Fortinet SSL-VPN Symlink Exploit: How Attackers Maintain Access Even After Patching

Fortinet has issued a critical warning about a sophisticated technique that allows threat actors to maintain access to FortiGate devices even after security vulnerabilities have been patched. This concerning development highlights the evolving nature of cybersecurity threats and the importance of comprehensive security measures.

 The Original Vulnerabilities

The initial attack vector involves several known vulnerabilities in FortiGate devices, including:

• CVE-2022-42475
• CVE-2023-27997
• CVE-2024-21762

These vulnerabilities allowed attackers to gain unauthorized access to FortiGate devices, particularly those with SSL-VPN functionality enabled.

  The Patch Bypass Technique

What makes this situation particularly alarming is how the attackers have found a way to maintain persistent access even after organizations apply security patches. Here’s how it works:

1. Attackers exploit one of the known vulnerabilities to gain initial access
2. They create a symbolic link (symlink) in the file system connecting the user filesystem to the root filesystem
3. This symlink is placed specifically in folders used to serve language files for the SSL-VPN
4. The modification occurs in the user filesystem, allowing it to evade detection
5. When the device is patched, the symlink remains in place
6. This grants the attacker ongoing read-only access to files on the device, including sensitive configurations

The read-only access is concerning as it allows attackers to view configuration files that may contain sensitive information like credentials, network topology details, and security policies.

  Impact and Scope

According to Fortinet, this attack technique:

• Only affects devices with SSL-VPN enabled
• Is not targeted at specific industries or regions
• Has been detected in organizations across multiple sectors
• Has potentially been active since early 2023

Security experts have noted that some affected organizations would be classified as critical infrastructure, raising the stakes of this security issue.

  Remediation Steps

Fortinet has taken several actions to address this issue and has provided guidance for organizations to protect themselves:

  Software Updates

Fortinet has released multiple updates to address the issue:

• FortiOS 7.4, 7.2, 7.0, 6.4: Updated to flag the symlink as malicious so the antivirus engine automatically removes it
• FortiOS 7.6.2, 7.4.7, 7.2.11, 7.0.17, and 6.4.16: These versions remove the malicious symlink and modify the SSL-VPN UI to prevent serving such symlinks

  Recommended Actions for Organizations

If you use FortiGate devices, Fortinet recommends:

1. Upgrade all devices to the latest versions (7.6.2, 7.4.7, 7.2.11, 7.0.17, or 6.4.16)
2. Review the configuration of all devices thoroughly
3. Treat all configurations as potentially compromised and perform appropriate recovery steps
4. Reset exposed credentials
5. Consider temporarily disabling SSL-VPN functionality until patches can be applied

 How CinchOps Can Help Secure Your Business

In today’s complex cybersecurity environment, threats like the FortiGate symlink exploitation demonstrate that traditional patching alone isn’t always sufficient. At CinchOps, we provide comprehensive security solutions that go beyond standard practices:

 

• Continuous Vulnerability Monitoring: We proactively scan for not just known vulnerabilities but also signs of exploitation and persistence techniques.
• Network Security Assessments: Our team analyzes your network infrastructure to identify potential security gaps and recommend comprehensive hardening measures.
• Incident Response Planning: We help develop and implement robust response protocols for when security incidents occur.
• Configuration Auditing: Regular review of device configurations to identify potentially dangerous settings or unauthorized changes.
• Security Architecture Consulting: Design your network with security at its foundation, minimizing the impact of any single compromise.

Don’t wait until your organization becomes the next victim of sophisticated persistence attacks. Contact CinchOps today to learn how our comprehensive security solutions can protect your critical infrastructure and business operations.

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Discover more about our enterprise-grade and business protecting cybersecurity services on our Cybersecurity page.

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For Additional Information on this topic, check out: Fortinet Warns Attackers Retain FortiGate Access Post-Patching

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