What is a Shimming Attack?

A shimming attack is a sophisticated cyber technique where an attacker inserts a small piece of code, known as a "shim," between two software layers. This inserted code intercepts and manipulates communications or operations that occur between these layers. Shimming is often used to bypass security mechanisms, capture sensitive data, or alter the behavior of a system without being detected. Unlike traditional malware, shims are more challenging to identify because they are designed to appear as legitimate parts of the software.

Shimming attacks can be particularly dangerous because they exploit the trust and dependencies within software ecosystems. For instance, a shim can be placed in the communication pathway between a legitimate application and its dependent library or operating system. This allows the attacker to modify inputs and outputs, potentially leading to unauthorized access, data breaches, or system compromises.

Origin of Shimming Attack

The concept of shimming attacks originated from the broader field of software manipulation and exploitation. The term "shim" itself refers to a thin layer or piece used to fill gaps, which perfectly describes the role of a shim in software manipulation. Early instances of shimming were seen in the form of compatibility layers used to run old software on new operating systems. However, malicious use of shimming techniques emerged as attackers recognized the potential to intercept and manipulate system operations covertly.

Historically, shimming gained prominence with the rise of complex software environments where multiple layers of dependencies and communication pathways became common. This complexity provided fertile ground for attackers to insert shims and exploit weaknesses in the interaction between software components.

Practical Application of Shimming Attack

A practical example of a shimming attack can be found in the realm of payment card security. Attackers can insert shims into the interface between a card reader and a point-of-sale (POS) terminal. These shims are designed to capture card details, including PINs and magnetic stripe data, as the information passes through the reader to the terminal.

For instance, a criminal could place a thin shim inside the card slot of an ATM or a gas pump. When a victim inserts their card, the shim records the card's information without disrupting the transaction process. The attacker later retrieves the shim and extracts the stolen data to create cloned cards or engage in fraudulent transactions.

Benefits of Shimming Attack

From the perspective of a malicious actor, shimming attacks offer several advantages:

Stealth: Shims operate quietly in the background, making them difficult to detect compared to more conspicuous forms of malware.

Precision: Shims can be tailored to target specific applications or system components, allowing for precise manipulation and data capture.

Versatility: Shimming techniques can be adapted to various platforms and devices, from desktop applications to embedded systems and IoT devices.

Persistence: Once installed, shims can remain effective over long periods, continuously providing the attacker with valuable information or access.

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