VPN Gremlin: User Impersonation Attack in Multiple SSL VPNs – Part 1

Apr 09, 2024

thumbnail-blog-VPN Gremlin: User Impersonation Attack in Multiple SSL VPNs - Part 1


Vulnerability Background

SSL VPN (Secure Sockets Layer Virtual Private Network) is an essential technology for enterprises, allowing users to securely connect to internal networks over insecure networks such as the internet. This is crucial for maintaining cybersecurity as it enables employees to work from home or remotely. However, despite the security features SSL VPN provides while maintaining business flexibility, it still has potential vulnerabilities.

A researcher at TXOne Networks, Ta-Lun Yen, has identified a vulnerability in SSL VPNs known as “VPN Gremlin”. The name stems from a whimsical term used by engineers when attributing inexplicable technical failures to “gremlins”, bringing mischievous little sprites to mind. However, in this context, they refer to elusive vulnerabilities hidden within technical details that could have serious consequences. The core of a “VPN Gremlin” attack involves impersonating a user by exploiting the trust placed in the SSL VPN protocol encapsulation. An active SSL VPN connection is a prerequisite for this attack. Using this connection, the attacker can interfere with the network protocol’s authentication mechanisms. In fact, this attack exploits a flaw within the SSL VPN connection, allowing the attacker to manipulate the header information of the encapsulation protocol before sending packets. This technique is used to bypass firewall rules (i.e., ACL policies), allowing the attacker to send data streams impersonating another user, although they cannot receive any responses.

Currently, Yen is actively working with SSL VPN technology providers to address these vulnerabilities. This serves as a reminder that in the digital world, continuous vigilance and prompt responses are key to protecting cybersecurity. For all enterprises and individuals using SSL VPN-related software, updating to the latest software version to patch these known vulnerabilities is necessary to prevent potential attacks.


Vulnerability Information

This attack exploits a vulnerability in SSL VPN connections, potentially enabling authorized remote attackers to use SSL VPN functionalities to masquerade as another VPN user and send packets, such as CVE-2023-20275, CVE-2024-3388. This issue arises because the system fails to properly validate the internal source IP address of packets after decryption. Attackers can exploit this vulnerability by crafting specific packets and sending them through the VPN tunnel, although this does not allow them to receive any returning data.


Table 1. Description of CVE-2023-46383
CVE-2023-20275 Cisco • Cisco Adaptive Security Appliance (ASA) Software
• Cisco Firepower Threat Defense (FTD) Software
December 5, 2023 4.1 Ta-Lun Yen of TXOne Networks


Table 2. Description of CVE-2024-3388
CVE-2024-3388 Palo Alto Networks GlobalProtect Gateway in PAN-OS Software April 11, 2024 5.1 Ta-Lun Yen of TXOne Networks


Moreover, a vulnerability in post-authentication privilege management within SSL VPN tunnels in certain products may allow attackers to elevate their privileges and potentially access restricted resources, such as CVE-2023-41715.

Table 3. Description of CVE-2023-41715
CVE-2023-41715 SonicWall SonicWall Gen7 Firewalls July 25, 2023 6.4 Ta-Lun Yen of TXOne Networks


In essence, the nature of these attacks is impersonation. The affected SSL VPN implementations not only amplify user access rights but may also inadvertently provide a means to breach network defenses. This technique is utilized to circumvent firewall rules (i.e., ACL policies), enabling attackers to send data streams while impersonating other users, although they cannot receive any responses. This vulnerability indicates that even communication protocols widely regarded as secure might harbor flaws that could be exploited by malicious actors.


Mitigation Recommendation

The attacks described in the previous section specifically require an active SSL VPN connection that has been compromised by an attacker. This can be achieved through one of the following two main methods:

  1. Direct System Compromise: The attacker gains command execution privileges on a computer that already has access to the SSL VPN gateway. This requires the attacker to be successfully authenticated and granted access to the gateway, presumably using legitimate credentials obtained illicitly or through prior compromise of the system.
  2. Credential Theft: The attacker acquires the necessary credentials to authenticate with the SSL VPN gateway via indirect methods such as phishing or other forms of social engineering. This method bypasses the need for initial direct access to the system by exploiting human factors or security lapses in credential management.


Apply Vendor Patches

Fortunately, vendors impacted by these vulnerabilities have issued patches to address the flaws. Updating systems to incorporate these patches is a primary defense mechanism against potential exploits.


Alternative Measures

In scenarios where immediate patching is not feasible:

  1. Implementation of Zero-Trust Principles: By adopting a zero-trust networking framework, every attempt to access resources is verified, authenticated, and validated repeatedly, regardless of the network origin. This approach minimizes the trust assumptions made within and outside the network perimeters.
  2. Enhanced Endpoint Security: Strengthening the security defenses of computers that can access the SSL VPN gateway is crucial. Our solution, Stellar, can detect malicious behavior through multi-method threat defense and also use the lockdown feature to strictly control access, thus reducing the risk of unauthorized access due to compromised devices.
  3. Protocol Switch: Temporarily disabling the SSL VPN protocol and switching to an alternative like IPSec could reduce exposure. IPSec provides robust features that might not be affected by the same vulnerabilities as SSL VPN.



[1] Cisco, “Cisco Security Advisory: Cisco Adaptive Security Appliance Software and Cisco Firepower Threat Defense Software Secure Socket Layer/Transport Layer Security Vulnerabilities,” Cisco Security, 2023.

[2] SonicWall, “Stack-Based Buffer Overflow and SonicOS SSL VPN Tunnel Vulnerability,” SonicWall Support, 2023.

[3] Palo Alto Networks, “CVE-2024-3388,” Palo Alto Networks Security, 2024.

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