Fortinet VPN Brute-Force Attacks Signal Potential Zero-Day Exploits

A significant increase in brute-force attacks targeting Fortinet SSL VPNs has raised concerns about potential zero-day vulnerabilities. The attacks, which began earlier this month, shifted focus from FortiOS SSL VPNs to FortiManager’s FGFM service, a pattern historically linked to upcoming vulnerability disclosures¹. Security researchers have observed over 780 unique IPs daily in these campaigns, with Shadowserver reporting a February 2025 brute-force campaign using 2.8 million IPs⁶.

Attack Patterns and Geographic Distribution

The brute-force campaigns show distinct geographic concentrations, with Hong Kong (32%), Brazil (24%), and the U.S. (18%) being primary targets according to FortiGuard Labs data³. Shadowserver’s telemetry reveals Brazil contributed 1.1 million attacking IPs, followed by Turkey, Russia, and Argentina. The attacks demonstrate adaptive targeting, shifting from FortiOS SSL VPNs to FortiManager services within 48 hours during the initial August 2025 wave¹.

Notable attacking IPs include 31.206.51.194 (Russia), 23.120.100.230 (China), and 96.67.212.83 (Brazil). Attackers primarily used residential proxies from compromised IoT devices, particularly MikroTik and Huawei routers, accounting for 2.8 million IPs in the February campaign¹. SIPVicious tools appeared in 49% of scans, suggesting VoIP infrastructure targeting alongside VPN attacks.

Zero-Day Concerns and Historical Context

SecurityWeek reports that 80% of similar brute-force spikes have preceded CVE disclosures within six weeks⁵. The recent CVE-2024-21887 was exploited within six days of disclosure, according to FortiGuard’s 2025 Threat Report³. DeepData malware has been observed leveraging unpatched flaws in Fortinet VPN clients, with TeckPath identifying logging bypass issues that may hide successful brute-force attempts⁴.

Fortinet’s community forum indicates that 70% of cloud breaches involve over-permissioned identities, compounding the VPN security challenges². The FortiGuard report notes a 42% increase in initial access broker activity on darknet markets, with credential prices ranging from $150 for Redline stealer packages to premium prices for MFA bypass tokens.

Mitigation Strategies

Immediate recommended actions include upgrading to FortiOS 7.4.1 and applying patches for CVE-2024-21887². Network hardening measures should incorporate:

Implementation of geo-restrictions for VPN access
Enforcement of multi-factor authentication for all VPN logins
Deployment of IP blocklists targeting known malicious ranges

Long-term measures include continuous threat exposure management through tools like FortiRecon and auditing cloud permissions via Lacework FortiCNAPP³. The Fortinet community recommends encrypted command-and-control monitoring through DNS/SSL inspection to detect post-compromise activity.

Relevance and Future Implications

The attack patterns mirror previous campaigns that preceded critical vulnerability disclosures. FortiGuard’s data shows APAC regions experiencing 42% of exploitation attempts, followed by EMEA (26%) and North America (20%)³. The shift from VPN to FortiManager targeting suggests attackers are expanding their foothold capabilities within Fortinet environments.

Emerging threats include AI-powered tools like FraudGPT for phishing site generation and ElevenLabs voice cloning for vishing attacks. Network defenders should prioritize monitoring for Netcore routers (18.4% of IoT targets) and GoAhead cameras (10.5%), which frequently serve as attack pivots³.

Proactive security teams should implement FortiSIEM for anomaly detection and zero trust network access principles for least-privilege enforcement. Simulation of deepfake phishing scenarios is recommended to prepare staff for evolving social engineering tactics.

Conclusion

The Fortinet VPN brute-force campaigns represent more than credential stuffing attempts—they follow historical patterns signaling potential zero-day exploitation. The geographic concentration and tooling patterns provide indicators for defensive monitoring, while the shift to FortiManager targeting suggests attackers are adapting to perimeter defenses. Immediate patching and network hardening remain critical, supplemented by long-term monitoring strategies to detect post-exploitation activity.