Responding to Critical Vulnerabilities in FortiOS SSL VPN (CVE-2023-27997)

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A severe vulnerability recently disclosed in Fortinet’s FortiOS SSL VPN product enables unauthenticated remote code execution, allowing hackers to bypass authentication and gain full system control. With a critical CVSS score of 10, all organizations using affected versions of FortiOS SSL VPN should take prompt action to detect vulnerable systems and mitigate risks. This article provides an in-depth examination of the vulnerability, how to identify affected systems, and best practices for mitigation and remediation.

Understanding the Impacts of CVE-2023-27997

The vulnerability, tracked as CVE-2023-27997, exists in the SSL VPN pre-authentication module of FortiOS. It enables a heap overflow, which is a type of buffer overflow that causes excess data to spill over from an allocated memory region into adjacent regions in the heap memory area. By overflowing the memory buffer, attackers can overwrite adjacent memory locations to insert malicious code and alter program execution.

Successful exploitation of this vulnerability essentially removes the authentication barrier, allowing unauthenticated remote attackers to gain arbitrary code execution on the target system. Any networks or resources protected by the compromised SSL VPN would then be accessible to attackers, enabling serious data breaches, ransomware attacks, system damage, and other malicious activities.

The ease with which this vulnerability can be exploited and the severe potential impacts are why it has received a CVSS score of 10, the maximum on the scale. A CVSS 10 vulnerability is incredibly dangerous, difficult to defend against, and should be remediated immediately.

While SSL VPNs are designed to provide secure remote access solutions for organizations, a pre-authentication vulnerability completely subverts this security. It allows any attacker on the internet to gain access without credentials, since the authentication layer is bypassed. This makes attacks much more difficult to detect based on anomalous logins or credential misuse.

The vulnerability disclosure indicates the flaw can be exploited remotely without user interaction, increasing the avenues for attack. Attackers would simply need network access and the ability to send specially crafted packets to a vulnerable SSL VPN interface to trigger the heap overflow and achieve remote code execution.

With the authentication gate removed, attacks can proceed quickly and stealthily directly to data exfiltration, encryption for ransom, destruction of systems, or lateral movement further into the network. Organizations relying on these SSL VPNs for secure remote access need to address this vulnerability immediately to avoid potential catastrophe.

Determining Affected FortiOS Versions

According to the vulnerability details released by Fortinet, the following FortiOS and FortiProxy versions are affected by CVE-2023-27997:

  • FortiOS 6.0.x – 6.0.17 and below are vulnerable
  • FortiOS 6.2.x – 6.2.14 and below are vulnerable
  • FortiOS 6.4.x – 6.4.13 and below are vulnerable
  • FortiOS 7.0.x – 7.0.12 and below are vulnerable
  • FortiOS 7.2.x – 7.2.5 and below are vulnerable
  • FortiOS 7.4.x – 7.4.0 and below are vulnerable
  • FortiProxy 2.0.x – 2.0.13 and below are vulnerable
  • FortiProxy 7.0.x – 7.0.10 and below are vulnerable
  • FortiProxy 7.2.x – 7.2.4 and below are vulnerable

Any organization using these FortiOS or FortiProxy releases in their environment could have vulnerable, exploitable systems that need to be addressed. The first step is to confirm which versions are actively in use across the infrastructure.

Detecting Vulnerable FortiOS Systems

To detect potentially affected systems, there are a few approaches that can be taken:

  • Check the firmware version reported in the FortiOS CLI using the command:
diagnose sys fortiguard-service status
  • Review any available FortiOS release notes or update records to determine historical versions
  • Scan device inventories and configuration management databases to identify SSL VPN products and firmware versions where available
  • Perform non-invasive scans of the external attack surface to detect SSL VPN interfaces that may be affected
  • Query FortiOS administrators to confirm versions in use across the environment

Once vulnerable versions are identified, action must be taken to eliminate exposure. Any systems still running one of the affected versions listed should be considered at imminent risk until patched or mitigated.

Mitigating the Risks of CVE-2023-27997

For any vulnerable SSL VPN appliances and systems detected, organizations should take these steps to mitigate risks:

Upgrade to a Patched FortiOS Firmware Version

The most effective mitigation is to upgrade vulnerable devices to a patched firmware version that addresses CVE-2023-27997. The following FortiOS releases have been fixed according to Fortinet:

  • FortiOS 6.0.x – Upgrade to 6.0.17 or later
  • FortiOS 6.2.x – Upgrade to 6.2.15 or later
  • FortiOS 6.4.x – Upgrade to 6.4.13 or later
  • FortiOS 7.0.x – Upgrade to 7.0.12 or later
  • FortiOS 7.2.x – Upgrade to 7.2.5 or later
  • FortiOS 7.4.x – Upgrade to 7.4.0 or later

And these patched FortiProxy versions are available:

  • FortiProxy 2.0.x – Upgrade to 2.0.13 or later
  • FortiProxy 7.0.x – Upgrade to 7.0.10 or later
  • FortiProxy 7.2.x – Upgrade to 7.2.4 or later

Upgrading to these latest patched firmware versions will fully resolve the vulnerability and secure any affected appliances. Organizations should follow their change management processes to upgrade devices, test functionality, and schedule maintenance windows with minimal disruption.

Disable SSL VPN Services

If it is not possible to immediately upgrade a vulnerable FortiOS device, organizations should disable SSL VPN services on the appliance to reduce the attack surface.

Refer to Fortinet’s knowledge base for instructions on fully disabling SSL VPN connections for your firmware version. This typically involves removing the Remote Access VPN > SSL-VPN Web Portal from the GUI or setting either the “vpn certificate setting” or “sslvpn service” to “disable” via CLI commands.

Successful disablement can be validated when remote users are no longer able to access the SSL VPN login page for the appliance. This eliminates the attack vector until firmware upgrades can be performed. However, note that other vulnerabilities may still exist, so upgrading is still the best defense.

Harden Device Configurations

After upgrading vulnerable appliances, organizations should harden their FortiOS configurations by following Fortinet’s recommended best practices for security hygiene. These cover steps like:

  • Physical security of appliances
  • Enabling vulnerability monitoring through PSIRT
  • Firmware update policies
  • Use of encrypted protocols
  • Updated FortiGuard database services
  • Routine penetration testing
  • DDoS prevention
  • Password policies and storage

Proactively applying these security hardening measures reduces the overall attack surface and risk of exploits. Fortinet provides detailed instructions for each recommendation that organizations can reference and implement.

Monitor VPN Traffic

Until mitigation steps can be fully implemented, IT security teams should closely monitor the SSL VPN applications, network traffic, and system logs for any signs of exploit attempts or other suspicious activity.

Sudden spikes in traffic, abnormal connection patterns, unknown user agents, or unfamiliar IP addresses interacting with the SSL VPN could indicate exploitation is being attempted or already underway. Gathering this forensic evidence can help identify if a successful compromise has already occurred.

Add Compensating Controls

While upgrading devices and services is the primary remediation, additional compensating controls can also be considered. For example, implementing multi-factor authentication (MFA) for VPN connections would still require attackers to have an additional factor beyond just exploiting the vulnerability.

MFA won’t patch the vulnerability but could prevent some compromises. Other controls like enhanced logging, alerting, microsegmentation to limit lateral movement, and allowing VPN traffic only from managed endpoints could also help limit damage until systems are fully patched.

Continuously Monitoring for Vulnerable Assets

To prevent blind spots around emerging vulnerabilities like CVE-2023-27997, organizations should implement continuous monitoring and non-intrusive scanning of their external attack surface and unknown exposed assets.

Cloud-based attack surface management platforms can automatically discover internet-facing systems and match them against known vulnerable software. This allows security teams to identify and remediate at-risk assets faster.

By maintaining real-time visibility of exposed systems and the versions of software in use, organizations can vastly improve response time when new threats are disclosed. They can instantly check for any impact and take appropriate mitigation steps before incidents occur.

Proactively building this level of situational awareness around vulnerabilities in the environment can greatly strengthen the organization’s risk posture and resiliency. Relying on periodic scanning or manual review of configurations won’t provide the same speed of detection and response.

Integrating attack surface visibility and monitoring into existing workflows and tools like service request tickets, Slack channels, SIEM platforms etc also ensures vulnerable systems don’t get overlooked. Automated alerts can be channeled through the appropriate processes to drive rapid remediation.


The critical severity and ease of exploitability of the CVE-2023-27997 FortiOS SSL VPN vulnerability mean it poses severe risk to any organization with a vulnerable deployment. All affected systems could provide an open door for attackers to infiltrate networks and cause damage.

By understanding how this vulnerability bypasses authentication, identifying affected versions, and applying mitigations like upgrading, disabling services, and hardening configurations, companies can eliminate the exposure created by this flaw.

Continuous proactive monitoring for these types of risks is also crucial for rapid response. With increasing hybrid environments and growing reliance on appliances like SSL VPNs for remote access, vulnerabilities will continue to arise. Developing strong programs for vulnerability management, attack surface visibility, log analysis and patching will help reduce business risk.

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