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.
YARA Rule
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.