Understanding DLL Sideloading in Cybersecurity: Threats, Real-World Cases, and Prevention

In today’s cybersecurity landscape, attackers use increasingly sophisticated tactics to infiltrate systems, steal data, and compromise security. One method that has emerged as a favored tactic among cybercriminals is DLL sideloading. This approach takes advantage of legitimate software vulnerabilities, allowing malicious actors to sneak malware into systems under the guise of authentic files. Understanding DLL sideloading is essential for businesses to protect against these underhanded threats, as it highlights the importance of a strong defense against lateral movement attacks.

In this article, we’ll explore what DLL sideloading is, examine real-world cyberattacks where it was used, and discuss how businesses can prevent this insidious method of attack.

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What is DLL Sideloading?

DLL (Dynamic-Link Library) sideloading is a technique where an attacker places a malicious DLL file in a location where legitimate applications mistakenly load it. Typically, when a program needs to call a DLL file, it follows a search order to find the file. In sideloading attacks, an attacker places their malicious DLL in a location prioritized in this search order, tricking the application into loading the harmful file instead of the intended one.

This exploitation technique is particularly dangerous because it capitalizes on the system’s trust in legitimate software, allowing the attacker to bypass some security protocols. Once sideloaded, the malicious DLL can execute code with the same privileges as the trusted application, making it difficult to detect as it operates under the radar of traditional antivirus solutions.

Real-World Examples of Cyber Attacks Using DLL Sideloading

1. APT41 and Targeted Attacks on Video Game Companies (2019)
   Advanced Persistent Threat (APT) group APT41, a well-known China-based cyber-espionage group, used DLL sideloading in attacks on video game companies. They leveraged vulnerabilities in legitimate gaming applications to inject malicious DLL files. Once the files were loaded, APT41 could access user data, execute code remotely, and maintain persistence in compromised networks. This allowed them to exfiltrate proprietary information and spy on operations, causing significant financial and reputational damage to affected companies.
2. ShadowPad Incident (2017)
   ShadowPad is another sophisticated malware platform that used DLL sideloading as part of its attack vector. The attackers embedded malicious DLLs within software updates for widely-used products, including software in supply chain management systems. When organizations installed updates, they unknowingly ran malicious DLLs alongside legitimate software. This campaign went undetected for months, giving attackers ample time to monitor, log keystrokes, and extract sensitive information from hundreds of organizations worldwide.
3. CCleaner Compromise (2017)
   In the infamous CCleaner attack, hackers exploited the software update process of the popular PC optimization tool to distribute malware via DLL sideloading. When users updated CCleaner, the compromised version installed malicious DLLs, which then allowed attackers to gain access to the systems of thousands of users. This high-profile attack affected millions, demonstrating the dangerous reach DLL sideloading can achieve when distributed through widely-used software.
4. Operation Ke3chang (2016)
   Ke3chang, another sophisticated APT group, used DLL sideloading in attacks against organizations in the Middle East and Europe. They crafted malware embedded within fake document files, which sideloaded malicious DLLs when opened. This approach allowed them to bypass security measures, establish a foothold in government and corporate networks, and collect valuable intelligence.

How to Prevent DLL Sideloading in a Business

DLL sideloading is a dangerous attack vector, but businesses can take proactive steps to mitigate the risk:

1. Implement Application Whitelisting
   Application whitelisting allows only approved software to run on systems. By specifying which applications and processes can be executed, businesses can limit the risk of malicious DLLs being loaded into trusted programs. Regularly updating and auditing the whitelist is essential to ensure that it reflects the organization’s current software needs.
2. Harden Endpoint Security with Detection Tools
   Advanced endpoint detection and response (EDR) solutions can detect anomalies in system behavior, including unexpected DLL loads. Businesses should implement security solutions that monitor and log the loading of DLLs, alerting administrators to any suspicious behavior indicative of sideloading attacks.
3. Update Software Regularly
   Keeping software up-to-date is vital in preventing DLL sideloading. Many cyberattackers exploit outdated software with known vulnerabilities, so businesses should have a consistent patch management program in place to ensure all applications are current. Automatic updates can help, but critical systems should also be manually reviewed.
4. Establish DLL Load Order Controls
   IT teams can configure applications and systems to control the DLL load order. By setting custom load paths or using Windows’ Safe DLL Search Mode, businesses can help prevent applications from unintentionally loading malicious DLLs placed in non-standard locations.
5. Educate Employees on Safe Software Practices
   Phishing remains a major avenue for delivering malicious files, including DLLs. Educating employees on recognizing phishing attempts and the risks of downloading software from unverified sources is crucial. Additionally, enforcing policies on software downloads and installation can help limit the number of third-party applications that could introduce vulnerabilities.

Conclusion

DLL sideloading is a complex and stealthy form of attack that exploits trusted software to bypass security measures. As illustrated by numerous high-profile attacks, the impact of DLL sideloading can be devastating, leading to data breaches, espionage, and severe financial losses.

For organizations, a multi-layered defense strategy is key. By implementing robust application controls, utilizing advanced endpoint security, and maintaining up-to-date systems, businesses can reduce the likelihood of a DLL sideloading attack. Staying vigilant and proactive in the ever-evolving cybersecurity landscape is essential to keeping company data and infrastructure secure from this and other similar threats.