Tenda AC5 Stack-Based Buffer Overflow Vulnerability (CVE-2026-4906)

CVE-2026-4906 is a critical vulnerability affecting Tenda AC5 version 15.03.06.47. This vulnerability is a stack-based buffer overflow located in the decodePwd function of the /goform/WizardHandle file, specifically within the POST Request Handler component. An unauthenticated, remote attacker can exploit this vulnerability by sending a crafted POST request with a manipulated WANT or WANS argument. Publicly available exploit code exists, increasing the risk of widespread exploitation. Successful exploitation allows attackers to execute arbitrary code on the affected device, potentially leading to complete system compromise. This poses a significant risk to organizations and home users relying on this Tenda router model.

Attack Chain

1. The attacker identifies a vulnerable Tenda AC5 router running firmware version 15.03.06.47.
2. The attacker crafts a malicious POST request targeting the /goform/WizardHandle endpoint.
3. The POST request includes the WANT or WANS argument, carefully crafted to cause a buffer overflow in the decodePwd function.
4. The router processes the POST request and calls the decodePwd function.
5. Due to the insufficient bounds checking in decodePwd, the oversized WANT or WANS argument overwrites adjacent memory on the stack.
6. The attacker overwrites critical data, such as the return address, with a malicious address pointing to injected shellcode.
7. The decodePwd function returns, but instead of returning to the legitimate caller, it jumps to the attacker-controlled shellcode.
8. The shellcode executes with the privileges of the web server process, allowing the attacker to perform arbitrary actions on the system, such as gaining a reverse shell or modifying router settings.

Impact

Successful exploitation of CVE-2026-4906 allows a remote attacker to execute arbitrary code on the vulnerable Tenda AC5 router. This could lead to complete compromise of the device, allowing the attacker to modify router settings, intercept network traffic, or use the router as a pivot point to attack other devices on the network. Given the widespread use of Tenda routers in home and small business environments, this vulnerability poses a significant risk to a large number of users.

Recommendation

• Monitor web server logs for POST requests to /goform/WizardHandle with unusually long WANT or WANS parameters. Deploy the Sigma rule Tenda AC5 Suspicious WizardHandle POST Request to detect this activity.
• Apply any available patches or firmware updates released by Tenda to address CVE-2026-4906.
• Consider deploying a web application firewall (WAF) with rules to block requests that exploit buffer overflow vulnerabilities, mitigating the risk even if a patch is not immediately available.
• Implement network segmentation to limit the impact of a compromised router on other devices on the network.