Attack Chain

1. The attacker sends a specially crafted HTTP request to the /user_group.asp endpoint on the D-Link DI-8100 router.
2. The CGI Handler processes the request and passes user-supplied data to the sprintf function.
3. The sprintf function, without proper bounds checking, copies the user-supplied data into a fixed-size buffer.
4. The attacker provides input exceeding the buffer’s capacity, triggering a buffer overflow.
5. The overflow overwrites adjacent memory regions, potentially including critical program data or function pointers.
6. By carefully crafting the overflow data, the attacker can inject malicious code into memory.
7. The attacker manipulates the execution flow to redirect control to the injected code.
8. The injected code executes with the privileges of the CGI Handler process, allowing the attacker to potentially gain control of the device.

Impact

Successful exploitation of this buffer overflow vulnerability could allow a remote attacker to execute arbitrary code on the D-Link DI-8100 router. This can lead to a complete compromise of the device, allowing the attacker to intercept network traffic, modify router settings, or use the compromised device as a foothold for further attacks on the local network. Given the widespread use of D-Link routers, a large number of devices are potentially vulnerable.

Recommendation

• Apply available firmware updates from D-Link to patch CVE-2026-7857.
• Monitor web server logs for suspicious requests targeting the /user_group.asp endpoint, as this could indicate exploitation attempts.
• Deploy the Sigma rule detecting suspicious requests to /user_group.asp to your SIEM and tune for your environment.
• Implement strong password policies and regularly update router credentials to mitigate the risk of unauthorized access.