Suspicious File Write to SharePoint Layouts Directory

Severity: high
Author: Swachchhanda Shrawan Poudel (Nextron Systems)
Source: upstream

Detects suspicious file writes to SharePoint layouts directory which could indicate webshell activity or post-exploitation. This behavior has been observed in the exploitation of SharePoint vulnerabilities such as CVE-2025-49704, CVE-2025-49706 or CVE-2025-53770.

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1190` Exploit Public-Facing Application
Persistence	`T1505.003` Server Software Component: Web Shell

Event coverage

Provider	Event ID	Title
Sysmon	11	FileCreate

Stages and Predicates

Stage 1: `selection`

or:
Image|endswith: '\cmd.exe'
Image|endswith: '\powershell.exe'
Image|endswith: '\powershell_ise.exe'
Image|endswith: '\pwsh.exe'
Image|endswith: '\w3wp.exe'
or:
TargetFilename|endswith: .asax
TargetFilename|endswith: .ascx
TargetFilename|endswith: .ashx
TargetFilename|endswith: .asmx
TargetFilename|endswith: .asp
TargetFilename|endswith: .aspx
TargetFilename|endswith: .bat
TargetFilename|endswith: .cer
TargetFilename|endswith: .cmd
TargetFilename|endswith: .config
TargetFilename|endswith: .hta
TargetFilename|endswith: .js
TargetFilename|endswith: .jsp
TargetFilename|endswith: .jspx
TargetFilename|endswith: .php
TargetFilename|endswith: .ps1
TargetFilename|endswith: .vbs
or:
TargetFilename|contains: '\15\TEMPLATE\LAYOUTS\'
TargetFilename|contains: '\16\TEMPLATE\LAYOUTS\'
or:
TargetFilename|startswith: 'C:\Program Files (x86)\Common Files\Microsoft Shared\Web Server Extensions\'
TargetFilename|startswith: 'C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\'

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.