The hypothesis is that the detection of the msvcrt_Win7AMD64 YARA rule may indicate the presence of malicious code leveraging Windows 7-specific runtime libraries, potentially used for code obfuscation or execution in a restricted environment. SOC teams should proactively hunt for this behavior in Azure Sentinel to identify potential advanced persistent threats or malware that may be evading traditional detection methods.
rule msvcrt_Win7AMD64 {
meta:
author = "Jaume Martin"
condition:
hash.md5(0, filesize) == "c8fc794cc5a22b5a1e0803b0b8acce77"
}
This YARA rule can be deployed in the following contexts:
Scenario: Windows Update Installation
Description: A legitimate Windows Update installation may include files from the msvcrt library, which can trigger the rule.
Filter/Exclusion: Check the file path against known Windows Update directories, e.g., C:\Windows\Temp\ or C:\Windows\SoftwareDistribution\.
Scenario: Microsoft Visual C++ Redistributable Installation
Description: Installing Microsoft Visual C++ Redistributable packages (e.g., vcredist.exe) may include msvcrt components.
Filter/Exclusion: Exclude files signed by Microsoft with the publisher “Microsoft Corporation” or check the file hash against known good hashes for the redistributable.
Scenario: Scheduled Task Running a Legacy Application
Description: A scheduled task running a legacy application (e.g., notepad.exe, cmd.exe, or older .NET applications) may load msvcrt DLLs.
Filter/Exclusion: Filter based on the process name or use the process.parent field to identify tasks initiated by the Task Scheduler.
Scenario: System File Check (sfc /scannow)
Description: The System File Checker utility may temporarily load or replace msvcrt DLLs during system integrity checks.
Filter/Exclusion: Exclude events where the process is svchost.exe or the command line includes sfc /scannow.
Scenario: Admin Task Using PowerShell to Manage System Files
Description: An administrator may use PowerShell to copy or replace system files, including msvcrt components, during maintenance.
Filter/Exclusion: Exclude processes with powershell.exe and a command line containing Copy-Item or Move-Item with known system paths.