AMSI Bypass: Patching Technique

( Original text ) Abstract In this blog post, we introduce a technique that can help attackers run malicious code over Microsoft Windows 10 (Version 1607) using PowerShell (version 5). CyberArk alerted Microsoft to the weakness, and while Microsoft issued a patch in version 1709, organizations that haven’t implemented the fix remain at risk. The technique can be […]

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AmsiScanBuffer Bypass — Part 3

( Original text by @_RastaMouse ) In Part 2, we engineered a delivery method for the AmsiScanBuffer Bypass discussed in Part 1. In this post, we’ll make some modifications to the bypass itself. If you read Part 1 and the original posts from CyberArk, you will know that the bypass works by patching the AMSI DLL in memory. But before […]

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AmsiScanBuffer Bypass — Part 2

( Original text by @_RastaMouse ) In Part 1, we had a brief look at the AmsiScanBuffer bypass technique. We found some circumstances where the bypass code would be identified as malicious before it could be executed (which turned out to be a simple string detection), and modified the code to circumvent this. In this post, we’ll […]

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AmsiScanBuffer Bypass — Part 1

( Original text by @_RastaMouse ) Andre Marques recently posted a pretty nice write-up for circumventing AMSI, based on previous work by CyberArk. Please read these for all the technical details — we’re launching this post with the C# code from Andre: using System; using System.Runtime.InteropServices; namespace Bypass { public class AMSI { [DllImport(«kernel32»)] public static extern IntPtr GetProcAddress(IntPtr hModule, string procName); […]

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