CVE-2026-121600 in A950RG
Summary
by MITRE • 11/04/2026
A flaw has been found in TOTOLINK A950RG 4.1.2cu.5268_B20221104. Affected by this vulnerability is the function setMacFilterRules of the file /cgi-bin/cstecgi.cgi of the component MAC Filter Handler. This manipulation of the argument addEffect causes stack-based buffer overflow. The attack can be initiated remotely. The exploit has been published and may be used.
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Analysis
by VulDB Data Team • 11/04/2026
The vulnerability identified in TOTOLINK A950RG firmware version 4.1.2cu.5268_B20221104 represents a critical security flaw within the device's web management interface, specifically located in the MAC Filter Handler component. The root cause of this issue resides in the setMacFilterRules function found within the /cgi-bin/cstecgi.cgi script. This CGI endpoint is responsible for processing configuration requests related to network access control lists, which are essential for restricting or allowing devices based on their hardware addresses. When a user submits a request to modify these rules, the system parses the input arguments and passes them to internal functions that handle memory allocation and data copying operations. The specific defect lies in how the addEffect argument is processed during this operation.
The technical nature of this flaw is classified as a stack-based buffer overflow, corresponding to CWE-121 in the Common Weakness Enumeration standard. This type of vulnerability occurs when a program writes more data to a fixed-length block of memory on the call stack than it can hold. In the context of the TOTOLINK router, the setMacFilterRules function fails to adequately validate or bound-check the length of the addEffect argument before copying it into a pre-allocated buffer on the stack. If an attacker provides a payload that exceeds the expected size limits, the excess data overflows beyond the boundaries of the intended buffer and corrupts adjacent memory locations. This corruption can overwrite critical return addresses, saved frame pointers, or other control flow data structures stored on the stack.
Because this vulnerability is located in a CGI script accessible via HTTP requests, it allows for remote exploitation without requiring prior authentication to many router configurations that do not enforce strict access controls on specific administrative endpoints, or if default credentials are still active. The ability to initiate an attack remotely significantly increases the risk profile of this flaw. An attacker can craft malicious HTTP POST requests containing a specially crafted addEffect parameter designed to trigger the buffer overflow condition. By carefully constructing the payload, it is possible to overwrite the return address with shellcode or pointers to existing executable code within the router's firmware, such as system() calls or other utility functions commonly found in embedded Linux environments.
The operational impact of successfully exploiting this stack-based buffer overflow is severe and typically results in full remote code execution on the target device. Once an attacker gains control over the instruction pointer through memory corruption, they can execute arbitrary commands with the privileges of the web server process running on the router. This level of compromise allows for complete takeover of the network infrastructure connected to the TOTOLINK A950RG. Attackers may use this access to intercept traffic, perform man-in-the-middle attacks, redirect users to malicious sites, or pivot into other devices on the local area network. Furthermore, compromised routers are frequently enlisted as nodes in botnets for distributed denial-of-service attacks or used to mine cryptocurrency, degrading performance and increasing power consumption.
This vulnerability aligns with several tactics within the MITRE ATT&CK framework, particularly those related to initial access and execution via web services. The exploitation technique involves leveraging a software flaw to gain unauthorized entry into the system environment. Given that exploit code has already been published in public repositories, the window for mitigation is narrow. Organizations relying on this specific model of router must prioritize immediate remediation efforts to prevent potential compromise by automated scanning tools or opportunistic attackers who monitor vulnerability databases for newly disclosed exploits affecting widely deployed consumer-grade networking equipment.
Mitigation strategies should focus primarily on applying vendor-provided firmware updates that address the buffer overflow issue through proper input validation and bounds checking in the setMacFilterRules function. If an update is not immediately available, administrators can attempt to mitigate risk by disabling remote management features if they are enabled, ensuring that administrative interfaces are only accessible from trusted internal networks rather than exposed directly to the internet. Additionally, changing default administrator passwords and restricting access to CGI endpoints through firewall rules on upstream network devices can reduce the attack surface. Regular monitoring of vendor security advisories is essential to ensure timely patching as new firmware versions become available to resolve this critical stack-based buffer overflow vulnerability.