![]() ![]() Well one thing that works in an attacker’s favour is that the virtual memory space of processes is huge being 2 GB in 32 bits and 128 TB in 64 bits. Another option would be to obfuscate the code in memory, either by adding/modifying instructions or dynamically encrypting/decrypting our payload in memory when a scan a detected. In the next few sections we’ll discuss potential bypass techniques in more detail.Īfter confirming Windows Defender memory scanning was being triggered by specific APIs, the next question was how can we bypass it? One simple approach would be to avoid the APIs that trigger Windows Defender’s runtime scanner but that would mean manually rewriting Metasploit payloads which is far too much effort. Suspicious Behaviour – AV will often monitor for suspicious behaviour (usually API calls) and use this to trigger a scan, again this could be of local files or process memory. ![]() This concept also applies to scanning the memory of running processes. Periodic – AV will periodically scan systems, daily or weekly scans are common and this can involve all or just a subset of the files on the system. It’s also worth mentioning how scans can be triggered:įile Read/Write – Whenever a new file is created or modified this can potentially trigger the AV and cause it to initiate a scan of the file. This can be more challenging for attackers as it can be harder to obfuscate code in memory as its executing and off the shelf payloads are easily detected. Process Memory/Runtime Analysis – Similar to the static analysis except running process memory is analysed instead of files on disk. A newer variation of this technique is machine learning based file classification which essentially compares static features against known good and bad profiles to detect anomalous files. While this is effective against known malware, static signatures are often easy to bypass meaning new malware is missed. ![]() Static Analysis – Involves scanning the contents of a file on disk and will primarily rely on a set of known bad signatures. Before diving into Windows Defender we wanted to quickly introduce the main analysis methods used by most modern AV engines: ![]()
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