CVE-2018-3953 in E1200
Summary
by MITRE
Devices in the Linksys ESeries line of routers (Linksys E1200 Firmware Version 2.0.09 and Linksys E2500 Firmware Version 3.0.04) are susceptible to OS command injection vulnerabilities due to improper filtering of data passed to and retrieved from NVRAM. Data entered into the 'Router Name' input field through the web portal is submitted to apply.cgi as the value to the 'machine_name' POST parameter. When the 'preinit' binary receives the SIGHUP signal, it enters a code path that continues until it reaches offset 0x0042B5C4 in the 'start_lltd' function. Within the 'start_lltd' function, a 'nvram_get' call is used to obtain the value of the user-controlled 'machine_name' NVRAM entry. This value is then entered directly into a command intended to write the host name to a file and subsequently executed.
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Analysis
by VulDB Data Team • 05/30/2023
The CVE-2018-3953 vulnerability affects Linksys ESeries routers including the E1200 and E2500 models with specific firmware versions, representing a critical operating system command injection flaw that undermines the security posture of these networking devices. This vulnerability stems from inadequate input validation mechanisms within the router's web management interface, specifically in how user-supplied data is processed and stored in non-volatile memory. The flaw manifests when administrators or users interact with the router's web portal and enter data into the 'Router Name' field, which then gets transmitted as the 'machine_name' parameter to the apply.cgi script through a POST request. The vulnerability operates through a chain of events beginning with the preinit binary's handling of the SIGHUP signal, which triggers execution of code at offset 0x0042B5C4 within the start_lltd function, ultimately leading to the dangerous direct execution of user-controllable data through the nvram_get function.
The technical exploitation of this vulnerability follows a well-defined path that aligns with CWE-77 and CWE-78 categories, representing command injection flaws that permit arbitrary command execution on the affected device. When the router processes the user-provided machine name value, it retrieves this data from NVRAM storage without proper sanitization or validation, allowing malicious input to be interpreted as shell commands. The vulnerability specifically targets the start_lltd function where the retrieved machine_name value gets embedded directly into a command string meant to write the hostname to a file, creating a direct execution path for attacker-controlled payloads. This represents a classic case of improper input handling where user-controllable data flows through multiple processing layers without adequate sanitization, enabling attackers to execute arbitrary commands with the privileges of the router's system processes.
The operational impact of this vulnerability extends beyond simple command execution, as it provides attackers with potential access to critical router functionalities and network control capabilities. An attacker who successfully exploits this vulnerability could gain complete control over the affected router, potentially enabling them to modify network configurations, redirect traffic, establish persistent backdoors, or use the device as a pivot point for attacking other systems within the network. The vulnerability affects devices that are typically deployed in home and small office environments where network security may be less stringent, making them attractive targets for exploitation. Additionally, since the vulnerability exists in the firmware level and affects core networking functions, the impact could extend to network availability and integrity, potentially disrupting services or allowing unauthorized access to sensitive network traffic.
Mitigation strategies for CVE-2018-3953 should focus on immediate firmware updates from Linksys to address the root cause of the vulnerability, as well as network-level protections such as restricting access to the router's web management interface and implementing proper network segmentation. Organizations should also consider disabling unnecessary services and features that might expose the vulnerable components, particularly the web management interface. The vulnerability demonstrates the importance of proper input validation and the principle of least privilege in embedded systems, where user-controllable inputs should never be directly executed as commands without thorough sanitization and validation. Network administrators should monitor for exploitation attempts and implement intrusion detection systems to identify potential attacks targeting this specific vulnerability. The ATT&CK framework categorizes this vulnerability under T1059.001 (Command and Scripting Interpreter: PowerShell) and T1068 (Exploitation for Privilege Escalation) as it enables arbitrary command execution and potential privilege escalation within the network infrastructure.