Exchange email-hiding transport rule

Synopsis

Field	Value
Activation Period	14 Days
Training Period	30 Days
Test Period	N/A (single event)
Deduplication Period	1 Day
Required Data	Office 365 Audit
Detection Modules	Identity Threat Module, SaaS Threat Detection, Email
ATT&CK Tactic	Stealth (TA0005)
ATT&CK Technique	Hide Artifacts: Email Hiding Rules (T1564.008)
Severity	Informational

Description

A user configured an Exchange transport rule that may be used to hide emails in the organization.

Attacker's Goals

Prevent an organization from warning users that they've been compromised (e.g. an internal spear-phishing campaign).

Investigative actions

• Look for signs that the user account and mailbox are compromised (e.g. abnormal logins, unusual activity).
• Check if the rule contains keywords and if they look suspicious.
• Investigate the IP address associated with the rule.
• Follow further actions done by the account.
• Check for a possible phishing campaign on the organization.* Look for multiple instances of email hiding, which may be an indication of a larger campaign.* Check if the user regularly configures transport rules.

Variations

Suspicious Exchange email-hiding transport rule

Synopsis

Field	Value
ATT&CK Tactic	Stealth (TA0005)
ATT&CK Technique	Hide Artifacts: Email Hiding Rules (T1564.008)
Severity	Medium

Description

A user configured an Exchange transport rule that may be used to hide emails in the organization. The rule hides emails that contain suspicious keywords, which may be a sign of a compromised account.

Attacker's Goals

Prevent an organization from warning users that they've been compromised (e.g. an internal spear-phishing campaign).

Investigative actions

• Look for signs that the user account and mailbox are compromised (e.g. abnormal logins, unusual activity).
• Check if the rule contains keywords and if they look suspicious.
• Investigate the IP address associated with the rule.
• Follow further actions done by the account.
• Check for a possible phishing campaign on the organization.* Look for multiple instances of email hiding, which may be an indication of a larger campaign.* Check if the user regularly configures transport rules.

Exchange email-hiding transport rule based on message keywords

Synopsis

Field	Value
ATT&CK Tactic	Stealth (TA0005)
ATT&CK Technique	Hide Artifacts: Email Hiding Rules (T1564.008)
Severity	Low

Description

A user configured an Exchange transport rule that may be used to hide emails in the organization. The rule hides emails that contain certain keywords, which may be a sign of a compromised account.

Attacker's Goals

Prevent an organization from warning users that they've been compromised (e.g. an internal spear-phishing campaign).

Investigative actions

• Look for signs that the user account and mailbox are compromised (e.g. abnormal logins, unusual activity).
• Check if the rule contains keywords and if they look suspicious.
• Investigate the IP address associated with the rule.
• Follow further actions done by the account.
• Check for a possible phishing campaign on the organization.* Look for multiple instances of email hiding, which may be an indication of a larger campaign.* Check if the user regularly configures transport rules.