Executive Summary

[{'type': 'paragraph', 'content': 'CVE-2026-2871 is a critical stack-based buffer overflow vulnerability found in the Tenda A21 router firmware version 1.0.0.0, specifically in the fromSetIpMacBind function of the /goform/SetIpMacBind endpoint.'}, {'type': 'paragraph', 'content': 'The vulnerability arises because the function processes a "list" parameter containing IP-MAC binding rules without validating the input length before copying it into a fixed-size 128-byte stack buffer using the unsafe strcpy function.'}, {'type': 'paragraph', 'content': 'Additionally, the parsed data is passed to the set_device_name function, which uses unsafe sprintf calls on a 256-byte buffer without bounds checking, creating multiple points for stack corruption.'}, {'type': 'paragraph', 'content': 'This improper input validation and unsafe string handling allow an attacker to cause a stack-based buffer overflow remotely, potentially overwriting the stack frame and enabling arbitrary code execution or denial of service.'}, {'type': 'paragraph', 'content': 'A proof-of-concept exploit is publicly available, demonstrating how sending an excessively large "list" parameter via HTTP POST to the vulnerable endpoint can crash the service or allow remote code execution.'}] [1, 2, 3, 4]

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Impact Analysis

This vulnerability can have severe impacts on the affected device and its users.

• Remote Code Execution (RCE): An attacker can exploit the buffer overflow to execute arbitrary code with root privileges on the router.
• Denial of Service (DoS): Malformed or oversized input can crash the HTTP daemon, causing the router to become unresponsive.
• Compromise of Confidentiality, Integrity, and Availability: The attacker can manipulate device behavior, intercept or alter network traffic, or disrupt network services.

Because the exploit is remotely executable without authentication, attackers can target vulnerable devices over the network, increasing the risk of widespread compromise.

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Compliance Impact

I don't know

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Detection Guidance

[{'type': 'paragraph', 'content': 'This vulnerability can be detected by monitoring HTTP POST requests sent to the endpoint /goform/SetIpMacBind on Tenda A21 routers running firmware version 1.0.0.0.'}, {'type': 'paragraph', 'content': 'Specifically, detection involves identifying unusually large or malformed "list" parameters in the POST data, which are used to trigger the stack-based buffer overflow.'}, {'type': 'paragraph', 'content': 'A proof-of-concept exploit sends an excessively large string (e.g., 10,000 \'A\' characters) in the "list" parameter to cause the overflow.'}, {'type': 'list_item', 'content': 'Use network monitoring tools (e.g., Wireshark or tcpdump) to capture HTTP POST requests to /goform/SetIpMacBind and inspect the "list" parameter size and content.'}, {'type': 'list_item', 'content': "Example tcpdump command to capture relevant traffic: tcpdump -i <interface> -A 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'"}, {'type': 'list_item', 'content': 'Use curl or similar tools to test the endpoint by sending crafted POST requests with large "list" parameters to observe if the device crashes or behaves abnormally.'}, {'type': 'list_item', 'content': 'Example curl command to test: curl -X POST http://<router-ip>/goform/SetIpMacBind -d \'list=$(python3 -c "print(\'A\'*10000)")\''}] [1]

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Mitigation Strategies

[{'type': 'paragraph', 'content': 'Immediate mitigation steps include restricting access to the vulnerable endpoint /goform/SetIpMacBind to trusted networks only, such as by firewall rules or network segmentation.'}, {'type': 'paragraph', 'content': "Disabling remote management or access to the router's web interface from untrusted networks can reduce exposure."}, {'type': 'paragraph', 'content': 'Monitor the device for abnormal behavior such as crashes or reboots, which may indicate exploitation attempts.'}, {'type': 'paragraph', 'content': 'Since no official patches or countermeasures currently exist, consider replacing the affected device with a non-vulnerable alternative.'}, {'type': 'paragraph', 'content': 'Avoid using the IP-MAC binding feature or the vulnerable function until a fix is available.'}] [1, 4]

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