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A recently disclosed vulnerability in Tenda AC7 routers, identified as CVE-2023-41559, has put millions of devices at risk. The flaw, which affects routers running firmware version V15.03.06.44, allows remote attackers to execute arbitrary code or trigger denial-of-service (DoS) conditions. This vulnerability is tied to improper input validation in the formSetFirewallCfg function, leaving devices vulnerable to exploitation. In this article, we will break down the technical details of the vulnerability, its potential impact, and what users can do to mitigate the risk.
CVE-2023-41559
The vulnerability in Tenda AC7 routers originates from improper handling of user input. Specifically, the formSetFirewallCfg function allows data from the firewall parameter to be copied into a fixed-size buffer without checking its size. This oversight results in a stack overflow when a payload larger than 1,000 bytes is sent to the device, causing memory corruption and enabling attackers to hijack the control flow of the router.
Vulnerability Mechanism
The vulnerability is due to the unchecked copying of user data into the firewall_buf array, which is only 256 bytes in size. The malicious data, when it exceeds this size, overflows the stack and corrupts critical memory structures, such as return addresses, which can then be exploited by attackers to execute arbitrary code or crash the device.
Here is a simplified version of the vulnerable code:
“`c
void formSetFirewallCfg() {
char firewall_buf[256];
strcpy(firewall_buf, firewall_value); // No bounds checking
}
“`
Exploitation Requirements
– Firmware Version: V15.03.06.44 (unpatched)
- Network Access: Local or remote access to the router’s web interface (port 80/443)
– Attack Vector: HTTP POST request to `/goform/SetFirewallCfg`
A proof-of-concept (PoC) attack involves sending a payload that triggers the stack overflow, causing a DoS condition where the router becomes unresponsive. The following Python script demonstrates how attackers can exploit the vulnerability:
“`python
import requests
url = http://192.168.0.1/goform/SetFirewallCfg
headers = {Host: 192.168.0.1, Content-Type: application/x-www-form-urlencoded}
payload = {firewallEn: a 1000} Overflow trigger
response = requests.post(url, headers=headers, data=payload)
print(fServer response: {response.status_code}) Expected crash (500 error)
“`
When executed, this code sends an oversized payload that causes the router’s HTTP service to crash, making the device unresponsive.
Risk Assessment
| Risk Factor | Details |
|–|–|
| Remote Code Execution | Attackers can overwrite return addresses and execute arbitrary code via crafted payloads. |
| Denial-of-Service (DoS) | The buffer overflow causes critical services to crash, requiring a hardware reset to restore functionality. |
| Firmware Version Spread | Firmware version V15.03.06.44 is widely deployed, exposing many routers to potential attacks. |
| Lack of Input Validation | The vulnerability arises due to the misuse of strcpy, highlighting the need for proper input validation. |
Replication Environment Setup
To replicate the vulnerability in a controlled environment, the following steps are required:
1. Network Bridge Configuration: Set up a network bridge on an Ubuntu 20.04 system.
2. Emulation with QEMU: Use QEMU to emulate the router’s environment and test the vulnerability.
3. Patched Binary: Use a patched version of httpd to bypass environment checks during exploitation.
Mitigation Recommendations
To protect against this vulnerability, users should take the following actions:
1. Firmware Updates: Tenda has released a patch for this vulnerability, which can be downloaded from their official portal. Users should update their devices to the latest firmware version.
2. Input Validation: Developers should replace the unsafe strcpy function with strncpy and implement length checks to prevent buffer overflows.
3. Network Segmentation: Isolating vulnerable routers from untrusted networks can reduce the risk of remote exploitation.
This incident highlights the critical need for better input sanitization in the firmware of Internet of Things (IoT) devices. Administrators should prioritize applying patches and closely monitor network traffic for any unusual activity that may indicate an attack.
What Undercode Say:
This vulnerability serves as a stark reminder of the inherent risks associated with poor input validation practices in device firmware. Buffer overflows remain one of the most common, yet easily preventable, vulnerabilities, yet they continue to plague devices across various industries. With millions of Tenda AC7 routers still running the vulnerable firmware version, the potential for widespread exploitation is concerning.
The lack of proper bounds checking on user-supplied data, particularly in critical functions such as firewall configuration, makes the device an easy target for attackers. As seen in the proof-of-concept example, a simple HTTP POST request can trigger a DoS condition, rendering the device unresponsive. This highlights the ease with which attackers can disrupt services without even needing to execute arbitrary code.
Furthermore, the risk of remote code execution (RCE) opens the door for more serious exploits. With the ability to overwrite return addresses, attackers could gain full control of the affected devices, potentially using them for botnets, espionage, or other malicious purposes. The vulnerability is especially concerning because of the widespread deployment of the vulnerable firmware version across many AC7 routers.
In terms of mitigation, updating firmware is the immediate and most effective action to take. However, input validation should be considered a priority not just for this case, but for future IoT devices. The use of safer functions such as strncpy and enforcing length checks could prevent a wide range of similar vulnerabilities from being exploited.
The fact that this vulnerability was discovered by security researchers underscores the importance of regular security audits for IoT devices. While some vendors have already taken steps to address such vulnerabilities, many others still fail to implement basic security measures, leaving their devices exposed to attacks. In light of this, administrators must stay vigilant and proactive in patching devices and protecting their networks.
Fact Checker Results
- The vulnerability in Tenda AC7 routers (CVE-2023-41559) is real and has been verified by independent security researchers.
- The flaw stems from improper input validation, allowing attackers to exploit buffer overflow vulnerabilities for DoS and remote code execution.
- Tenda has issued patches to address the vulnerability, but the affected firmware version remains widely used in the field.
References:
Reported By: https://cyberpress.org/vulnerability-tenda-ac7-routers/
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