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 Windows follows a specific search order when loading DLLs. It looks at directories that include the application’s folder, system directories, and environment paths. Attackers exploit this by placing a malicious DLL file in a directory that is searched before the legitimate one.
Since Windows loads the first matching DLL it finds, the attacker’s file is executed instead of the intended DLL. This method works particularly well when an application does not specify an absolute path for its required DLLs.
Attackers often target outdated or misconfigured applications that automatically load external libraries.
2. DLL Proxying In this technique, an attacker places a rogue DLL with the same name as a legitimate DLL in a location that gets loaded first. The malicious DLL acts as a “proxy” by forwarding legitimate function calls to the real DLL, while also injecting malicious code.
Since the application still functions as expected, users may not notice any abnormal behavior. The attacker can use DLL proxying to inject spyware, steal credentials, or escalate privileges without triggering security alerts.
3. Persistence via Scheduled Tasks or Registry Changes Once a malicious DLL is successfully loaded, attackers use various persistence techniques to ensure their payload is executed repeatedly. This can include:
Modifying registry keys to reference the malicious DLL so that it loads automatically at startup. Creating scheduled tasks that execute a trusted application that, in turn, loads the attacker’s DLL. Hijacking Windows services that automatically load DLLs from specified directories.
Embedding malicious DLLs in places where they are continuously reloaded allows attackers to maintain access and evade detection for extended periods.
4. Privilege Escalation If an application vulnerable to DLL hijacking attacks runs with administrator privileges, any sideloaded DLL will inherit the same permissions. This enables attackers to:
Gain system-level access with full control over the compromised machine. Disable security tools, such as antivirus programs, firewalls, or endpoint detection systems. Execute additional payloads, such as installing rootkits, keyloggers, or ransomware.
Privilege escalation via DLL sideloading is particularly dangerous in enterprise environments, as attackers can move laterally within a network and compromise critical infrastructure.
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.
