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On a seemingly normal morning in the home office, several employees try to log into the company network via VPN as usual. But instead of working productively on projects, they are greeted by an error message: “Connection failed”. The company’s VPN gateway has fallen victim to an organized DDoS attack and remote access is completely paralyzed. But what exactly is behind such an attack? What risks does it pose for companies and how can you protect yourself effectively against it?
What exactly is a VPN?
A Virtual Private Network (VPN) enables data to be exchanged securely over the internet as if you were in the office. While private users often use VPNs to change their IP address or bypass geo-restrictions, companies focus on secure access to internal resources. As soon as the connection is established, the user receives an internal or company IP address via which they can seamlessly access file shares, email servers and applications.
In technical terms, a VPN client establishes a peering session on the user’s end device to a dedicated VPN gateway in the data center or in the cloud. This gateway – often a firewall, a specialized router or a virtual server – acts as a gateway to the internal network. All traffic is transmitted in an encrypted connection so that third parties can only observe unreadable data, but not the content.
Vulnerability of VPN Gateways
The encrypted tunnel itself is hardly vulnerable to attack – it is only a logical connection layer based on protocols such as IPSec or SSL/TLS. The actual target is not the tunnel, but the endpoint at which the tunnel terminates: the VPN gateway. This gateway, often a firewall, a specialized router or a virtual server, has a public IP address and is therefore the only visible endpoint of a company. It is the publicly visible part that protrudes into the Internet and must be protected.
An attacker launching a DDoS attack does not target the tunnel itself, but this public IP address of the gateway. As soon as the firewall or load balancer collapses under the flood of requests, all VPN sessions break off and remote work comes to a standstill. It is important to understand that VPN gateways are often also firewalls, which makes them a critical point in the network infrastructure. This is why it is so important to protect them
The mechanics of a DDoS attack on VPNs
Distributed Denial-of-Service (DDoS) attacks attempt to flood their target with mass requests, using large botnets for example. When attacking a VPN gateway, cyber criminals often combine various techniques. Volumetric floods such as UDP or ICMP bumps clog the bandwidth, protocol exhaustion attacks (e.g. SYN or SSL handshake floods) exhaust resources such as session tables or CPU cycles and more sophisticated application attacks can directly exploit gaps in the VPN stack.
The result is always the same: the infrastructure can no longer process legitimate connections and stops the service. At the same time, the attacker remains largely anonymous because he only hits the IP of the gateway – not that of the individual end devices.
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When a cyber attack threatens the survival of the company
If the VPN gateway fails, all employees outside the company premises are usually affected. They will not be able to retrieve emails or access protected applications. The result is an immediate drop in productivity. With every minute of downtime, hourly or even daily rates are lost, and in industries with service level agreements (SLAs), contractual penalties can be incurred. The financial impact can be significant, including lost revenue, direct recovery costs and potential reputational damage.
Internal support is inundated with a flood of helpdesk tickets, while it can hardly remedy the situation itself – after all, the attack surface is outside the infrastructure directly accessible to it. It becomes particularly explosive when managers are also affected: As soon as management notices the outage, the pressure to escalate increases considerably. The fact that employees are unable to log in leads to a situation in which a company with a high proportion of employees working from home is almost completely unable to work.
Risks and side effects
A successful DDoS attack can not only lead to immediate downtime, but can also serve as a distraction for further attacks. While the IT team is busy restoring VPN access, attackers may try to capture access data or infiltrate malware at the same time. In addition, such an incident can undermine your employees’ trust in the IT department and weaken the general perception of IT security in the company. If remote working itself is questioned, this also has an impact on employee satisfaction.
How can companies protect themselves?
Individual firewalls are often powerless against large-volume DDoS attacks as they are not designed for this. Effective protection therefore relies on several levels:
Upstream DDoS mitigation
Specialized systems, whether cloud-based or on-premise, are installed upstream of the VPN gateway. They analyze the traffic to filter out harmful packets before they reach the firewall. A “Firewall-as-a-Service” (FWaaS) is a cloud-based solution that can also include DDoS protection.
Dynamic routing techniques
Methods such as Remotely Triggered Black Hole Filtering (RTBH) or BGP FlowSpec allow malicious traffic to be routed to a black hole in real time or specific filter rules to be distributed across the network. In this way, legitimate traffic remains undisturbed while the attack is isolated.
Georedundancy and anycast
By distributing your VPN gateway across multiple locations – such as Frankfurt and New York – and using anycast routes, you ensure that VPN clients are always connected to the closest and healthiest gateway. This means that a failure at one location does not lead to a total outage.
Strict access controls
IP whitelisting for known internal networks and multi-factor authentication (MFA) further reduce the attack surface. Open only the absolutely necessary ports and protocols to create structural barriers for potential attackers.
Continuous monitoring and testing
Realistic traffic baselines and real-time analyses help to detect anomalies at an early stage. Regular DDoS simulations and failover exercises verify that the protection mechanisms really work in an emergency – long before a real attack occurs.
Infrastructure protection
Protection of the infrastructure hosting the VPN gateway (server, router, firewall).
DDoS attacks on VPNs: Detect, Prevent, Protect
In a world where remote working has become the norm, DDoS attacks on VPN gateways pose a serious threat. While the tunnel itself remains encrypted and secure, the visible gateway – the VPN gateway – remains the most vulnerable element. Only with a multi-layered defense consisting of upstream DDoS mitigation services, redundant gateways, dynamic routing techniques, strict access controls and permanent monitoring can you guarantee that your employees can work safely and reliably even on an inconspicuous morning – even if the attackers do everything they can to prevent exactly that.
With Link11, you get comprehensive DDoS protection that efficiently protects you against many DDoS attack methods. Would you like to find out more? Contact our security experts.
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