Bypassing Perimeter Defenses via Unencrypted Tunneling Protocols

TLDR: This research exposes how misconfigured, unencrypted tunneling protocols like GRE and VXLAN allow attackers to bypass perimeter firewalls and gain unauthorized access to internal networks. By spoofing source IP addresses and manipulating routing protocols, an attacker can trick internal infrastructure into accepting malicious traffic as legitimate. Security teams must audit their network configurations for these protocols and enforce strict egress filtering to prevent this class of lateral movement.

Network security often relies on the assumption that the perimeter is a hard shell. We spend countless hours hardening firewalls and configuring complex access control lists, yet we frequently overlook the "plumbing" that connects our infrastructure. When that plumbing consists of unencrypted, stateless tunneling protocols, the perimeter becomes a suggestion rather than a barrier. Recent research presented at DEF CON 2025 demonstrates that attackers can exploit these protocols to turn a single compromised device into a gateway for full internal network access, effectively rendering traditional IP-based filtering useless.

The Mechanics of Tunnel Hijacking

The core of this vulnerability lies in the inherent trust placed in tunneling protocols like Generic Routing Encapsulation (GRE) and VXLAN. These protocols are designed for efficiency and scalability, not security. Because they are stateless and lack native encryption, they do not verify the identity of the sender beyond the outer IP header.

An attacker who identifies an unencrypted tunnel can craft packets that appear to originate from a trusted peer. By spoofing the source IP address, the attacker can inject traffic directly into the internal network. If the target router or switch is configured to trust traffic arriving via the tunnel interface, it will decapsulate the packet and forward the inner payload to the destination, bypassing any firewall rules that would have blocked the traffic had it arrived from the public internet.

The research highlights that this is not just a theoretical risk. Many enterprise-grade devices, including those from Cisco, Fortinet, and Palo Alto Networks, support these protocols by default. When these devices are misconfigured—specifically when they fail to validate the source of tunnel traffic or when they allow unauthorized interfaces to participate in routing protocols—the entire network is exposed.

Exploiting Routing Protocols

The impact of this technique scales significantly when combined with routing protocol manipulation. Many organizations use BGP or OSPF to manage internal traffic. If an attacker can inject themselves into these routing exchanges, they can announce malicious routes or redirect traffic to systems they control.

By sending crafted OSPF "Hello" packets or BGP updates through a hijacked tunnel, an attacker can force the victim's routers to update their routing tables. This allows the attacker to intercept traffic destined for sensitive internal assets, such as domain controllers or virtualization management interfaces. The CVE-2025-6443 advisory underscores the severity of these misconfigurations, particularly in environments where legacy settings remain enabled.

To test for this, you can use custom scanning tools to identify open tunnel endpoints. The GRE Tunnel Scanner and the VXLAN Scanner released alongside this research are excellent starting points for identifying exposed infrastructure. During an engagement, you should look for evidence of these protocols in your network captures:

# Example of identifying OSPF traffic in a packet capture
tcpdump -i any proto ospf

If you see OSPF or BGP traffic originating from unexpected sources, you have likely found a misconfigured tunnel that can be leveraged for lateral movement.

Real-World Applicability and Defense

For a pentester, this is a goldmine for initial access and privilege escalation. Once you have a foothold on a single device, you don't need to find a complex exploit to move laterally. You simply need to identify the tunnel configuration and start injecting traffic. The lack of authentication in these protocols means that if you can reach the tunnel endpoint, you can control the traffic flow.

Defending against this requires a shift in how we view network infrastructure. First, audit your environment for any unencrypted tunnels. If you must use them, ensure that they are protected by IPsec or another form of cryptographic authentication. Second, implement strict egress filtering on your firewalls. Do not allow internal devices to initiate connections to arbitrary public IP addresses, and specifically block traffic that attempts to spoof internal IP ranges.

Finally, ensure that your routing protocols are configured with authentication. OSPF and BGP both support MD5 or SHA-based authentication, which prevents unauthorized devices from injecting routing updates. If your routers are accepting routing information from unauthenticated sources, you are essentially inviting an attacker to take control of your network topology.

The era of trusting internal network traffic is over. As these techniques become more widely understood, the barrier to entry for network-level attacks will continue to drop. Start by scanning your own infrastructure—you might be surprised by what you find.