Abnormal Security Account Takeover Detection

Overview

Abnormal Security's Account Takeover Protection monitors sign-in activity and mailbox behavior to detect compromised internal accounts. By analyzing user behavior patterns, device fingerprints, sign-in locations, and mailbox rule changes, Abnormal identifies accounts that have been taken over by attackers. This skill covers ATO case management, investigation workflows, and remediation actions.

Account Takeover Indicators

Indicator	Description	Risk Level
Impossible Travel	Sign-ins from geographically distant locations in short time	High
Unusual Sign-in Location	Sign-in from a country or region not seen before	Medium
New Device	Sign-in from an unrecognized device or browser	Medium
Suspicious Mailbox Rules	Auto-forward, delete, or move rules targeting sensitive emails	Critical
Bulk Email Sending	Account sending mass emails to internal or external recipients	High
Password Change	Unexpected password or MFA changes	High
Lateral Phishing	Compromised account sending phishing to internal users	Critical
Data Exfiltration	Large file downloads or email forwarding to external addresses	Critical
Token Theft	Session token stolen and used from different location/device	High

ATO Case Field Reference

Core Fields

Field	Type	Description
caseId	string	Unique account takeover case ID
affectedUser	string	Email address of the compromised account
severity	string	Critical, High, Medium, Low
detectedAt	datetime	When the takeover was first detected
status	string	Open, Investigating, Remediated, Closed

Sign-in Activity Fields

Field	Type	Description
signInEvents	object[]	List of suspicious sign-in events
signInEvents[].timestamp	datetime	When the sign-in occurred
signInEvents[].ipAddress	string	IP address of the sign-in
signInEvents[].location	string	Geographic location (city, country)
signInEvents[].device	string	Device or browser fingerprint
signInEvents[].status	string	Success, Failed, MFA Challenged
signInEvents[].riskLevel	string	Risk assessment of the sign-in

Mailbox Activity Fields

Field	Type	Description
mailboxRules	object[]	Suspicious mailbox rules created
mailboxRules[].type	string	Forward, Delete, Move
mailboxRules[].target	string	External email or folder target
mailboxRules[].createdAt	datetime	When the rule was created
emailsSent	int	Number of emails sent during compromise
emailsForwarded	int	Number of emails auto-forwarded

MCP Tools

Tool	Description	Key Parameters
abnormal_ato_cases_list	List account takeover cases	pageSize, pageNumber, filter, fromDate, toDate
abnormal_ato_cases_get	Get ATO case details	caseId
abnormal_ato_activity	Get sign-in and mailbox activity for a user	email, fromDate, toDate
abnormal_ato_remediate	Take remediation action on an ATO case	caseId, action

Tool Usage Examples

List open ATO cases:

{
  "tool": "abnormal_ato_cases_list",
  "parameters": {
    "filter": "status eq 'Open'",
    "pageSize": 25
  }
}

Get ATO case details:

{
  "tool": "abnormal_ato_cases_get",
  "parameters": {
    "caseId": "ato-abc123"
  }
}

Get sign-in activity for a user:

{
  "tool": "abnormal_ato_activity",
  "parameters": {
    "email": "[email protected]",
    "fromDate": "2026-03-20T00:00:00Z",
    "toDate": "2026-03-27T00:00:00Z"
  }
}

ATO Investigation Workflows

Standard ATO Investigation

1. Review case details:
   • What indicators triggered the detection?
   • Which user is affected?
   • When was the compromise detected?
2. Analyze sign-in activity:
   • Check for impossible travel (locations too far apart in timeframe)
   • Identify the attacker's IP address and location
   • Check if MFA was bypassed or not enrolled
3. Review mailbox changes:
   • Check for auto-forwarding rules to external addresses
   • Check for delete rules targeting security notifications
   • Check for inbox rules hiding incoming emails
4. Assess damage:
   • How many emails were sent from the compromised account?
   • Were any internal users phished (lateral movement)?
   • Was sensitive data forwarded or downloaded?
5. Remediate:
   • Force password reset
   • Revoke active sessions and tokens
   • Remove malicious mailbox rules
   • Re-enable MFA if it was disabled
6. Post-incident:
   • Notify affected users who received emails from the compromised account
   • Check if credentials were used elsewhere
   • Update security policies if gaps found

Impossible Travel Investigation

1. Map sign-in locations - Plot the geographic locations and timestamps
2. Calculate travel feasibility - Could the user physically travel between locations?
3. Check VPN usage - Verify if the organization uses VPNs that could explain the location change
4. Identify legitimate sign-in - Contact the user to confirm which sign-in was theirs
5. Block attacker access - Revoke the session from the illegitimate location

Mailbox Rule Investigation

1. List all mailbox rules - Get current rules for the affected account
2. Identify suspicious rules:
   • Forwarding to external addresses
   • Deleting emails from security team or IT
   • Moving emails to hidden folders
3. Remove malicious rules - Delete any rules created during the compromise window
4. Check forwarded data - Determine what emails were forwarded to the attacker

Remediation Actions

Action	Description	When to Use
FORCE_PASSWORD_RESET	Reset the user's password	Confirmed account compromise
REVOKE_SESSIONS	Terminate all active sessions	Active compromise with attacker logged in
DISABLE_ACCOUNT	Temporarily disable the account	Severe compromise with ongoing activity
REMOVE_RULES	Remove suspicious mailbox rules	Malicious forwarding/delete rules found

Severity Assessment

Factor	Medium	High	Critical
Indicator Type	New device, unusual location	Impossible travel, password change	Lateral phishing, data exfiltration
User Role	Standard user	Manager, finance	Executive, admin
Activity	Suspicious sign-in only	Mailbox rules created	Emails sent to internal users
MFA Status	MFA active, blocked	MFA bypassed	MFA disabled by attacker
Data Access	No sensitive access	Read sensitive emails	Forwarded sensitive data

Error Handling

Common API Errors

Code	Message	Resolution
400	Invalid email format	Check email address format
401	Unauthorized	Check API token
403	Insufficient permissions	Token needs ATO scope
404	Case not found	Verify case ID
429	Rate limited	Wait and retry

Best Practices

1. Respond to ATO cases immediately - Account takeovers are time-sensitive
2. Always revoke sessions first - Stop active attacker access before investigating
3. Check mailbox rules early - Forwarding rules can silently exfiltrate data
4. Verify with the user - Confirm which sign-ins were legitimate
5. Check for lateral movement - Compromised accounts often target internal users
6. Enforce MFA - Accounts without MFA are prime takeover targets
7. Monitor post-remediation - Attackers may attempt to re-compromise the account
8. Correlate with vendor risk - Account takeover may originate from a compromised vendor

Related Skills

• Abnormal Threats - Threat detection and analysis
• Abnormal Vendors - Vendor risk assessment
• Abnormal Messages - Message analysis
• Abnormal API Patterns - API authentication and usage