When we think about file-based threats, we tend to think of the more obvious culprits. Malicious Word documents, suspicious PDFs, and ZIP archives loaded with insecure content steal the spotlight.

However, the reality is that attackers are just as eager to exploit file types that most users never see or interact with directly. One of the most dangerous examples of this is the Dynamic-Link Library (DLL) file.

In this article, we’ll explain what DLL files are, and we’ll review how they function within Windows systems. We’ll explain why they’re a target for threat actors, and we’ll look at the obfuscation techniques attackers use to sneak DLLs past basic network edge defenses.

At the end, we’ll explore how Cloudmersive’ Advanced Virus Scan API detects obfuscated DLL threats and blocks them before they can cause harm.

What is a DLL File?

A DLL is a system file used by Windows applications to store shared functionality. That includes code, data, and other resources that multiple programs can use.

Users don’t run DLL files directly, which differentiates them from EXE executables. Instead, they’re accessed and loaded by running programs only when they’re needed. This allows programs to avoid duplication by modularizing key features and reusing specific core components.

A good example of this is spell-checking for MS Office applications. Since most MS Office applications provide the same spell-checking feature, it makes sense to store that feature as a modular component in a DLL file so all MS Office applications can access it. This avoids duplicate code and ensures consistency across all MS Office programs.

The dynamic loading system makes Windows more flexible, but it also opens the door for abuse. DLLs, like EXE files, must follow the Windows Portable Executable file structure – and that means they can carry executable machine code, embedded resources, and export functions. All these features are attractive for attackers to exploit.

Why DLLs are High-Impact Threat Vectors

The fact that DLLs can contain executable code isn’t the only thing that makes them dangerous. By maneuvering malicious DLLs into the right locations, attackers can feasibly trick applications into loading malicious content. This creates an opportunity to execute arbitrary code in the context of that trusted program.

The consequences of this kind of attack could be anything from privilege escalation to stealthy persistence – or even system compromise.

Sideloading via DLL is another possibility. If a legitimate application is tricked into looking for required DLLs in an insecure location, they can load malicious resources they were never meant to have access to in the first place.

This is a classic DLL hijacking scenario. After the application loads malicious DLL resources, the file effectively has the same power as any executable. The difference is that DLLs tend to get less scrutiny from antivirus or system defenses.

Common DLL Attack Techniques

Attackers use numerous techniques to deliver malicious DLLs, including insecure file upload, phishing emails, and infected software bundle downloads. Below, we’ll walk through some of the most common tactics we can expect threat actors to use in DLL-based attacks.

DLL Sideloading (Hijacking)

Applications usually look for DLL dependencies in local directories before system paths. Attackers can exploit this by placing a malicious DLL in the same folder as a legitimate EXE. This “hijacks” the usual process, introducing the possibility that the application might load the DLL’s executable instead of the real one.

Real-world attackers have abused Microsoft Teams and Adobe tools by packaging malicious DLLs with renamed versions of legitimate applications. In such cases, launching the application EXE loads the malicious DLL without suspicion.

Embedded DLLs in Archives

Since DLLs are rarely an expected file type in file upload workflows or email attachments, it’s common for security workflows to blacklist the .dll extension. To get around that, attackers often obfuscate malicious DLLs within ZIP, RAR, and other compressed archive formats. This can hide the insecure content from detection – especially if security policies don’t look deep enough into the archive.

Phony or Disguised Extensions

Renaming extensions is another common workaround for security workflows that blacklist DLL. It’s simple enough for attackers to disguise DLLs by renaming them with less suspicious extensions like JPG, TXT, or DOCX.

Attackers often craft scripts or malware droppers separately to rename their files back to DLL after they’ve entered the target system. These adjacent payloads typically load the DLLs dynamically via PowerShell or batch scripts.

Remote DLL Loading

More sophisticated attackers are capable of initiating DLL-based attacks without storing actual DLL files on the target disk at all. Instead, they pull DLLs from remote attacker-controlled domains, load them into memory, and execute them using tools like rundll32.exe or PowerShell.

Real-World Example: PlugX and DLL Sideloading

The PlugX remote access trojan (RAT) is one of the more well-known DLL-based threats in the wild. It’s often packaged as a legitimate signed executable, a malicious DLL, and an encrypted payload file. The legitimate executable loads the DLL via sideloading, and the DLL decrypts and runs the final malware payload.

Basic endpoint defenses often fail to detect this attack because it leverages a signed, trusted application. It’s been used against telecommunications, government, and even defense sectors. This highlights the need for security policies with deeper content inspection capabilities.

How Cloudmersive Detects and Blocks DLL Threats

Cloudmersive’s Advanced Virus Scan API is purpose-built to identify malware signatures and uncover obfuscated threats with deep content verification.

That means unpacking compressed archives (ZIP, RAR, 7Z, etc.), disk images, Office files, and other complex file formats to identify buried DLLs – even when they’re named with phony extensions, nested several layers deep, or bundled together with legitimate software – and investigating them thoroughly. Heuristic analysis ensures suspicious behavioral patterns are identified regardless of whether explicit malware signatures are present.

This combination of strategies offers comprehensive protection against one of the most stealthy and damaging file types in the malware toolkit.

Conclusion

DLL files are devastating attack vectors in the wrong hands. They’re a popular choice for attackers given their ability to run silently in the background, piggyback with trusted software, and avoid detection through standard obfuscation techniques. It’s critically important that our security policies do not trust DLL files and tirelessly investigate complex file types capable of hiding them.

To learn more about blocking DLL-based threats with Cloudmersive’s Advanced Virus Scan API, please contact a member of our team.