Coordinated Brute Force Campaign Targets Fortinet SSL VPN

SUMMARY :

A significant spike in brute-force traffic targeting Fortinet SSL VPNs was observed on August 3, with over 780 unique IPs triggering the Fortinet SSL VPN Bruteforcer tag. The activity was deliberate and precise, focusing on FortiOS. Two distinct waves of attacks were identified: a long-running set of brute-force activity and a sudden burst beginning August 5. The second wave shifted from targeting FortiOS to FortiManager - FGFM profile. Historical data revealed a potential residential origin or proxy use. The analysis suggests evolving attack patterns and potential reuse of tooling. Research indicates that such spikes often precede new vulnerability disclosures within six weeks. Defenders are advised to use GreyNoise to search for and block malicious IPs associated with this campaign.

OPENCTI LABELS :

brute-force,vulnerability,fortimanager,fortinet,ssl vpn,fortios,ip blocking,fgfm


AI COMMENTARY :

1. In recent threat intelligence observations, a coordinated brute force campaign has been uncovered targeting Fortinet SSL VPN instances. The report titled [report] Coordinated Brute Force Campaign Targets Fortinet SSL VPN highlights a significant surge in malicious login attempts detected on August 3. Security sensors flagged over 780 unique IP addresses as they triggered the Fortinet SSL VPN Bruteforcer tag in our monitoring platform, underscoring the precision and scale of this operation.

2. The campaign unfolded in two distinct waves of activity. The first wave represented a prolonged series of login attempts against FortiOS, stretching over several days. The second wave emerged on August 5 as a sudden burst of traffic that shifted its focus from FortiOS to the FortiManager FGFM profile. This pivot suggests that threat actors are adapting their tactics to exploit multiple management interfaces in the Fortinet ecosystem, broadening their attack surface.

3. Technical analysis of the brute-force methodology reveals systematic password spraying using distributed IP addresses. Historical data indicates that many source addresses may originate from residential ISPs or leverage proxy services to obfuscate their true origin. Such tactics complicate attribution efforts and allow attackers to evade conventional IP reputation filters by dynamically cycling through clean proxies.

4. Threat actor infrastructure patterns point to the possible reuse of tooling across campaigns. Repeated code signatures within the brute-force scripts and consistent timing patterns imply that the same group or closely coordinated multiple actors are behind both waves. This evolving behavior demonstrates a cycle of continuous improvement, where feedback from one campaign informs the next iteration of their exploitation framework.

5. Intelligence insights reveal that sharp spikes in brute-force traffic often precede public disclosures of new vulnerabilities. Historical correlation suggests that similar surges have appeared within six weeks before zero-day announcements impacting related VPN or network management products. Security teams should treat anomalous authentication failures as early warning indicators of pending exploit releases.

6. Defenders are advised to leverage GreyNoise to search for and block the identified malicious IP addresses associated with this campaign. Implementing strict rate limits on SSL VPN portals, enforcing multi-factor authentication, and monitoring logs for failed FGFM login attempts will mitigate the risk. Regular patch management for both FortiOS and FortiManager, combined with proactive IP blocking and threat intelligence integration, will strengthen defenses against this coordinated brute force offensive.


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Coordinated Brute Force Campaign Targets Fortinet SSL VPN