Suspicious ASPX File Drop by Exchange

Severity: high
Author: Florian Roth (Nextron Systems), MSTI (query, idea)
Source: upstream

Detects suspicious file type dropped by an Exchange component in IIS into a suspicious folder

MITRE ATT&CK coverage

Tactic	Techniques
Persistence	`T1505.003` Server Software Component: Web Shell

Event coverage

Provider	Event ID	Title
Sysmon	11	FileCreate

Stages and Predicates

Stage 1: `all of selection`

or:
TargetFilename|contains: 'FrontEnd\HttpProxy\'
TargetFilename|contains: '\inetpub\wwwroot\aspnet_client\'
CommandLine|contains: MSExchange
Image|endswith: '\w3wp.exe'

Stage 2: `all of selection_types`

or:
TargetFilename|endswith: .ashx
TargetFilename|endswith: .asp
TargetFilename|endswith: .aspx

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.

Field	Kind	Values
`CommandLine`	match	`MSExchange` corpus 3 (sigma 3)
`Image`	ends_with	`\w3wp.exe` corpus 6 (sigma 6)
`TargetFilename`	ends_with	`.ashx` corpus 3 (sigma 3) `.asp` corpus 4 (sigma 4) `.aspx` corpus 5 (sigma 5)
`TargetFilename`	match	`FrontEnd\HttpProxy\` `\inetpub\wwwroot\aspnet_client\`

Neighbors

Broader alternatives (more inclusive than this rule)

These rules match a superset of what this rule catches. They cover the same events plus more. Use them if you want wider coverage and can absorb more false positives.

Suspicious File Drop by Exchange [sigma] (drops 1 filter this rule applies) (first-stage match only; downstream stages may differ)