Fortinet SSL VPNs Hit by Global Brute-Force Wave Before Attackers Shift to FortiManager

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Fortinet SSL VPNs Hit by Global Brute‑Force Wave Before Attackers Shift to FortiManager

In the first half of August 2025, cybersecurity researchers and network defenders across the globe witnessed a significant surge in brute‑force attacks targeting Fortinet SSL VPN infrastructure — a critical remote access solution used by thousands of enterprises, governments, and managed service providers. Within days, the attack patterns evolved, with threat actors pivoting from FortiOS‑based SSL VPN endpoints to targeting FortiManager, Fortinet’s centralized device management platform. The sequence of events serves as a stark warning that brute‑force campaigns can be harbingers of deeper reconnaissance or exploitation of undisclosed vulnerabilities, and highlights how attackers adapt their tactics to maximize impact. The Hacker News+1

What Happened: The Brute‑Force Surge

On August 3, 2025, threat intelligence firm GreyNoise — which continuously monitors wide swaths of internet traffic — detected a coordinated brute‑force login campaign targeting Fortinet SSL VPNs. Over 780 unique IP addresses were involved in making repeated login attempts against exposed VPN portals; attackers were systematically guessing credentials in hopes of gaining unauthorized access. The Hacker News+1

The geographic scope of the traffic was global. IPs involved in the attack originated in countries including the United States, Canada, Russia, and the Netherlands, and scanning or brute‑force attempts were observed against targets in Hong Kong, Brazil, Spain, Japan, and the U.S. — evidence of a widespread and persistent campaign rather than a localized experiment. The Hacker News

Crucially, the brute‑force activity was not opportunistic, according to GreyNoise. The traffic was highly targeted at Fortinet’s SSL VPN profile, suggesting deliberate intent rather than random credential guessing. Many of the IPs exhibited distinct TCP fingerprints and were tagged by GreyNoise sensors specifically as Fortinet‑focused probing. GreyNoise

Two Waves of Activity

Analysis of the traffic revealed two distinct waves of brute‑force attempts:

  1. A prolonged, steady wave tied to one TCP signature, indicating scanning and repeated login attempts over time.

  2. A sudden burst of attack traffic starting around August 5, with a different TCP signature — and notably, a shift in focus from SSL VPN endpoints to FortiManager’s FGFM service. NetmanageIT CTO Corner

This behavioral shift — from attacking SSL VPN login interfaces to probing the FortiManager FGFM protocol — suggests attackers are adjusting their targeting strategy. Rather than focusing solely on individual VPN access points, they may be probing management infrastructure that could unlock access to multiple Fortinet devices at once.

Why This Matters: Threat Signals and Potential Zero‑Days

Several industry observers have noted that spikes in brute‑force activity often precede vulnerability disclosures affecting the same vendor’s products, typically within a six‑week window. That means defenders should treat such large, focused brute‑force campaigns as potential early indicators of forthcoming exploit activity or zero‑day vulnerability research. Infosecurity Magazine

Indeed, not long after the initial brute‑force surge, Fortinet issued an emergency patch for a critical remote command execution vulnerability (CVE‑2025‑25256) in its FortiSIEM platform, confirming that exploit code was circulating in the wild. Although that particular flaw affected FortiSIEM rather than SSL VPN or FortiManager directly, its discovery reinforced the notion that attackers were stepping up activity across multiple Fortinet product lines. The Cyber Express

Separately, defenders and researchers have tracked other recent critical Fortinet vulnerabilities, including multiple unauthenticated authentication bypass and remote admin access flaws in FortiOS, FortiProxy, FortiSwitchManager, and FortiWeb platforms, some of which are being actively exploited in the wild and tied to configuration theft and privilege escalation. IT Pro+1

Taken together, the pattern of early brute‑force sweeps followed by critical vulnerability activity suggests attackers may be performing credential harvesting, reconnaissance, and coverage testing ahead of or in conjunction with developing or deploying exploit code. This type of blend of broad scanning and opportunistic probing has been seen in other contexts where threat actors map out access surfaces before launching high‑impact exploits. NetmanageIT CTO Corner

Understanding the Targets: SSL VPN and FortiManager

Fortinet SSL VPN

Fortinet’s SSL VPN — part of its flagship FortiOS product, often running on FortiGate firewalls — provides encrypted remote access for users connecting from outside corporate networks. Because SSL VPN exposes a login interface directly to the internet, it naturally becomes a prime target for credential guessing, especially when weak, reused, or default credentials are in place. SC Media

Brute‑force campaigns are designed to exploit such weak authentication: attackers script repeated login attempts using common usernames and password lists to find valid combinations. When successful, a brute‑force compromise can give an attacker remote access into internal networks — often the first step in a broader intrusion campaign.

Brute‑force attacks on VPNs also exploit the fact that many organizations fail to enforce strong external access controls, do not rate‑limit login attempts effectively, and may not utilize multi‑factor authentication (MFA). This leaves exposed interfaces vulnerable to automated credential guessing from distributed sources. NetmanageIT CTO Corner

FortiManager

FortiManager is a centralized management and orchestration platform that administrators use to configure and control Fortinet devices at scale, including FortiGate firewalls, FortiAP access points, and related infrastructure. Compromising FortiManager can be far more valuable to attackers than targeting individual SSL VPN endpoints because a successful breach could allow them to:

  • Export configurations and credentials for multiple devices

  • Inject policy changes across estate

  • Pivot to connected FortiOS devices silently

  • Disable logging and security monitoring centrally

The shift in attacker focus from SSL VPN to FortiManager — via the FGFM protocol — likely reflects an escalation in ambition and opportunity. Rather than repeatedly trying to guess VPN passwords, attackers may be probing for weaknesses in management interfaces that could yield broader control and persistence. NetmanageIT CTO Corner

The Bigger Picture: Elevated Risk for Network Security Infrastructure

Fortinet products are widely deployed across industries and geographies, from SMBs to large enterprises. Because of their prevalence, any focused incentive activity against Fortinet infrastructure — whether brute‑force login campaigns, vulnerability exploration, or exploitation of management platforms — has systemic risk implications for network security across sectors.

Data from threat monitoring and device exposure studies have shown thousands of Fortinet devices are internet‑accessible, including those with SSL VPN functionality. Past research has even found that compromised devices linger with persistent backdoors and symbolic link manipulations, even after patching, due to improper cleanup of post‑exploitation artifacts. vpnMentor+1

This broader exposure suggests that brute‑force and exploit campaigns don’t just threaten remote access services; they can serve as entry points for deeper compromise in networks where SSL VPN serves as the external access gatekeeper. Combined with credential stuffing, phishing, and other identity attacks, brute force becomes one tool in an arsenal of access vectors for attackers. WithSecure™

Defensive Measures and Best Practices

Organizations using Fortinet SSL VPN, FortiManager, or related infrastructure must treat this threat seriously and adopt layered defensive measures:

Strong Authentication and MFA

One of the most effective ways to blunt brute‑force attacks is to require multi‑factor authentication on all external login portals. MFA significantly raises the bar for attackers who may obtain a valid username but still must overcome a second authentication factor.

Rate Limiting and Lockouts

Implement rate limiting for login attempts and enforce temporary lockouts for repeated failures. This greatly slows brute‑force campaigns and reduces the feasibility of automated guessing. Fortinet devices provide options to configure thresholds and login block intervals to mitigate abuse.

Geographic and IP-Based Restrictions

Where possible, restrict VPN access to known IP ranges or geographic regions relevant to your user base, blocking traffic from high‑risk or unknown sources. Tools such as threat intelligence feeds can help maintain dynamic block lists for malicious sources.

Enhanced Monitoring and Logging

Monitor authentication logs and look for patterns of failed logins, especially from distributed or unusual IP sources. Correlate events with threat intelligence to distinguish malicious signals from benign activity.

Patch Management and Configuration Hardening

Keep Fortinet products — including FortiOS, FortiManager, FortiSIEM, and others — up to date with the latest patches. Many vulnerabilities can be mitigated by applying vendor‑provided fixes and recommended configuration changes.

Zero‑Trust Network Architecture

Adopting zero‑trust principles — where no connection is implicitly trusted and access decisions are made continuously based on context — can reduce reliance on broad perimeter defenses like SSL VPN. This includes microsegmentation and identity‑based access policies.

Why Bruteforce Campaigns Should Not Be Ignored

While a brute‑force attack might seem like only credential guessing, it serves as an important intelligence signal. Significant brute‑force traffic directed at specific vendors’ products often:

  • Exposes attacker intent and prioritization

  • Helps map which interfaces and service endpoints are being probed

  • Precedes the emergence or exploitation of vulnerabilities

  • Assists attackers in credential harvesting or fallback access routes

GreyNoise and other threat intelligence sources have noted that patterns like the Fortinet SSL VPN wave — especially when followed by a targeting shift to FortiManager — are not random, but likely part of a broader reconnaissance and exploitation lifecycle. BleepingComputer

Conclusion: Stay Vigilant as Attacks Evolve

The global brute‑force wave against Fortinet SSL VPNs — followed by a shift toward FortiManager — illustrates how attackers scale their efforts across product families and interfaces, experimenting with access and probing for weak points. This blend of credential‑based attacks and management surface exploration signals a more adaptive, persistent reconnaissance strategy that could accompany future zero‑day exploits or deeper breaches. NetmanageIT CTO Corner

Defenders must adopt a multi‑layered approach to harden authentication, monitoring, patching, and access control — while remaining aware that brute‑force spikes are more than just noisy login attempts. They can be the first sign of more advanced campaigns aiming to compromise security infrastructure at scale.

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