Shimming

Shimming is a technique used in driver manipulation to intercept and modify the behavior of legitimate device drivers without modifying their original code. It involves inserting additional code, known as a shim, between the operating system and the driver. Shimming can be used for both legitimate purposes, such as compatibility fixes, and malicious activities, such as bypassing security controls or injecting malicious code.

Shims act as a layer between the operating system and the driver, allowing for the interception and modification of driver calls and behaviors. Some common examples of shim-based attacks include:

1. Function Hooking: Shims can intercept calls made to specific functions within a driver and redirect them to modified code. This enables attackers to modify the behavior of the driver, potentially bypassing security checks or injecting malicious instructions.
2. API Redirection: Shims can redirect calls made to specific application programming interfaces (APIs) to alternative or malicious versions of those APIs. This can be used to manipulate data, capture sensitive information, or perform unauthorized actions.
3. Kernel-Level Manipulation: Shims can operate at the kernel level, intercepting and modifying system calls and interactions between the operating system and the driver. This allows attackers to gain elevated privileges, perform unauthorized operations, or bypass security mechanisms.

Shimming attacks can be challenging to detect because they do not require modifying the original driver files. Instead, they leverage the dynamic modification capabilities provided by the shim layer. To mitigate the risks associated with shimming attacks, the following preventive measures can be implemented:

• Use Driver Signing: Employ digital signatures for drivers to ensure that only trusted and signed drivers are loaded. This helps prevent the installation and execution of malicious or tampered drivers.
• Implement Code Integrity Checks: Implement mechanisms to verify the integrity of loaded drivers, ensuring that their code has not been modified. This can include using secure boot mechanisms, driver signature checks, or checksum verification.
• Monitor Driver Behavior: Implement runtime monitoring and behavior analysis of drivers to detect any abnormal or malicious activities. This can involve the use of endpoint protection solutions or intrusion detection systems.
• Regularly Update Drivers: Keep drivers up to date with the latest versions provided by the hardware manufacturer. Updates often include security patches and bug fixes that address known vulnerabilities, reducing the risk of shimming attacks.