DLL Side-Loading
DLL side-loading is a sophisticated technique attackers use to escalate privileges on a system by exploiting how Windows handles the loading of Dynamic Link Libraries (DLLs). This technique takes advantage of the way applications search for and load DLLs, allowing attackers to introduce malicious DLLs that are executed with elevated privileges.
Detailed Steps in a DLL Side-Loading Attack
Identifying a Target Application: Attackers begin by identifying applications that search for DLLs in directories that are accessible to the attacker. These applications are often ones that run with higher privileges, such as system utilities, software installers, or popular third-party applications.
Crafting a Malicious DLL: The attacker creates a malicious DLL with the same name as a legitimate one the target application is expected to load. This DLL is designed to perform harmful actions, such as executing additional malware, harvesting credentials, or manipulating system configurations.
Placing the Malicious DLL: The attacker places the crafted DLL in a directory from which the application is likely to load it. This could be the application’s installation directory, the current working directory, or another location that is searched before the system directories (e.g., %TEMP%, %APPDATA%).
Executing the Application: When the target application runs, it inadvertently loads the malicious DLL instead of the legitimate one, due to the order in which directories are searched for DLLs. This allows the attacker's code to execute within the context of the target application, potentially with elevated privileges.
Privilege Escalation Mechanisms
Once the malicious DLL is loaded by the target application, it inherits the privileges of the application. If the application runs with administrative rights, the malicious DLL can perform any action that an administrator can. The primary objectives of privilege escalation through DLL side-loading include:
Executing Additional Malware: The attacker can drop and execute additional payloads to further compromise the system.
Gaining Persistent Access: The malicious DLL can create persistence mechanisms to ensure the attacker maintains access to the system.
Stealing Sensitive Data: The attacker can access and exfiltrate sensitive information, such as passwords, encryption keys, or personal data.
Modifying System Configurations: With elevated privileges, the attacker can alter system configurations, disable security features, or add backdoors for future access.
Example Scenarios
Compromising Software Installers: Attackers often target software installers that require administrative privileges. By placing a malicious DLL in the installer’s directory, the installer inadvertently loads the malicious DLL during the installation process, granting the attacker elevated privileges.
Hijacking Legitimate Applications: Attackers may target commonly used applications by privileged users. For example, if a system administrator frequently uses a particular application, placing a malicious DLL in a location that the application checks first ensures that the attacker’s code is executed with administrative privileges whenever the application is run.
Indicators to Detect DLL Side-Loading
Detecting DLL side-loading involves a multifaceted approach that combines careful monitoring of file systems, processes, network activity, and event logs. Here’s an in-depth look at the specific indicators that can help identify DLL side-loading attempts:
Unexpected DLLs in Application Directories
One of the most telling signs of DLL side-loading is the presence of unexpected or suspicious DLL files in application directories. Legitimate applications usually have a predefined set of DLLs they rely on, stored in specific directories. Anomalies can include:
Unusual File Names and Locations: Malicious DLLs often mimic the names of legitimate DLLs to avoid detection. However, they may be found in directories where these legitimate DLLs are not typically located. For example, finding a system DLL in a user’s profile directory or a temporary folder can be a red flag.
Recently Added or Modified DLLs: Attackers frequently add or modify DLLs to introduce malicious code. Monitoring file creation and modification dates in application directories can help identify recently added DLLs that could be malicious.
Unusual File Attributes
Malicious DLLs often have attributes that distinguish them from legitimate files. These attributes include:
Missing or Incorrect Metadata: Legitimate DLLs usually contain metadata such as version information, company name, and digital signatures. Malicious DLLs may lack this information or have incorrect details that don’t match the legitimate DLL they are impersonating.
Suspicious File Size: Comparing the file size of a suspect DLL with the known size of the legitimate DLL it is mimicking can reveal discrepancies. Malicious DLLs might be larger or smaller due to the addition or omission of certain code sections.
Anomalous Application Behavior
Applications that have been compromised through DLL side-loading may exhibit behavior that deviates from their normal operation. Indicators include:
Unexpected Requests for Elevated Privileges: If an application that typically runs with standard user privileges suddenly requests administrative rights, it may have been compromised to load a malicious DLL that requires higher privileges to execute its payload.
Frequent Crashes or Performance Issues: Malicious DLLs can cause stability issues. Applications that start crashing frequently, becoming unresponsive, or showing degraded performance without a clear cause might be loading malicious DLLs.
Suspicious Network Activity
Once a malicious DLL is loaded, it often tries to communicate with external servers to exfiltrate data or receive further instructions. Indicators of suspicious network activity include:
Unusual Outbound Connections: Monitoring network traffic for unexpected outbound connections from applications that normally do not communicate over the network can help identify compromised applications. These connections might be directed to known malicious IP addresses or domains associated with command and control (C2) servers.
Data Exfiltration Attempts: Large volumes of data being sent out of the network, especially from applications that should not be transmitting such data, can indicate that a malicious DLL is trying to exfiltrate sensitive information.
Process Monitoring
Monitoring active processes on a system can reveal abnormal behaviors indicative of DLL side-loading. Specific signs include:
Processes Loading DLLs from Non-Standard Directories: If processes are seen loading DLLs from directories that are not typically used for storing DLLs, this can indicate a side-loading attack. For instance, system processes loading DLLs from user profile directories or temporary folders are suspicious.
Unusual Parent-Child Process Relationships: Anomalous parent-child relationships, where a normally benign application spawns processes that exhibit suspicious behavior, can suggest that the application has loaded a malicious DLL. For example, if a word processor suddenly spawns a command shell, this could indicate malicious activity.
Log Events
To effectively detect DLL side-loading attacks, it is essential to monitor a subset of Windows Event IDs that provide critical insights into process activities, file modifications, and system changes. Here are the most important Event IDs for detecting DLL side-loading:
Event ID 4688: New Process Creation
This event logs whenever a new process is created on the system. Monitoring Event ID 4688 is crucial as it tracks the creation of processes, which can reveal when an application starts and potentially loads a DLL from an unusual location. Details such as the parent process, command line arguments, and the user account under which the process runs can provide valuable context for identifying suspicious activities.
Event ID 4697: Service Installation
Event ID 4697 is logged when a new service is installed on the system. Attackers may use DLL side-loading to execute malicious code via newly installed services. By monitoring this event, security teams can identify unauthorized or suspicious services that might be utilized to load malicious DLLs.
Event ID 4657: Registry Value Change
This event is logged when a registry value is modified. Attackers often change registry settings to facilitate DLL side-loading, such as modifying the AppInit_DLLs or KnownDlls registry keys. Monitoring these changes can reveal attempts to persistently load malicious DLLs through registry modifications.
Sysmon Event ID 8003: DLL Search Order Hijacking
Event ID 8003, specific to Microsoft Sysmon (System Monitor), logs when a process loads a DLL using a search order that can be hijacked. This event is critical for identifying deviations from standard, secure DLL loading methods, indicating potential side-loading attempts.
Event ID 7045: A New Service Was Installed
This event, part of the Service Control Manager events, provides information about new services installed on the system. Monitoring Event ID 7045 helps track the installation of services that might be leveraged to load malicious DLLs, providing insights into potential side-loading activities.
Last updated