Analysis Summary

A newly discovered zero-day vulnerability, tracked as CVE-2025-9961, has been identified in TP-Link routers, posing a severe security risk to users. Security Researcher revealed that the flaw resides in the Customer Premises Equipment (CPE) WAN Management Protocol (CWMP) binary, part of the TR-069 protocol used by ISPs for remote management. The vulnerability is a stack-based buffer overflow in the cwmp process that enables attackers to achieve remote code execution (RCE). Researchers successfully developed a proof-of-concept (PoC) exploit demonstrating how attackers can bypass protections and gain full control over vulnerable devices.

The technical exploitation hinges on the ability to overwrite the program counter (PC) after triggering the overflow, thereby seizing execution flow. While Address Space Layout Randomization (ASLR) typically thwarts such attacks by randomizing memory addresses, the absence of an information leak forced researchers to adopt a brute-force approach. By repeatedly guessing the base address of the libc library and targeting the system() function, they crafted a working exploit. Although failed attempts crash the cwmp service, attackers with web panel access could restart it, making brute-forcing feasible in practice.

The attack scenario requires the router to be set to accept a malicious Auto Configuration Server (ACS). The payload, delivered via a crafted SetParameterValues request, uses a ret2libc (return-to-libc) technique to execute system commands. These commands can instruct the router to fetch and run a malicious binary such as a reverse shell from an attacker-controlled server. Once executed, the attacker gains complete remote access, enabling them to intercept user traffic, launch internal network attacks, or conscript the router into a botnet. This highlights the high-impact risk of network-facing management protocols like TR-069, where small flaws in request handling can lead to severe compromise.

During their testing, Researcher encountered issues with the standard GenieACS platform, which corrupted payloads, forcing them to develop a custom ACS emulator to transmit the exploit correctly. Their technical write-up and PoC exploit have been released on GitHub for research and testing purposes, stressing that unauthorized use is illegal. This disclosure underscores not only the dangers of misconfigurations and protocol weaknesses but also the reality that ASLR alone is not sufficient when attackers employ persistent brute-force strategies. TP-Link users are strongly advised to apply upcoming firmware patches immediately to mitigate exploitation risks, as attackers could weaponize this flaw to gain complete control over affected routers.

Impact

• Code Execution
• Gain Access
• Buffer Overflow

Indicators of Compromise

CVE

• CVE-2025-9961

Affected Vendors

Remediation

• Apply the latest firmware updates from TP-Link as soon as they are released to patch CVE-2025-9961.
• Disable TR-069 (CWMP) remote management if not required, or restrict it to trusted service provider servers only.
• Block or limit access to the router’s web management panel from untrusted networks (especially the internet).
• Monitor router logs and network activity for unusual SetParameterValues requests or repeated service crashes, which may indicate brute-force exploitation attempts.
• Use firewall rules to restrict communication with unknown or suspicious Auto Configuration Servers (ACS).
• Replace or segment potentially exposed routers until patched, especially in enterprise or ISP environments.