This hypothesis targets the execution of crypter binaries that utilize the SLESH obfuscation technique, a method often employed by threat actors to hide malicious payloads from static analysis tools. Proactively hunting for this specific crypter signature in Azure Sentinel allows the SOC to identify low-severity, stealthy initial access or second-stage implants that may evade standard behavioral detections.
rule Crypter31SLESH
{
meta:
author="malware-lu"
strings:
$a0 = { 68 FF 64 24 F0 68 58 58 58 58 FF D4 50 8B 40 F2 05 B0 95 F6 95 0F 85 01 81 BB FF 68 }
condition:
$a0
}This YARA rule can be deployed in the following contexts:
This rule contains 1 string patterns in its detection logic.
Scenario: A developer or DevOps engineer uses a custom Python or C# script to obfuscate a small utility or configuration loader before deploying it to a staging environment. The script applies a simple XOR or RC4 encryption layer (often referred to as “crypting”) to the binary, which matches the Crypter31SLESH signature.
python.exe, dotnet.exe, or node.exe and the file path contains directories like \scripts\, \build\, or \staging\. Additionally, exclude if the file extension is .py, .cs, or .js and the size is under 50KB.Scenario: An IT administrator runs a legitimate, signed third-party installer (e.g., from a vendor like Adobe, Oracle, or VMware) that includes a self-extracting archive or a crypter-based payload to handle license verification or component staging. The crypter is a standard part of the vendor’s distribution mechanism.
Adobe Inc., VMware, Inc., Oracle Corporation) and the signer matches the expected vendor for the application. Also, exclude if the process name matches known installer binaries like setup.exe, install.exe, or uninstall.exe from trusted vendor directories.Scenario: A security team or red team member executes a custom-built tool or a modified version of a common utility (like mimikatz or psexec) that has been lightly obfuscated to bypass basic AV during a controlled test or migration task. This is a legitimate, authorized activity in a test or staging zone.