Attack Chain

1. The attacker identifies a Tenda CX12L router running firmware version 16.03.53.12 exposed to the internet.
2. The attacker crafts a malicious HTTP request targeting the /goform/SetPptpServerCfg endpoint.
3. The crafted request includes an overly long string as input to the formSetPPTPServer function.
4. The formSetPPTPServer function copies the attacker-supplied string into a fixed-size buffer on the stack without proper bounds checking.
5. The buffer overflow overwrites adjacent stack memory, including the function’s return address.
6. When the formSetPPTPServer function returns, it attempts to jump to the overwritten return address, now controlled by the attacker.
7. The attacker-controlled return address points to shellcode injected as part of the malicious HTTP request.
8. The shellcode executes with the privileges of the affected process, allowing the attacker to execute arbitrary commands on the router.

Impact

Successful exploitation of this vulnerability allows a remote attacker to execute arbitrary code on the Tenda CX12L router. This could allow the attacker to gain complete control over the device, potentially leading to the theft of sensitive information (such as Wi-Fi passwords), modification of router settings (such as DNS servers), or the use of the router as a bot in a larger botnet. Given the widespread use of Tenda routers, this vulnerability could impact a significant number of users.

Recommendation

• Deploy the Sigma rule “Detect CVE-2026-8138 Exploitation Attempt — Tenda CX12L Buffer Overflow” to your SIEM to detect exploitation attempts targeting the vulnerable endpoint.
• Apply the Sigma rule “Detect Suspicious HTTP POST to SetPptpServerCfg Endpoint” to identify unusual activity.
• Monitor web server logs for POST requests to /goform/SetPptpServerCfg with abnormally long parameter values to identify potential exploitation attempts.