LNK Stomping (Exploit Kit) – Malware

Overview

LNK stomping is a sophisticated technique that targets the way Windows operating systems handle shortcut files, known as LNK files, to bypass security mechanisms such as Smart App Control (SAC) and SmartScreen. These security features are designed to protect users from potentially harmful files by analyzing their metadata, particularly the “Mark of the Web” (MotW), which signals that a file may have originated from an untrusted source, such as the internet. However, LNK stomping exploits a flaw in how these files are processed, allowing malicious actors to remove or bypass the MotW label, thus evading detection from these reputation-based protections.

Targets

Individuals

How they operate

At its core, LNK stomping exploits a mechanism within Windows known as “canonicalization.” This process involves how Windows resolves file paths when accessing resources. A standard LNK file typically includes the full path to the target executable, along with associated metadata such as the MotW. However, Windows allows some flexibility in how file paths are formatted, particularly with regard to trailing characters like spaces or dots. In a typical situation, the presence of such characters does not affect the function of the file path, but the Windows Explorer process treats it as part of the file system. When the LNK file is opened, the operating system normalizes the path to its canonical form, which can strip away or “stomp” any associated MotW markers embedded within the LNK’s metadata.

The crux of the exploit lies in the fact that during this canonicalization process, the MotW can be removed without triggering a warning. Attackers can manipulate this behavior by altering the internal structure of the LNK file, such as appending extraneous spaces or dots to the path or using relative paths. These modifications cause Windows Explorer to reformat the file, removing the MotW before any security mechanisms can inspect the file. Consequently, the malicious file executes as if it were from a trusted source, bypassing SmartScreen and SAC, both of which rely on the MotW to determine the file’s safety.

This technique is particularly effective because it does not require any changes to the underlying security framework of the operating system. Instead, it leverages existing behavior in the Windows file system, making detection more difficult for traditional security measures. Anti-virus solutions and endpoint protection tools typically rely on signature-based detection methods, which may not detect the manipulations of the LNK file’s structure, especially when no new vulnerabilities are introduced by the exploit. Additionally, because the technique involves leveraging existing OS functionality, it’s more difficult to patch compared to traditional vulnerabilities.

In a typical LNK stomping attack, the attacker would craft a malicious LNK file, possibly as part of a larger phishing campaign or a drive-by download, and deliver it to the victim. Upon opening the file, the exploit kicks in as Windows Explorer resolves the file path and strips the MotW, allowing the payload to execute without triggering any security warnings. The malware can then establish persistence on the system, download additional malicious code, or exfiltrate sensitive information without detection.

As defenders, understanding how LNK stomping works is crucial for detecting and mitigating this technique. One potential mitigation is the implementation of more robust inspection mechanisms that look beyond file metadata to analyze the actual behavior of files. Additionally, user education and a heightened awareness of phishing campaigns can help reduce the risk of falling victim to such attacks. Finally, while tools like SAC and SmartScreen are effective at flagging files based on MotW, their reliance on this metadata can be circumvented by attackers who understand how to manipulate LNK files, making the implementation of additional behavioral-based security measures necessary to protect systems from these kinds of stealthy evasion tactics.

Reference: 

• Dismantling Smart App Control