In mid-2025, cybersecurity threats continue to evolve with alarming speed. A recent surge in brute-force attacks targeting Fortinet SSL VPNs globally serves as a stark reminder of the persistent and adaptive nature of cyber adversaries. This coordinated assault, initially observed by threat intelligence firms, highlights a critical pivot by attackers, shifting their focus from VPN entry points to management interfaces. Understanding the intricate details of this campaign is paramount for security professionals safeguarding their organizations’ perimeters.

The Initial Barrage: Fortinet SSL VPNs Under Siege

On August 3, 2025, a significant spike in brute-force traffic was detected, specifically aimed at Fortinet SSL VPN devices. Threat intelligence firm GreyNoise reported this coordinated activity, identifying over 780 unique IP addresses participating in the initial wave. Within a mere 24-hour window, 56 distinct IP addresses continued to engage in this persistent effort. The target? Fortinet’s widely deployed Secure Sockets Layer Virtual Private Network solutions, essential for remote access and secure communication. The implications of such a broad-scale attack are severe, as successful brute-force attempts can lead to unauthorized network access, data breaches, and further lateral movement within compromised environments.

Attackers Pivot: From FortiGate to FortiManager

Initial reports focused heavily on the brute-forcing of FortiGate SSL VPNs. However, cybersecurity researchers quickly noted a strategic shift in the attackers’ methodology. Post-initial compromise attempts on VPNs, or perhaps in parallel, adversaries began directing their brute-force efforts towards FortiManager instances. FortiManager is a centralized management platform for Fortinet devices, offering comprehensive control over network security policies, configurations, and updates. Gaining unauthorized access to a FortiManager instance could grant attackers an unparalleled level of control over an organization’s entire Fortinet security infrastructure, enabling them to disable security features, deploy malicious configurations, or exfiltrate sensitive data with devastating efficiency.

Understanding the Brute-Force Tactic

Brute-force attacks are a fundamental and persistent threat in the cyber landscape. This method involves systematically trying every possible combination of characters until the correct password or passphrase is discovered. While seemingly simplistic, the effectiveness of brute-force attacks lies in their scale and persistence, especially when targeting weak, default, or easily guessable credentials. The use of a vast array of unique IP addresses in these Fortinet attacks suggests a distributed effort, likely leveraging botnets or compromised infrastructures, making detection and blocking more challenging. The objective remains the same: guess valid credentials to gain unauthorized access.

Remediation Actions and Proactive Defenses

Organizations leveraging Fortinet products, especially SSL VPNs and FortiManager, must take immediate and decisive action to mitigate the risks posed by these ongoing brute-force attacks. Proactive defense strategies are crucial to prevent compromise.

• Strong, Unique Passwords: Enforce strong, complex, and unique passwords for all user accounts, especially for VPN access and FortiManager logins. Implement multi-factor authentication (MFA) as mandatory.
• Multi-Factor Authentication (MFA): Enable MFA for all VPN users and, most critically, for administrative access to FortiManager. This is the single most effective defense against brute-force attacks.
• Account Lockout Policies: Configure aggressive account lockout policies on both FortiGate and FortiManager to temporarily block IP addresses or users after a set number of failed login attempts.
• Geographic IP Restrictions: Restrict VPN access and FortiManager access to specific geographical regions or trusted IP ranges relevant to your organization’s operations.
• Regular Patching and Updating: Ensure all Fortinet devices, including FortiGate and FortiManager, are running the latest firmware and software versions. Stay informed about security advisories from Fortinet.
• Monitor Logs and Alerts: Implement robust logging and monitoring for failed login attempts, suspicious activity, and configuration changes on FortiGate and FortiManager. Integrate these logs with a Security Information and Event Management (SIEM) system for real-time analysis.
• Audit User Accounts: Regularly review and audit user accounts with access to Fortinet devices, removing inactive or unnecessary accounts.
• Network Segmentation: Implement network segmentation to isolate critical management interfaces like FortiManager from public-facing networks.
• Intrusion Prevention Systems (IPS): Ensure IPS signatures on your FortiGate devices are up-to-date and configured to detect and block brute-force attempts.

Relevant Tools for Detection and Mitigation

Leveraging appropriate tools is essential for effectively defending against and responding to brute-force attacks.

Tool Name	Purpose	Link
FortiAnalyzer	Centralized logging, analytics, and reporting for Fortinet devices. Crucial for identifying suspicious login patterns.	FortiAnalyzer
FortiAuthenticator	Provides centralized authentication, identity management, and multi-factor authentication for Fortinet environments.	FortiAuthenticator
SIEM Solutions (e.g., Splunk, Elastic Stack)	Security Information and Event Management systems for aggregating, analyzing, and alerting on security logs from various sources.	Splunk
Nmap	Network scanner to identify open ports and services, helping to discover exposed FortiManager interfaces.	Nmap
FortiGuard Labs Threat Intelligence	Real-time threat intelligence from Fortinet to update IPS/AV signatures and stay ahead of emerging threats.	FortiGuard Labs

Conclusion

The coordinated brute-force campaign targeting Fortinet SSL VPNs and the subsequent pivot to FortiManager instances underscore the relentless nature of cybercrime. Organizations must remain vigilant, apply robust security hygiene, and implement multi-layered defenses, particularly focusing on strong authentication mechanisms. Proactive monitoring, timely patching, and adherence to security best practices are not merely recommendations; they are critical imperatives for maintaining a strong cybersecurity posture against sophisticated and evolving threats.