How Gremlin Stealer Refines Its Attack: Obfuscation, Crypto Clipping, and File-Based Stealth

Introduction

Cybersecurity researchers at Unit 42 have uncovered a sophisticated evolution of the Gremlin stealer, a malware strain that has adapted new techniques to compromise sensitive data. This variant employs advanced obfuscation, crypto clipping, and session hijacking while cleverly hiding malicious components inside resource files. This article explores how Gremlin stealer has evolved and what it means for both defenders and potential victims.

The Rise of Gremlin Stealer: From Simple Theft to Sophisticated Espionage

Originally, the Gremlin stealer was a relatively straightforward credential theft tool. Over time, its authors added layers of complexity to evade detection and maximize damage. The latest version represents a significant leap—incorporating techniques typically seen in high‑end banking trojans and advanced persistent threats.

Origins and Early Capabilities

Early variants of Gremlin focused on harvesting passwords, cookies, and browser autofill data from infected systems. It spread through phishing campaigns and exploit kits, often leaving only light forensic traces. However, security tools began flagging its static signatures, driving the need for evolution.

Key Evolved Tactics

The modern Gremlin stealer now integrates three core advanced methods that make it more dangerous and harder to analyse.

Advanced Obfuscation

Gremlin’s code is heavily obfuscated at multiple levels. It uses string encryption, API call hiding, and control‑flow flattening to frustrate reverse engineering. The malware also employs dynamic code generation—only decrypting critical functions when they are needed in memory, leaving the core payload indistinguishable from benign data. This makes static analysis nearly impossible.

Crypto Clipping

One of the stealer’s most profitable features is crypto clipping. It monitors the clipboard for cryptocurrency wallet addresses and automatically replaces them with addresses controlled by the attackers. When victims paste and send funds, they unknowingly transfer cryptocurrency to the adversary. This technique works across multiple coin types including Bitcoin, Ethereum, and Monero.

Session Hijacking

Gremlin can also hijack active online sessions by stealing authentication cookies and tokens. It extracts these from browsers, email clients, and VPN applications. Attackers then import them into their own environment, gaining immediate access to accounts without needing passwords. This allows for prolonged, stealthy compromise.

Hiding in Plain Sight: The Role of Resource Files

A particularly clever aspect of Gremlin’s evolution is its use of PE resource files to conceal malicious payloads and configuration data.

What Are Resource Files?

Windows Portable Executable (PE) files include a section called .rsrc (resource) that typically stores icons, version info, and non‑executable data. Most security scanners treat resource data as inert, making it an ideal hiding place. Gremlin packs its encrypted payload and other artifacts inside this section.

How Gremlin Encodes Payloads

The malware embeds its core modules as custom resource entries with innocent‑sounding names (e.g., “BIN” or “DAT”). During execution, the main dropper reads the resource data, decrypts it using a hard‑coded key or algorithm, and loads it directly into memory. This fileless execution technique bypasses many on‑disk signature checks and leaves fewer artefacts for investigators.

Evasion from Security Tools

Because resource files are often excluded from behavioural analysis in sandbox environments (due to their perceived benign nature), Gremlin can operate undetected for extended periods. Anti‑malware engines that rely solely on scanning executables for suspicious imports or strings will miss the hidden code. Only runtime memory inspection can reveal the stealer in action.

Impact and Targets

Gremlin stealer’s evolved tactics have been observed predominantly in attacks on financial institutions, cryptocurrency platforms, and enterprise credential stores. The combination of crypto clipping and session hijacking can lead to direct financial loss, while the stolen session tokens enable lateral movement within corporate networks. Unit 42 researchers warn that the use of resource files makes Gremlin a template for future malware families aiming to hide in plain sight.

Mitigation and Defensive Measures

Organisations can defend against Gremlin by:

• Implementing behavioural detection tools that monitor process memory and API calls rather than static signatures.
• Enabling application control policies that block execution of unsigned binaries or those that modify resource sections.
• Training users to recognise phishing lures that often deliver the initial dropper.
• Using runtime file analysis tools that inspect PE resource entries for abnormalities, such as encrypted or unusually large data blocks.

Additionally, regular patching of browsers and operating system components reduces the attack surface for session hijacking vulnerabilities.

Conclusion

The evolution of Gremlin stealer demonstrates how persistent malware authors adapt to security advances. By leveraging advanced obfuscation, crypto clipping, session hijacking, and resource‑file hiding, this stealer has become a formidable threat. As Unit 42’s analysis shows, staying ahead requires a shift from static checks to dynamic, memory‑aware defenses. The lessons from Gremlin will likely influence both next‑generation malware and the security tools designed to stop them.