CVE-2026-121618 in E5600
Summary
by MITRE • 11/04/2026
A weakness has been identified in Linksys E5600 1.1.0.26. This affects an unknown part of the file /cgi-bin/fqc.cgi of the component Factory Quality Control. This manipulation causes information disclosure. Remote exploitation of the attack is possible. The exploit has been made available to the public and could be used for attacks. The vendor was contacted early about this disclosure but did not respond in any way. This vulnerability only affects products that are no longer supported by the maintainer.
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Analysis
by VulDB Data Team • 11/04/2026
The Linksys E5600 router, specifically running firmware version 1.1.0.26, contains a critical information disclosure vulnerability within its Factory Quality Control component. The flaw resides in the CGI script located at /cgi-bin/fqc.cgi, which is designed to handle diagnostic and quality control tasks during manufacturing or maintenance phases. This specific weakness allows an attacker to extract sensitive system data that should remain inaccessible under normal operational conditions. Because the affected firmware version represents a legacy release no longer supported by Linksys, users of this device are left without official patches or security updates from the vendor. The lack of response from the manufacturer upon notification further exacerbates the risk, leaving deployed devices in an unpatched state vulnerable to exploitation.
From a technical perspective, the vulnerability stems from improper access control and insufficient input validation within the fqc.cgi script. This CGI endpoint likely exposes internal system variables, configuration files, or hardware diagnostic outputs that are intended for authorized personnel only. The nature of this flaw aligns with CWE-200: Information Exposure, where security-sensitive information is disclosed to an unauthorized actor without malicious intent from the application itself but due to a design weakness. An attacker can interact with this endpoint remotely over the network, leveraging standard HTTP requests to trigger the disclosure mechanism. Since the script processes user-supplied input or simply serves data based on query parameters without verifying the requester's privileges, it becomes a direct vector for data leakage.
The operational impact of this vulnerability is significant due to its remote exploitability and public availability. Attackers can utilize publicly available exploits to harvest sensitive information from targeted routers. This disclosed information may include network configuration details, internal IP addresses, administrative credentials stored in plain text or weak hashes, device serial numbers, and other metadata that aids in further reconnaissance. Such data is invaluable for planning more sophisticated attacks, such as man-in-the-middle assaults, credential stuffing, or lateral movement within a local area network. The public availability of the exploit lowers the barrier to entry for threat actors, increasing the likelihood of automated scanning and opportunistic attacks against vulnerable devices on the internet.
This vulnerability maps directly to MITRE ATT&CK techniques related to Discovery and Collection. Specifically, it relates to T1082: System Information Discovery, where an adversary gathers information about the target environment to facilitate subsequent actions. Additionally, depending on the specific data leaked, it may also touch upon T1539: Steal Web Session Cookie if session tokens are exposed, or general credential theft mechanisms found in router firmware dumps. The persistence of this vulnerability is compounded by the fact that many users continue to operate older hardware beyond its end-of-life period due to cost constraints or lack of awareness regarding security risks associated with unsupported devices.
Mitigation strategies for organizations and individuals still relying on Linksys E5600 routers must focus on network-level controls since patching is not an option. Implementing strict firewall rules to restrict access to the router's management interface from untrusted networks is essential. If remote administration is disabled, ensure that only trusted internal subnets can reach the device. Additionally, segmenting IoT devices into isolated VLANs limits the blast radius of any potential compromise derived from this information disclosure. Upgrading to a supported model with regular security updates remains the most effective long-term solution. Until such an upgrade occurs, continuous monitoring for anomalous traffic patterns targeting the /cgi-bin/fqc.cgi endpoint can help detect exploitation attempts in real-time.