You’ve crafted the perfect exploit, but it gets caught by antivirus the moment it runs. This is where you need to learn how to evade basic AV with payload obfuscation. Modern defensive tools use more than just simple signatures, but many can still be bypassed by transforming your payload’s code to make it unrecognizable. This tutorial will introduce the core concepts of why payloads get detected and how simple encoding can help you slip past static analysis.
Table of Contents
🧐 Why Do Payloads Get Detected by Antivirus?
Antivirus (AV) and Endpoint Detection and Response (EDR) solutions use several methods to catch malware. The most basic is signature-based detection. A security vendor analyzes a piece of malware (like a Metasploit shellcode) and creates a unique signature—a specific sequence of bytes—for it. The AV then scans all your files for this signature. If it finds a match, it flags the file as malicious. This is why running a well-known, unmodified payload almost always gets caught immediately.
🎭 How to Use Simple Encoding to Hide Your Payload
Since static AV is looking for a fixed sequence of bytes, the easiest way to bypass it is to change that sequence. This is where encoding comes in. By applying a simple algorithm like an XOR cipher to your payload, you completely transform its byte pattern. An XOR encoder takes your original shellcode and a secret key, and XORs each byte of the shellcode with the key. The resulting encoded payload will look like random garbage to a signature-based scanner and will not be detected.
🧠 How to Create a Decoder Stub for In-Memory Execution
Of course, the encoded payload can’t run by itself. It needs a small piece of code to decode it back to its original form right before execution. This is called a decoder stub. Your final executable will consist of this small decoder and the encoded payload. When you run the program, the decoder stub executes first. It will loop through the encoded payload in memory, apply the same XOR key to reverse the encoding, and then jump to the now-decoded shellcode to execute it. Because the original malicious signature never touches the disk and only appears in memory for a moment, this technique is highly effective at bypassing basic, static AV scans.
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Johnson, Richard. Kali Linux Essentials. NOBTREX LLC, 2025.
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