Introduction
DLL sideloading is a widely used attack technique that exploits how Windows applications load dynamic link libraries (DLLs). Threat actors use it to execute malicious payloads while evading traditional security measures.
This post explores how this attack technique works, why it is attractive to attackers, and the best methods for detection and mitigation.
What Is DLL Sideloading?
DLL sideloading (also known as DLL hijacking or DLL search order hijacking) is an attack method where a malicious DLL is placed in a location where a legitimate application expects to find a trusted DLL. When the application runs, it mistakenly loads the attacker’s DLL instead of the legitimate one.
How DLL Sideloading Works:
- Exploiting Search Order Hijacking: Windows searches for DLLs in a predefined order. Attackers place their malicious DLL in a directory checked before the legitimate system paths. Malware like PlugX often exploits sideloading vulnerabilities in software such as Adobe Reader.
- Injecting Malicious Code: The sideloaded DLL executes code to establish persistence, steal data, or download additional malware. For example, the CatB ransomware campaign used a DLL sideloading technique to execute its payload in the context of a trusted Windows service.
- Evading Security Mechanisms: Because the application loading the DLL is trusted, security solutions may not flag the execution as suspicious. APT groups like Mustang Panda have used this technique to evade detection while deploying Remote Access Trojans (RATs) like PlugX.
Common Use Cases for DLL Sideloading in Malware:
- Advanced Persistent Threats (APTs) use this attack technique to maintain persistence in compromised environments. For instance, Mustang Panda has used DLL sideloading with Adobe Reader executables to load malicious payloads.
- Ransomware groups leverage it to stealthily execute payloads. The CatB ransomware, for example, exploited a DLL sideloading vulnerability in the Windows MSDTC service to execute its malicious payload undetected.
- Banking Trojans employ it to evade detection by endpoint security solutions. The NetWire and Remcos trojans have been distributed via phishing campaigns using sideloaded malicious DLLs inside ZIP archives.
How Attackers Exploit DLL Sideloading
DLL sideloading is effective because it abuses legitimate software components. Here are the key attack methods:
1. Search Order Hijacking
- Attackers place a malicious DLL in a directory that Windows searches before system directories.
- This ensures the malicious DLL is loaded before the legitimate one.
2. DLL Proxying
- The attacker creates a fake DLL that forwards legitimate function calls to the original DLL while injecting malicious code.
- This makes detection more difficult since the application appears to function normally.
3. Persistence via Scheduled Tasks or Registry Changes
- Malicious DLLs can be loaded automatically at system startup via scheduled tasks or registry modifications.
4. Privilege Escalation
- If a vulnerable application runs with elevated privileges, the sideloaded DLL inherits those permissions, potentially granting attackers full system control.
How to Detect DLL Sideloading Attacks
Detecting DLL sideloading is challenging because attackers abuse trusted applications. However, the following methods improve detection rates:
1. Behavioral Analysis
- Identify unusual DLL loads and process injections.
- Use tools like Process Monitor to detect abnormal loading sequences.
Pro Tip: Set up custom Sysmon rules to flag DLLs executing from non-standard locations or unexpected processes.
2. Sandbox Analysis
Pro Tip: Use a detonation chamber or cloud-based malware analysis sandbox to automate behavioral detection and threat correlation.
3. Threat Intelligence Integration
- Platforms like VMRay TotalInsight help correlate known sideloading techniques with threat actor activity.
Pro Tip: Regularly update YARA rules and leverage MITRE ATT&CK mappings to track emerging DLL sideloading techniques.
4. YARA Rules for Static Analysis
- Use custom YARA rules to flag unsigned DLLs or suspicious exports.
Pro Tip: Implement recursive scanning of DLL files with static and dynamic indicators to detect hidden malware.
5. Monitoring Signed Executables Loading Unsigned DLLs
- Legitimate, signed executables should rarely load unsigned DLLs.
- Detect anomalies where a trusted process loads an unexpected DLL.
Pro Tip: Use File Integrity Monitoring (FIM) to alert you when trusted applications and their dependencies are modified.
How to Mitigate DLL Sideloading Risks
While detection is critical, proactive mitigation strategies help reduce exposure to DLL sideloading attacks.
1. Restrict Write Permissions
- Limit user and application write access to directories where DLLs are loaded.
Pro Tip: Configure Group Policy Objects (GPOs) to prevent unauthorized modifications to system directories and application folders.
2. Implement Application Allowlisting
- Use Windows Defender Application Control (WDAC) to prevent unauthorized DLL execution.
Pro Tip: Regularly update allowlists and blocklists to ensure only trusted DLLs are executed in high-risk environments.
3. Secure Boot and Code Signing Enforcement
- Ensure that only digitally signed DLLs from trusted sources can be loaded.
Pro Tip: Enable Microsoft Defender SmartScreen to block execution of untrusted and unsigned executables, reducing the risk of malicious DLL sideloading.
4. Regular Threat Hunting and Automated Detection
- Conduct periodic scans using VMRay DeepResponse to detect anomalies in DLL execution.
Pro Tip: Automate periodic threat-hunting scripts that scan for newly dropped DLLs and compare them against known legitimate files.
Conclusion
DLL sideloading is a highly effective technique attackers use to evade detection and establish persistence. Security teams must adopt advanced detection mechanisms, including behavioral analysis and sandboxing, to stay ahead of evolving threats. By integrating threat intelligence and enforcing strict security policies, organizations can reduce the risk posed by DLL sideloading attacks.
