Rooting DrayTek Routers: A Masterclass in Neglected Binary Hardening

TLDR: Researchers at Black Hat 2024 demonstrated that DrayTek Vigor routers remain a prime target for remote code execution due to a combination of legacy software, lack of modern binary protections, and insecure handling of user input. By exploiting a stack buffer overflow in the WebUI, they achieved unauthenticated root access on the host OS. This research serves as a stark reminder that even "enterprise-grade" network hardware often lacks basic exploit mitigations like ASLR, PIE, and stack canaries.

Network edge devices are the ultimate low-hanging fruit for anyone looking to establish a persistent foothold in a corporate environment. While we spend our days obsessing over complex cloud misconfigurations and zero-day chains in modern web frameworks, the humble router sitting at the perimeter is often running a decade-old stack with the security equivalent of a screen door. The recent research presented at Black Hat 2024 on DrayTek Vigor devices confirms that these routers are not just entry points; they are essentially unhardened mini-servers waiting to be compromised.

The Anatomy of an Unhardened Target

The researchers focused on the DrayTek Vigor 3910 and 3912, devices that market themselves as secure, feature-rich solutions for businesses. Under the hood, however, these routers run a proprietary operating system called DrayOS, which operates within a virtual machine on top of an Ubuntu 22.04 host. This architecture is intended to provide isolation and reliability, but it introduces a massive attack surface when the underlying binaries are not built with modern security mitigations.

During their analysis, the team identified that the primary binary, sch0d64.bin, lacks almost every standard binary protection. There is no Data Execution Prevention (DEP), no stack canaries, no Position Independent Executable (PIE), and no Address Space Layout Randomization (ASLR). For a pentester, this is a dream scenario. It means that once you find a memory corruption vulnerability, you do not need to worry about complex ROP chains or bypassing entropy. You can jump directly to your shellcode or perform a simple return-to-libc attack.

Exploiting the WebUI

The most critical finding was a stack buffer overflow in the WebUI, tracked as CVE-2024-41592. The vulnerability stems from how the router parses query string parameters. Many of the WebUI pages call a function that processes these parameters, but the destination buffer on the stack has a fixed length and lacks bounds checking.

When a user sends a request with an excessively long parameter, the buffer overflows, allowing an attacker to overwrite adjacent stack variables and, crucially, the return address. Because the binary is a "flat" executable without stack canaries, the overwrite is trivial. The researchers demonstrated that this can be triggered by navigating to almost any page in the WebUI, making it a highly reliable vector for unauthenticated remote code execution.

The exploit flow is straightforward:

1. Send a crafted GET request to a vulnerable endpoint.
2. Include a payload that fills the buffer and overwrites the return address.
3. Redirect execution to the desired shellcode or function.

The researchers also identified CVE-2024-41585, an OS command injection vulnerability that allows an attacker to execute arbitrary commands as root. This is particularly dangerous because the router's command-parsing logic is flawed; it checks if a command starts with a whitelisted string but does not validate the entire input, allowing for command chaining.

Why This Matters for Pentesters

If you are conducting an external infrastructure assessment, these devices are high-value targets. The presence of hardcoded credentials and the lack of Transport Layer Security (TLS) enforcement for the WebUI means that traffic interception is often enough to gain full administrative control. Even when TLS is enabled, the researchers noted that the router's implementation of the Pseudo-Random Number Generator (PRNG) is flawed, potentially allowing for the reconstruction of private keys, as detailed in the research on weak entropy in embedded systems.

When testing these devices, do not just look for the latest CVEs. Look for the "forgotten" vulnerabilities. Check if the WebUI is exposed to the internet, which is a violation of OWASP A07:2021 – Identification and Authentication Failures. If you find an exposed interface, assume it is vulnerable to command injection or buffer overflows until proven otherwise.

Defensive Takeaways

For those working with blue teams, the advice is simple but often ignored:

• Isolate management interfaces: Never expose the WebUI to the public internet. Use a VPN or a jump host to access these devices.
• Prioritize patching: While patching is standard, it is not a cure-all. The researchers noted that vendors often release patches that only address the specific reported variant, leaving other similar vulnerabilities in the same code path unpatched.
• Demand transparency: If a vendor has a history of recurring vulnerabilities in the same components, it is a sign of systemic issues in their development lifecycle.

The reality is that as long as manufacturers continue to prioritize features over fundamental binary hardening, these devices will remain a playground for attackers. If you are auditing a network, start by scanning for these routers. You might be surprised by how many are still running with default credentials or outdated firmware that is trivial to root. The next time you see a DrayTek device on a scan, do not just flag it as a finding; treat it as an open door.