider by letting it use privileged credentials. In such a case, what if the provider should impersonate or abuse the client’s privileged account to perform malicious activities?
The same idea can be applied in the named pipe context. If a process creates a pipe, this process will be the pipe owner or the pipe server. When another process connects to this pipe, it will be called the pipe’s client. Once connected, the pipe server can use the pipe client’s privilege level, the client’s security context or the client’s access rights. This is a Windows feature that helps perform activities based on the client’s privileges, since the server may have full access, but the client typically has more limited rights.
This feature can be exploited by creating a pipe server with limited or low privileges and then attempting to connect a much more privileged client to that pipe server. When that happens, the pipe server can abuse the client’s elevated privileges to perform activities based on those access rights.
Metasploit facilitates and automates the abusing process and allows penetration testers to perform it by executing a single command. Behind the scenes, the tool creates a pipe server with limited privileges, then configures a Windows service (the client) to connect to that pipe.
Abusing the Named Pipe Feature by Using Metasploit Meterpreter
The tool commonly used to test the potential of impersonating a pipe is the Metasploit Meterpreter module. Meterpreter is a modular part of the Metasploit penetration testing framework. It is an advanced, dynamically extensible payload that uses in-memory Dynamic Link Library (DLL) injection stagers and is extended over the network at runtime. This tool is typically used in schemes that bear on communications between Windows processes, which makes it useful to an attacker looking to impersonate a legitimate named pipe.
Now let’s say a threat actor executes a malicious process called myLove.exe under user MARY, who has limited privileges on the compromised host. This malicious process will connect back to the attacker’s host and can allow the threat actor to remotely execute other commands via MARY.
The malicious process starts out with MARY’s limited privileges, but the attacker’s goal is to escalate his or her privileges from MARY’s level to a system-level user. By executing a getsystem command, myLove.exe will create a pipe with a random name. In our example, that random name was “dqwfqx,” but it could have been another name as well. This pipe is originally created with MARY’s user privilege level.
Figure 1: Meterpreter session showing the malicious process created a reverse connection to the attacker’s host
By executing a getsystem command, myLove.exe will create a pipe with a random name. In our example, that random name was “dqwfqx,” but it could have been another name as well. This pipe is originally created with MARY’s user privilege level.
<img title="a windows sysmon event indicating a pipe has been created" src="https://securityintelligence.com/wp-content/uploads/2018/01/figure-2-a-windows-sysmon-event-indicating-a-pipe-has-been-created.png" alt="A Windows Sysmon e
Figure 2: A Windows Sysmon event indicating that a pipe has been created
The attacker’s challenge now is to convince a client with system-level privileges to connect to the new pipe. This is not hard to achieve, since Windows services can run as a system user and the attacker can install a new service or reconfigure an existing one to send any message to the named pipe “dqwfqx.”
Figure 3: Windows event indicating that a service was installed
The Windows Registry event shown below indicates that a service was installed and configured to connect to the named pipe “dqwfqx.”
Figure 4: Windows registry event indicating that a service has been installed in the system
When the service starts, this time it launches cmd.exe as a system-level user and connects to a pipe created by MARY, which now leads to a system-level client connecting to a pipe server that was created by a lower-privileged user.
Figure 5: A new process has been created and the process will use cmd.exe to connect to the pipe
This malicious activity may happen relatively smoothly, but it can be detected by using an SIEM solution to look for a pipe creation event followed by a service that connects to the same pipe.
This approach can be expanded by creating or modifying a scheduled task to connect to a lower-privilege pipe. In general, it’s important to check whether any service or scheduled task is configured to connect to a pipe and to examine the underlying activity’s source.
Exploiting Misconfigured Services to Escalate Privileges
There are many ways in which attackers can exploit misconfigured services to conduct privilege escalation schemes. Below are some of the most prominent ones I have worked with in the past year, as well as some tips that can be useful in detecting them.
Services With Unquoted Binary Paths
One of the easiest techniques to escalate privileges is to look for any service with an unquoted executable file location. For example, if we have a service and the service binary file is located in C:\users\MyCompanyService\myService.exe, then Windows will try to locate the service binary file and execute the following files:
C:\users\my company.exe; and
C:\users\my company Service\myService.exe.
If an attacker can place a malicious file with the name my.exe in C:\users\my.exe, he or she may be able to escalate to the service user’s privileges during the next service restart. The Windows OS service will start to search for the service executable file after a reboot and, while doing that, will execute the malicious file my.exe.
A proactive way to identify this type of activity is to use rules to detect any new service with an unquoted binary file location. Another great technique is to baseline or profile the processes that can run at the endpoint level and then check for any new unknown processes.
It’s also important to profile and baseline the processes that can run with system-level privileges to trigger an offense in cases where any new process attempts to run with system user privileges. This enables the security team to detect any privilege escalation attempts, even if a zero-day exploit is being used.
Folder Access Control Lists
Attackers can also abuse the permissions assigned to the service executable folder, since a poorly written access control list (ACL) may allow local users to add or override these files. Threat actors can look for a service configured to run with higher user privileges. If they can override the service binary executable file with another malicious service executable file, then they can escalate their privileges to the service user level.
Security professionals can detect this technique by profiling and mapping the process name to the process hash. This enables security teams to identify known processes that start with unknown or unseen hashes.
Service Object Permissions
A misconfigured permission may allow a local user to change service attributes or reconfigure a service, which can allow an attacker to change the service binary location to another malicious executable. A common technique is to change the service binary location to a set of commands to add a new or existing user to the local administrators group. This can enable the attacker to escalate his or her limited privileges by hijacking the new admin user.
Security teams can detect such activity by looking for a service binary executable change followed by the addition of a new user to the admin group. They can also profile the service binary file attributes (e.g., name, hash), then trigger an offense if a known service starts with a new attribute (e.g., a new binary file hash or a different location for the service binary file).
Getting a Command Line Shell as a System-Level User — Hacker Style
In most privilege escalation attacks, threat actors attempt to get a command line with the highest privileges possible. In Windows, a highly privileged user is the system user. Normal endpoint users and even Windows administrators do not launch command lines as system-level users, so determining whether a command line is launched as a system user is a good place to start when it comes to detecting various privilege escalation techniques.
In a typical attack that uses Metasploit, attackers launch a command line as a system user after escalating their privileges. They may also opt to use legitimate Windows utilities, such as Sysinternal PsExec, which would help them execute processes as a system user on a local or remote machine.
Monitoring for command lines launched with system privileges is a great method to detect malicious processes. Another good approach is to profile the processes that can run as system and trigger an alert when a new unseen or unknown system-level process starts. View the X-Force Exchange collection for a list of legitimate Windows processes that do start with system-level privileges.
Figure 7: Getting a command line shell as a system-level user using Metasploit
Nipping Privilege Escalation in the Bud
Privilege escalation activity is often a precursor to a potentially devastating data breach involving the enterprise’s most sensitive data. Detecting the above techniques before they cause harm and using well-managed SIEM solutions to monitor pipe creation events and other suspicious activity, security professionals can save their organizations time, money and negative headlines.