CVE-2021-39510 in DIR-816
Summary
by MITRE • 08/25/2021
An issue was discovered in D-Link DIR816_A1_FW101CNB04 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 • 01/26/2024
The vulnerability identified as CVE-2021-39510 represents a critical security flaw within the D-Link DIR816_A1_FW101CNB04 wireless router firmware, specifically manifesting in the handling of HTTP requests through the /goform/form2userconfig.cgi endpoint. This issue falls under the category of input validation and sanitization failures, where the router's web interface fails to properly validate or sanitize user-supplied input parameters, creating a pathway for unauthorized administrative actions.
The technical implementation of this vulnerability stems from improper parameter handling within the router's web server component, where the handler function for the form2userconfig.cgi route does not adequately filter or escape user-provided data. When an attacker crafts a malicious HTTP request containing specially formatted parameters, the router's internal processing logic can be manipulated to interpret these inputs as commands rather than simple data. This particular flaw allows for the construction of user name strings that can trigger unintended functionality within the router's user management system, specifically enabling the deletion of user accounts without proper authentication or authorization.
The operational impact of this vulnerability extends beyond simple privilege escalation, as it provides attackers with the ability to completely remove user accounts from the router's configuration, potentially disrupting legitimate network access and creating persistent security weaknesses. The vulnerability's exploitation requires minimal privileges and can be executed through standard web browser interactions, making it particularly dangerous in environments where unauthorized users might gain access to the router's administrative interface. This flaw directly violates the principle of least privilege and demonstrates inadequate input validation practices that are commonly addressed through CWE-20, which covers "Improper Input Validation" in the Common Weakness Enumeration catalog.
The attack surface for this vulnerability is significant, as it operates at the application layer of the router's web interface, making it accessible through standard HTTP protocols and requiring no specialized tools or deep technical knowledge to exploit. The vulnerability's presence in the firmware version 101CNB04 indicates a widespread issue affecting multiple installations, as D-Link routers with this firmware version are likely to be deployed in both residential and small office environments where network security may be insufficiently managed. This flaw aligns with ATT&CK technique T1078.004, which covers "Valid Accounts: Cloud Accounts," as unauthorized access to router administrative functions can effectively provide attackers with persistent access to network resources.
Mitigation strategies for this vulnerability should include immediate firmware updates from D-Link, which would address the underlying input validation issues within the web interface handler functions. Network administrators should also implement additional security measures such as restricting administrative access to the router through firewall rules, disabling unnecessary web management interfaces, and implementing strong authentication mechanisms. The vulnerability highlights the importance of proper parameter validation and input sanitization in embedded web applications, with recommendations aligning with industry best practices such as those outlined in the OWASP Top Ten and NIST Cybersecurity Framework. Organizations should also consider network segmentation and monitoring to detect anomalous administrative activities that might indicate exploitation attempts, as the deletion of user accounts would typically generate audit trail entries that could be monitored for security incidents.