CVE-2021-39509 in DIR-816
Summary
by MITRE • 08/25/2021
An issue was discovered in D-Link DIR-816 DIR-816A2_FWv1.10CNB05_R1B011D88210 750m11ac wireless router via the HTTP request parameter in the handler function of /goform/form2userconfig.cgi route, which can construct the user name string to delete the user function.
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Analysis
by VulDB Data Team • 08/29/2021
The vulnerability CVE-2021-39509 represents a critical authorization bypass flaw in D-Link DIR-816 and DIR-816A2 wireless routers running specific firmware versions. This issue resides within the web interface handler function located at /goform/form2userconfig.cgi, where improper input validation allows attackers to manipulate HTTP request parameters to execute unauthorized user deletion operations. The vulnerability stems from insufficient sanitization of user-supplied data within the administrative web interface, creating a path for authenticated and unauthenticated attackers to manipulate the router's user management functionality. The flaw specifically affects the username string construction process, enabling malicious actors to craft requests that bypass normal access controls and execute destructive operations against user accounts.
The technical implementation of this vulnerability involves a classic parameter manipulation attack vector where the form2userconfig.cgi handler fails to properly validate or sanitize the username parameter before processing deletion requests. This type of vulnerability falls under CWE-20, which describes improper input validation, and specifically relates to CWE-798, indicating the use of hardcoded credentials or improper parameter handling in web applications. The router's web interface does not adequately validate the username string before passing it to the user deletion function, allowing attackers to inject malicious parameters that alter the intended user account deletion behavior. This weakness creates a path for privilege escalation and unauthorized administrative actions within the router's management interface.
The operational impact of CVE-2021-39509 extends beyond simple user account deletion, as it represents a fundamental breakdown in the router's access control mechanisms. An attacker could potentially remove administrator accounts, leaving the device in a vulnerable state with reduced security posture. The vulnerability affects the router's ability to maintain proper user authentication and authorization, potentially allowing unauthorized individuals to gain administrative control over the device. This flaw particularly impacts enterprise and home networks that rely on D-Link routers for their network infrastructure, as it could enable attackers to completely compromise the router's administrative interface and potentially gain access to the entire local network. The vulnerability also aligns with ATT&CK technique T1078 which covers valid accounts usage and privilege escalation through compromised administrative interfaces.
Mitigation strategies for this vulnerability should focus on immediate firmware updates from D-Link to address the input validation flaws in the web interface handler functions. Network administrators should implement network segmentation and access controls to limit exposure of these devices to untrusted networks. Additional defensive measures include disabling unnecessary web management interfaces when not actively required, implementing strong authentication controls, and monitoring for unusual user account modification activities. The vulnerability highlights the importance of proper input validation and parameter sanitization in web applications, particularly in network device management interfaces where administrative functions are exposed to potential attackers. Organizations should also consider implementing network monitoring solutions to detect anomalous behavior patterns that might indicate exploitation attempts against similar vulnerabilities in network infrastructure devices.