The detection rule identifies potential malicious activity involving the msvcrt library on Windows XP x86 systems, which may indicate the use of legacy or obfuscated malware. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify and mitigate threats that leverage outdated or less commonly monitored environments.
YARA Rule
rule msvcrt_WinXPx86 {
meta:
author = "Jaume Martin"
condition:
hash.md5(0, filesize) == "b68f72d77754f8b76168ced0924a4174"
}
This YARA rule can be deployed in the following contexts:
Scenario: Windows Update Installer (wusa.exe)
Description: The Windows Update Installer may use msvcrt.dll during installation on Windows XP systems.
Filter/Exclusion: Check the process name (ImageFileName == "wusa.exe") or use a file hash of the legitimate Windows Update installer.
Scenario: Microsoft .NET Framework Installation
Description: Installing or repairing the .NET Framework on a Windows XP machine may trigger the rule due to the use of msvcrt.dll.
Filter/Exclusion: Filter by process name (ImageFileName == "msiexec.exe") or check the command line arguments for .NET installation.
Scenario: Scheduled Task Running a Legacy Application
Description: A scheduled task that runs a legacy application (e.g., a VB6 or old C++ app) may load msvcrt.dll legitimately.
Filter/Exclusion: Use a process name filter (e.g., ImageFileName == "notepad.exe" or the specific application name) or check the task scheduler context.
Scenario: System File Checker (sfc.exe) Scanning msvcrt.dll
Description: The System File Checker utility may scan or repair msvcrt.dll on a Windows XP system.
Filter/Exclusion: Filter by process name (ImageFileName == "sfc.exe") or check the command line for scan or repair operations.
Scenario: Microsoft Office Application (e.g., Excel, Word) on Windows XP
Description: Legacy versions of Microsoft Office (e.g., Office 2003) may use msvcrt.dll and trigger the rule.
Filter/Exclusion: Filter by process name (ImageFileName == "EXCEL.EXE" or `ImageFileName == “WINWORD.EXE