Defending Your Network Edge Against the Next Zero-Day Exploit

Key Insights

• Network edge devices are prime targets for both nation state and opportunistic adversaries, with recent attacks leveraging critical flaws in well-known vendor systems, such as those of Fortinet, Ivanti, and Citrix.
• Adopting Zero Trust methodologies significantly reduces the risk of compromised edge devices in an ever-evolving threat landscape. While Zero Trust is not a new concept, it is still highly effective, and sometimes underestimated.
• Given the long journey towards achieving Zero Trust, organizations should consider prioritizing hardening measures and vigilant monitoring, to significantly boost resilience in the short term.
• Sygnia’s proactive strategies offer achievable, battle-tested robust defense measures, bolstering your network’s resilience against such emerging cyber threats.

Background

The relentless cycle of zero-day exploits has turned network edge security management into a familiar game of whack-a-mole. For decades, the IT industry has grappled with this vicious loop: vulnerabilities are exposed, patches are delayed, and adversaries exploit these weaknesses – often amidst a flurry of media attention and technical scrutiny. This ongoing pattern persists, with no vendor immune to the pervasive challenge of securing network edge devices against vulnerability exploitation.

Network edge devices such as perimeter firewalls, VPN gateways, and externally-facing load balancers, are by nature directly exposed to the internet with limited defenses, making them inherently vulnerable. Such devices cannot run endpoint protection tools like EDR, limiting the ability to promptly detect compromise or other types of anomalous behavior, or to efficiently perform forensic analysis. Moreover, misconfigurations such as weak access controls, poor password practices, and the absence of multifactor authentication are common. Keeping up with patching is a challenge, as firmware or software can quickly become outdated – especially when updates require taking devices offline, and interrupting business workflows. Furthermore, as appliances age and fall out of support, organizations face the daunting task of purchasing, designing, configuring, deploying, and hardening new appliances.

Recent sophisticated attacks, including the Ivanti and Fortinet zero-day exploits and the Citrix Bleed vulnerability, underscore the systemic threats to network edge devices. In these attacks, attackers were able to exploit vulnerabilities using living-off-the-land technique and malware to maintain persistence, sometimes despite system upgrades and resets. For instance, the Fortinet FortiOS SSL VPN compromise and the Citrix Bleed flaw enabled attackers to establish persistent access and extract sensitive data, while circumventing multifactor authentication. These incidents reveal a stark reality: nation-backed actors invest significant resources into uncovering zero-day vulnerabilities, and following the public disclosure of these vulnerabilities, common exploit tools are swiftly developed. These tools are then rapidly adopted by various ransomware groups, targeting unpatched devices.

Incorporating insights from our incident response cases, it is evident that exploiting vulnerabilities in network edge devices is a primary method for attackers seeking initial access to enterprise networks (T1190). This evolving threat landscape demands a vigilant and proactive defense strategy that both addresses immediate vulnerabilities, and fortifies the network against the next wave of attacks.

Contrary to common belief, and as highlighted in a previous Sygnia publication, the risk of a network being breached due to newly-discovered vulnerabilities in network edge devices is not a ‘black swan’ event that cannot be preemptively anticipated and mitigated. The notion that zero-day vulnerabilities in the network perimeter leave us powerless, inevitably leading to full network compromise upon infiltration, and that patching is our only defense against these threats, are misconceptions. A comprehensive defense strategy that goes beyond mere patching can effectively address and mitigate these risks, by including other immediate measures – such as hardening management interfaces and continuous monitoring, and adopting Zero Trust methodologies that modernize remote enterprise access and reduce the adverse impact of a perimeter breach. By adopting such a holistic approach, organizations can preempt potential vulnerabilities, transforming what are perceived as inevitable points of failure into well-defended and resilient assets.

Traditional VPN vs. Zero-Trust Network Access

Traditional enterprise VPN (Virtual Private Network) infrastructure has long been the cornerstone of enterprise remote access, providing a secure tunnel over public networks to the organization’s internal systems, typically hosted on-premises. This solution requires exposing a VPN gateway, either incorporated within the external-facing firewall or not, that serves as an inbound entry point for remote users. As previously detailed, this architecture is inherently vulnerable to potential exploits, as VPN endpoints are easily discoverable and are susceptible to attacks. A severe vulnerability within the product may allow threat actors to gain persistent control over a gateway device, or bypass its defenses, such as authentication and authorization mechanisms.

On top of this, traditional VPNs pose additional security risks to an organization’s network. First, they typically extend the entire corporate network to the remote device, enabling unrestricted lateral movement and thus significantly broadening the attack surface once an endpoint is compromised. Furthermore, the ability for users and devices to connect from any location without consistent and continuous authorization validation opens the door to prolonged unauthorized access. Also, as the management interface is often publicly exposed and vulnerable to credential guessing and zero-day exploits, remote attackers could successfully gain privileged access. Adversaries frequently exploit these vulnerabilities, targeting VPN appliances to gain unauthorized access.

Enter Zero Trust Network Access (ZTNA), a remote access solution that adheres to the Zero Trust security principles of “never trust, always verify” and “assume breach”. With Zero Trust, entities within the internal network perimeter are not inherently trusted; instead, all traffic is treated as potentially dangerous, regardless of origin, and is verified according to a restrictive set of policies, while applying the principle of least privilege.

ZTNA solutions are typically cloud-based and are often offered by vendors as part of their Secure Access Service Edge (SASE) suite. As the name suggests, the goal of SASE solutions is to take the edge network security services that are required by enterprises, and provide them as fully managed cloud-hosted services; these include remote user access (i.e., ZTNA or VPN), inter-site SD-WAN connectivity, web browsing gateways, and Cloud Access Security Brokers (CASB), among others.

In a typical cloud-routed ZTNA architecture, a connector agent is deployed within the corporate network perimeter. This connector establishes outbound communications to the ZTNA service provider’s cloud infrastructure, keeping a persistent and encrypted tunnel. Clients initiate connections by authenticating to the cloud service, which in turn proxies the traffic to the designated internal service over this tunnel and via the internal connector.

With this architecture, the cloud-hosted service acts as the network edge, while the components within the internal network are not directly exposed to inbound traffic, effectively rendering the enterprise network perimeter invisible to basic scanning. In this way, the risk of vulnerability exploitation, remote code execution, and a persistent attacker foothold is transferred from the traditional on-premises infrastructure to a fully managed cloud-based service, as part of the well-known model of shared responsibility between customers and cloud service providers. The ability of the cloud service provider to mitigate potential vulnerabilities uniformly, promptly, and efficiently, even before they are publicly published, reduces the risk of vulnerability exploitation. Furthermore, the distributed nature of these cloud services inherently increases resilience to cyber threats on a particular organization, as it hinders adversaries’ attempts to single out and compromise specific enterprises.

ZTNA solutions also provide granular access control, only exposing specific applications rather than the entire network, and only allowing this access to authenticated and authorized clients, thus reducing the visible attack surface from a compromised endpoint. The adoption of Network Access Control (NAC) methodologies for ongoing device posture assessment is also becoming commonplace in ZTNA products, reinforcing the continuous verification of device integrity.

Immediate Defense Tactics

The following tactics can be employed to achieve short-term victories and increase defenses against network edge attacks, while preparing for more comprehensive strategic measures such as ZTNA and SASE solutions.

Limit the attack surface and exposure. Track and prioritize asset, vulnerability, and patch management efforts to regularly identify and update internet-facing network devices, and close off vulnerabilities that attackers can exploit as soon as possible. An External Attack Surface Management (EASM) solution can swiftly detect and assess exposed interfaces, facilitating the timely application of patches in line with vendor advisories. Automating patch management processes using solutions such as Cisco’s IOS Auto-Upgrade Manager can further solidify the network’s defenses, establishing a secure, well-monitored network perimeter. Although these practices may not safeguard against zero-day threats leveraged by highly sophisticated adversaries, they will form a comprehensive defense strategy against opportunistic threat actors, who use known exploits to leverage public vulnerabilities.

Protect management interfaces from exposure to the public internet. Several of the exploitable vulnerabilities found on network edge devices were within their management interfaces (e.g., CVE-2023-38035, CVE-2023-6548), and could be easily mitigated by eliminating internet exposure. Attackers actively search for exposed management interfaces, as these are often protected by guessable passwords, with no multifactor authentication, that can be bypassed to obtain unauthorized and often privileged access to network appliances. Permit access to management interfaces only from the internal network – or at the very least, shield the public-facing management interface with firewall rules that permit access solely from trusted public IP addresses. Ideally, segment management interfaces from the rest of the internal network, enforce multifactor authentication, and force connections to be made through a secure jump server – preferably controlled using a privileged access management solution.

Restrict egress traffic towards the internet, to thwart adversaries’ ability of maintaining persistent access over compromised devices through Command & Control (C2) channels. Perimeter firewall policies should be configured with restrictive policies, allowing outbound traffic only to specific destinations in accordance with business needs. Public-facing devices such as load balancers and VPN gateways should be placed behind a perimeter firewall, to enforce the restrictive firewall policies on their internet egress traffic. Restricting egress traffic that initiated directly from a compromised perimeter firewall appliance itself is somewhat more cumbersome; although these devices should be locally configured to restrict this traffic as well, a sophisticated adversary may seek ways to remove and thus circumvent such restrictions.

Continually monitor system file integrity in order to detect the presence of malware that may be disguised as legitimate files on edge network devices. Such monitoring includes flagging unauthorized modifications that could indicate breaches, and that require an immediate response. Many network appliance vendors provide complementary tools for this purpose, like Ivanti’s external Integrity Checker Tool, or Fortinet’s real-time file system integrity checking. Additionally, third-party File Integrity Monitoring (FIM) solutions, such as those offered by Varonis, Tripwire and Netwrix, can be integrated with compatible devices.

Enforce the principle of least privilege across all edge appliances – and everywhere else. Limiting user rights to only those that are necessary for task performance will significantly reduce the risk of attackers gaining extensive access through a compromised edge device. Admin and root accounts should be used sparingly, and with close supervision. Access control lists and role-based access controls are effective tools for maintaining minimum necessary access, and regular audits are essential to ensure these measures are properly enforced, thereby preserving the network’s edge integrity.

Enhance monitoring and detection capabilities to maintain comprehensive network visibility and enable timely action against threats. Configure network devices to forward system events, traffic logs, and threat logs to the corporate Security Information and Event Management (SIEM) system, or a suitable alternative solution. Specifically, ensure that HTTP traffic logs capture the X-Forwarded-For (XFF) header, to accurately determine caller IP addresses. Retain critical audit logs for a sufficient timeframe, to facilitate the ability to trace back breaches that may have been carried out before vulnerabilities were disclosed. Additionally, implementing Intrusion Detection and Prevention Systems (IDPS) enables swift identification and response to various suspicious activities. Many network appliances such as firewalls often have these features built in.

Configure network edge devices for high availability and redundancy, while establishing and testing failover protocols, to maintain continuous service availability and to minimize downtime. This level of preparedness is crucial for preventing service outages and data loss, whether due to denial-of-service attacks, hardware malfunctions, or planned maintenance. In particular, preparedness facilitates the application of patches with minimal or no service interruption, enabling quick patching to mitigate newly discovered vulnerabilities.

Strategic Defense Approaches

The strategic measures described below can help to diminish the risk of network edge attacks, by reducing the attack surface and enhancing resilience.

Migrate away from the traditional VPN to a cloud-based Zero Trust Network Access solution. As detailed in the previous section, typical ZTNA solutions reduce the risk of edge device exploitation by having a cloud provider serving the user request. Moreover, these solutions enhance security by ensuring granular and contextual access, and by requiring user and device identity verification prior to access. The transition to ZTNA involves assessing current internal network access needs, defining a strategic migration plan, selecting an appropriate ZTNA solution, and implementing it incrementally by gradually exposing internal services through it, to guarantee a seamless shift towards a more secure remote access model.

Fortify the internal network with additional segmentation. While organizations often focus on protecting the network perimeter, dividing the network into smaller, secure zones, can restrict an attacker’s ability to move laterally, and would limit the attack surface in the case of an exploited edge device. To regulate traffic between and within these zones, deploy firewalls and microsegmentation solutions, focusing on inhibiting movement over management protocols such as SSH, RDP, SMB, and WinRM. This strategic approach not only safeguards against edge device compromise, but also bolsters overall resilience to any type of initial breach, such as workstation compromise. This approach should be extended to cloud environments like Azure, AWS, and GCP.

Shift vendor applications from on-premises to SaaS, where feasible. Some of the critical vulnerabilities in recent years were found in business applications produced by vendors, such as Microsoft Exchange and Confluence. While some organizations may prefer keeping their business data self-hosted, for various reasons, migrating to Software-as-a-Service (SaaS) offerings can mitigate the risk of threat actors exploiting unpatched vulnerabilities within such applications, gaining privileged access to these systems, and potentially further moving laterally into the corporate network.

Conduct routine cyber security assessments to proactively identify misconfigurations, security gaps, and potential architectural enhancements before the next zero-day strikes, and adversaries can take advantage. Assessments come in many forms, such as enterprise security posture assessment, secure design review, penetration tests, red-teaming exercises and threat hunts. Sygnia provides all these services and more, to fortify your defenses in the face of the threat landscape’s continual evolution.

Conclusion

The complexity of cyber threats is constantly escalating in the ever-shifting digital terrain – particularly threats aimed at network edge devices. The recent discovery of zero-day vulnerabilities in Ivanti, Fortinet, and Citrix equipment serves as a vivid reminder of the relentless advancement of these threats. To combat these risks effectively and preemptively boost resilience, organizations must maintain a state of constant vigilance and adopt both immediate and long-term mitigation strategies.

Immediate tactical defenses such as identifying exposure, securing management interfaces, and conducting integrity checks are vital to mitigate the next zero-day. Meanwhile, strategic defenses like the adoption of Zero Trust approach and solutions, and the shift to cloud-based infrastructure lay the groundwork for enduring edge security. These proactive steps enable organizations to not only counter present threats, but also fortify their infrastructure against future perceived black swan cyber events that target network edge devices, ensuring sustained resilience and security.

This advisory and any information or recommendation contained herein has been prepared for general informational purposes and is not intended to be used as a substitute for professional consultation on facts and circumstances specific to any entity. While we have made attempts to ensure the information contained herein has been obtained from reliable sources and to perform rigorous analysis, this advisory is based on initial rapid study, and needs to be treated accordingly. Sygnia is not responsible for any errors or omissions, or for the results obtained from the use of this Advisory. This Advisory is provided on an as-is basis, and without warranties of any kind.