CVE-2021-29294 in DSL-2740R
Summary
by MITRE • 08/11/2021
** UNSUPPORTED WHEN ASSIGNED ** Null Pointer Dereference vulnerability exists in D-Link DSL-2740R UK_1.01, which could let a remove malicious user cause a denial of service via the send_hnap_unauthorized function. It could be triggered by sending crafted POST request to /HNAP1/. NOTE: The DSL-2740R and all hardware revisions are considered End of Life and as such this issue will not be patched.
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Analysis
by VulDB Data Team • 08/04/2024
The CVE-2021-29294 vulnerability represents a critical null pointer dereference flaw discovered in D-Link DSL-2740R UK_1.01 routers, specifically affecting the send_hnap_unauthorized function within the device's web interface. This vulnerability falls under the CWE-476 category, which addresses null pointer dereference conditions that can lead to system instability and denial of service. The affected device operates a web server that handles HNAP (Home Network Access Protocol) requests, making it susceptible to exploitation through crafted HTTP POST requests sent to the /HNAP1/ endpoint. The vulnerability is particularly concerning because it demonstrates how legacy network equipment can contain fundamental programming errors that persist even after product lifecycle end-of-life.
The technical exploitation of this vulnerability occurs when a remote attacker crafts a malicious POST request containing malformed data that triggers the send_hnap_unauthorized function to attempt dereferencing a null pointer. This programming error causes the router's web server process to crash or become unresponsive, effectively creating a denial of service condition that prevents legitimate users from accessing the device's management interface. The vulnerability is classified as a remote code execution risk within the context of the ATT&CK framework under the T1210 technique for exploitation of remote services, though in this specific case it manifests as a denial of service rather than code execution. The null pointer dereference is a classic software bug that occurs when a program attempts to access memory through a pointer that has not been properly initialized or has been set to null, leading to system termination or instability.
The operational impact of this vulnerability extends beyond simple service disruption as it affects network infrastructure that many users rely upon for internet connectivity and local network management. When the router becomes unresponsive due to this flaw, users lose access to their network configuration settings, internet connectivity, and potentially cannot manage connected devices through the router's web interface. This vulnerability is particularly dangerous in enterprise or home office environments where network reliability is critical, as it can be exploited by unauthorized parties to disrupt network services without requiring authentication or specialized knowledge beyond basic web request crafting. The issue affects all hardware revisions of the DSL-2740R model, indicating that this is a fundamental architectural flaw rather than a specific firmware bug that might have been corrected in later versions.
Given that D-Link has officially designated the DSL-2740R and all its hardware revisions as end of life, no official patches or firmware updates will be provided to address this vulnerability. This leaves affected users with limited remediation options, forcing them to either replace the device entirely or implement network-level mitigations. Organizations should consider network segmentation strategies to isolate these legacy devices, implement intrusion detection systems to monitor for exploitation attempts, and establish procedures for device retirement and replacement. The vulnerability highlights the importance of maintaining up-to-date network equipment and the risks associated with continuing to use unsupported devices in production environments. Users should be advised to upgrade to newer router models that are actively supported with security updates and patches to prevent exploitation of known vulnerabilities like CVE-2021-29294.