Suspicious File Created in Outlook Temporary Directory

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

Detects the creation of files with suspicious file extensions in the temporary directory that Outlook uses when opening attachments. This can be used to detect spear-phishing campaigns that use suspicious files as attachments, which may contain malicious code.

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1566.001` Phishing: Spearphishing Attachment

Event coverage

Provider	Event ID	Title
Sysmon	11	FileCreate

Stages and Predicates

Stage 1: `all of selection_extension`

or:
TargetFilename|endswith: .cpl
TargetFilename|endswith: .hta
TargetFilename|endswith: .iso
TargetFilename|endswith: .rdp
TargetFilename|endswith: .svg
TargetFilename|endswith: .vba
TargetFilename|endswith: .vbe
TargetFilename|endswith: .vbs

Stage 2: `all of selection_location`

or:
TargetFilename|contains: '\AppData\Local\Microsoft\Windows\'
TargetFilename|contains: '\Content.Outlook\'
TargetFilename|contains: '\AppData\Local\Microsoft\Olk\Attachments\'
TargetFilename|contains: '\AppData\Local\Packages\Microsoft.Outlook_'

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
`TargetFilename`	ends_with	`.cpl` corpus 2 (sigma 2) `.hta` corpus 12 (sigma 12) `.iso` corpus 5 (sigma 5) `.rdp` corpus 2 (sigma 2) `.svg` corpus 2 (sigma 2) `.vba` corpus 3 (sigma 3) `.vbe` corpus 13 (sigma 13) `.vbs` corpus 16 (sigma 16)
`TargetFilename`	match	`\AppData\Local\Microsoft\Olk\Attachments\` `\AppData\Local\Microsoft\Windows\` `\AppData\Local\Packages\Microsoft.Outlook_` `\Content.Outlook\`