Attack Chain

1. Attacker gains credentials through phishing or credential stuffing (Hypothetical).
2. Attacker successfully authenticates to the Fortigate SSL VPN using the compromised credentials.
3. Initial access to the internal network is established via the VPN tunnel.
4. Attacker performs reconnaissance activities, such as network scanning and enumeration, potentially using tools like Nmap or Bloodhound.
5. Malicious activity triggers a SIEM alert (e.g., unusual file access, suspicious process creation, or lateral movement).
6. The attacker attempts to escalate privileges on compromised systems using exploits or credential dumping tools like Mimikatz (Hypothetical).
7. Attacker moves laterally within the network to gain access to sensitive data or critical systems.
8. Data exfiltration is initiated using tools like rclone or scp (Hypothetical), or ransomware is deployed.

Impact

A successful attack following a Fortigate SSL VPN login can lead to significant consequences, including data breaches, financial losses, and reputational damage. By compromising a VPN account, attackers can bypass network perimeter security and gain access to sensitive internal resources. The impact can range from unauthorized access to sensitive data to the complete disruption of business operations through ransomware deployment. Targeted sectors could include any organization relying on Fortigate SSL VPN for remote access.

Attack Chain

1. An attacker gains unauthorized access to valid user credentials through phishing, credential stuffing, or other means (T1078).
2. The attacker uses the compromised credentials to successfully log in to a FortiGate SSL VPN, triggering a Fortinet authentication log event (event code "0101039426" or "0101039427").
3. After gaining VPN access, the attacker attempts to move laterally within the network (T1021).
4. The attacker executes malicious code on an internal system, such as a PowerShell script or a Cobalt Strike beacon (T1059).
5. The malicious activity triggers a SIEM alert due to suspicious behavior like unusual network connections, file modifications, or registry changes (T1562).
6. The SIEM alert is correlated with the preceding FortiGate VPN login based on the username within a 10-minute timeframe.
7. The attacker establishes persistence on the compromised system (T1547).
8. The attacker proceeds with their objectives, such as data exfiltration or ransomware deployment (TA0011).

Impact

A successful attack following this pattern can lead to significant damage, including data breaches, financial losses, and reputational damage. Compromised VPN access provides attackers with a privileged entry point into the internal network, allowing them to bypass perimeter security controls. Depending on the permissions of the compromised user, the attacker may be able to access sensitive data, disrupt critical systems, and move laterally to other high-value targets within the organization.

Recommendation

• Deploy the provided Sigma rule to your SIEM to detect FortiGate SSL VPN logins followed by SIEM alerts. Tune the rule's maxspan and risk_score thresholds to reduce false positives based on your environment.
• Investigate any alerts generated by the Sigma rule, focusing on the activities performed by the user after the VPN login. Use the "Investigation Guide" tag in the original rule as a starting point for analysis.
• Review FortiGate logs for unusual login patterns, such as logins from unfamiliar geographic locations or multiple failed login attempts followed by a successful login.
• Implement multi-factor authentication (MFA) for all VPN users to reduce the risk of credential compromise.
• Monitor SIEM alerts for indicators of lateral movement, such as unusual network connections, privilege escalation attempts, and suspicious process executions.