DLL Sideloading in Legitimate Apps: Hidden Dangers and How to Defend Against Them

🚨 Real-World Incident: DLL Sideloading Attack Hits Global Software Vendor

In early 2023, cybersecurity researchers uncovered a significant attack targeting a leading global software provider. The threat actors exploited a popular, trusted application by planting a malicious Dynamic Link Library (DLL) alongside the app's executable. When unsuspecting users ran the application, the legitimate software unknowingly loaded the trojanized DLL, granting attackers persistent access, data exfiltration capabilities, and network pivoting opportunities across multiple organizations worldwide.

This incident exposed just how insidious DLL sideloading can be, especially when attackers latch onto trusted, widely-deployed applications. The stealth and sophistication of the campaign shocked the industry and forced enterprises to rethink software trust models.

🕵️‍♂️ What is DLL Sideloading?

DLL sideloading is an attack technique where cybercriminals drop a malicious DLL file in a directory alongside a legitimate Windows application. Windows' default DLL search order causes the application to load the attacker's DLL instead of the intended one, providing a backdoor or launching malware under the disguise of a trusted process.

• 🐍 Many Windows executables blindly trust DLLs in their local folder, making them lucrative targets for sideloading.
• 🔍 Attackers reverse engineer legitimate apps to understand which DLLs they load and replicate those DLLs with malicious payloads.
• 🦠 Sideloaded DLLs can steal credentials, deploy ransomware, or evade EDR solutions since they operate within signed processes.

🔗 Deep Dive: Attack Flow of DLL Sideloading

The sequence of events in a typical DLL sideloading attack is highly effective, often bypassing conventional security controls. Here's how these attacks play out:

• 🧠 Step 1: Reconnaissance – Attackers analyze a popular software to identify which DLLs it loads at runtime.
• 📦 Step 2: Crafting the Payload – They develop a malicious DLL with the same filename as a legitimate one the app expects.
• 💾 Step 3: Placement – The legitimate app and malicious DLL are dropped into the same directory, typically via phishing or supply chain compromise.
• 🚦 Step 4: Execution – When the victim runs the app, it loads the attacker's DLL, executing malicious code under a trusted process' context.
• 🕳️ Step 5: Persistence and Exploitation – Attackers maintain access, escalate privileges, or deliver additional malware.

🎯 Technical Explainer: Why Does DLL Sideloading Happen?

DLL sideloading exploits fundamental design in how Windows resolves dependencies. Typically, Windows applications look for required DLLs in a specific order:

1. The application's directory
2. System directories (e.g., C:\Windows\System32)
3. PATH environment variables

If a malicious DLL is placed in the application's folder with the expected name, it is loaded first, before the legitimate version in the system directories. This order—meant for developer convenience—can be a security risk if apps do not enforce secure loading practices like specifying full DLL paths or using signed, strong-named DLLs only.

• 🐞 Vulnerable apps include misconfigured enterprise software, unmaintained tools, or any application missing security updates.
• 🔄 Attackers frequently bundle the legitimate executable to increase trust and evade detection.

📊 Industry Stats & Trends: The DLL Sideloading Surge

The rise in advanced persistent threats (APTs) using DLL sideloading is concerning. According to recent industry reports:

• 📈 Over 30% of new malware campaigns leveraged DLL sideloading in 2023, up from 18% in 2020 (CrowdStrike Threat Report).
• 🏢 Major APT groups like APT41, FIN7, and Lazarus have used DLL sideloading to compromise banks, governments, and tech giants.
• 🔬 Red-teaming frameworks and open-source utilities now include automated DLL sideloading modules, lowering the bar for less skilled attackers.
• 🛡️ Microsoft, in its Digital Defense Report, highlights DLL sideloading as one of the top three vectors for ransomware initial access.

Clearly, the trend shows organizations cannot ignore this threat, as attackers increasingly pivot to sideloading to bypass EDRs and traditional AV solutions.

👨‍💻 Case Study Deep Dive: SolarMarker Malware Campaign

The "SolarMarker" malware family exploded in 2022-2023, utilizing sophisticated DLL sideloading techniques. Attackers distributed legitimate installers (like Adobe Reader or Microsoft Teams), but snuck a malicious DLL with each bundle.

• 🧬 SolarMarker's DLLs harvested passwords, browser data, and captured keystrokes.
• 🕵️‍♂️ The attackers blended their malicious DLLs with legitimate apps, making detection extremely difficult.
• 🌍 Victims spanned law firms, schools, healthcare providers, and more, all tricked into running trusted apps that secretly loaded malware.

SolarMarker demonstrated how crafty adversaries weaponize DLL sideloading to achieve massive scale, persistence, and stealth.

💡 Why Are Applications at Risk?

Several common coding and deployment practices inadvertently facilitate DLL sideloading:

• 🧑‍💼 Developers often use weak or default DLL load paths, letting apps accept any DLL in the working directory.
• 🚪 Publishers may forget to digitally sign all dependencies or fail to validate signature chains.
• 🛠️ Lack of regular security audits of third-party and in-house software.
• 🔗 Overreliance on user privileges without strict access controls.

All these create a fertile ground for attackers to insert themselves into the supply chain.

🌀 Attacker Techniques & Tricks

• 🚚 Portable Executables: Attackers drop both the vulnerable app and malicious DLL for maximum "legitimacy".
• 🅰️ Masquerading: Sideloaded DLLs are given near-identical names to trusted originals (e.g., mscoree.dll vs. mscore123.dll).
• 💉 Process Injection: Gaining code execution from the sideloaded DLL enables further in-memory attacks.
• 👁️ EDR/AV Bypass: Because malware runs inside a trusted process, detection is challenging.

🛡️ How To Prevent DLL Sideloading in Applications

Securing against DLL sideloading requires a mix of technical countermeasures, organizational policies, and robust monitoring. Here are proven strategies:

• 🧑‍💻 Developers:
  • 🔏 Always use fully qualified paths when loading DLLs.
  • 📜 Whitelist allowed DLL dependencies in the app config.
  • 🔑 Digitally sign all executables and DLLs; validate signatures at load time.
  • 📅 Perform regular dependency checks and vulnerability management.
• 🏢 Organizations:
  • 🛡️ Deploy EDR/XDR with DLL monitoring/alerts on all endpoints.
  • 🚦 Enforce least privilege and application whitelisting.
  • 🔎 Conduct security assessments on all third-party software before deployment.
  • 🔧 Regularly update and patch software vulnerabilities.
• 🙋 End Users:
  • 🎣 Avoid downloading executables from untrusted sites – always use official vendor sources.
  • ⚠️ Beware of unexpected file attachments and software bundles.
  • 🔒 Enable OS-level protections like Windows Defender SmartScreen.

🔗 Codesecure's Proactive Defense Solutions

At Codesecure, we specialize in cutting-edge application security assessments and threat hunting to uncover hidden risks like DLL sideloading at every layer of your tech stack. Our approach includes:

• 🛡️ In-depth code reviews for sideloading vulnerabilities.
• 🔬 Red teaming to simulate real-world DLL sideloading attacks.
• 🔍 Continuous monitoring and bespoke incident response playbooks.
• 👥 Security training for developers and IT teams on preventing sideloading techniques.
• 🏢 Enterprise risk assessments including all software and third-party supply chains.

Protect your organization from stealthy DLL sideloading attacks—before they happen.

📣 Secure Your Apps: Contact Codesecure Today!

• 📞 Call us: +91 7358463582
• 📧 Email: osint@codesecure.in
• 🌐 Visit: www.codesecure.in

Don't let your trusted apps become unwitting accomplices in cybercrime. The threat of DLL sideloading will only keep growing—talk to Codesecure's experts for a comprehensive security assessment today!

🔒 Final Thoughts: Facing the DLL Sideloading Storm

The threat landscape is evolving, and techniques like DLL sideloading are more than just clever tricks—they’re a top priority for attackers seeking stealth, persistence, and access to critical assets.

By understanding the root causes, keeping software up to date, and deploying advanced monitoring, you can protect your business from one of the most insidious forms of software supply chain attack.

Codesecure stands ready to help—contact us for a safer, more resilient future.