Executive Summary

[{'type': 'paragraph', 'content': 'CVE-2026-2906 is a critical stack-based buffer overflow vulnerability found in the Tenda HG9 router firmware version 300001138, specifically in the Samba Configuration Endpoint at /boaform/formSamba.'}, {'type': 'paragraph', 'content': 'The vulnerability occurs when the argument sambaCap is set to 1, causing the function to retrieve the user-supplied serverString parameter and use sprintf to format a system command string into a small fixed-size stack buffer of 64 bytes.'}, {'type': 'paragraph', 'content': "Because sprintf does not perform bounds checking, if serverString is longer than about 40 bytes, it overflows the buffer, overwriting the function's return address on the stack."}, {'type': 'paragraph', 'content': 'This unsafe use of sprintf without boundary checks leads to the stack-based buffer overflow.'}] [1, 2, 3]

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Impact Analysis

[{'type': 'paragraph', 'content': 'This vulnerability can have two main impacts:'}, {'type': 'list_item', 'content': "Denial of Service (DoS): Overwriting the return address or stack can cause the router's HTTP daemon process to crash, making the web interface unresponsive."}, {'type': 'list_item', 'content': 'Remote Code Execution (RCE): An attacker can craft a payload to overwrite the return address with a pointer to malicious code, enabling arbitrary command execution with root privileges on the router.'}, {'type': 'paragraph', 'content': 'Because the exploit can be launched remotely without local access, it poses a severe security risk, potentially allowing attackers to fully compromise the device.'}] [1, 2, 3]

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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 sending a crafted POST request to the vulnerable endpoint `/boaform/formSamba` with the parameter `sambaCap=1` and a `serverString` parameter containing a long string (e.g., 200 characters) to trigger the buffer overflow.'}, {'type': 'paragraph', 'content': 'A proof-of-concept Python script exists that demonstrates exploitation by sending such a POST request, which can be adapted for detection purposes.'}, {'type': 'paragraph', 'content': "For manual testing, you can use the following curl command to check if the system is vulnerable by observing if the router's web interface crashes or becomes unresponsive:"}, {'type': 'list_item', 'content': 'curl -X POST http://[router-ip]/boaform/formSamba -d "sambaCap=1&serverString=$(python3 -c \'print("A"*200)\')"'}, {'type': 'paragraph', 'content': 'If the router crashes or the web interface becomes unresponsive after this request, it indicates the presence of the vulnerability.'}] [1]

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Mitigation Strategies

[{'type': 'paragraph', 'content': 'Immediate mitigation steps include:'}, {'type': 'list_item', 'content': "Avoid exposing the router's web interface to untrusted networks to reduce the risk of remote exploitation."}, {'type': 'list_item', 'content': 'Implement network-level filtering or firewall rules to block access to the `/boaform/formSamba` endpoint from unauthorized sources.'}, {'type': 'list_item', 'content': 'If possible, disable the Samba service or the vulnerable endpoint until a firmware update or patch is available.'}, {'type': 'list_item', 'content': "Monitor for unusual crashes or instability in the router's web interface, which may indicate exploitation attempts."}, {'type': 'paragraph', 'content': 'Long-term remediation involves updating the firmware to a version where the vulnerability is fixed by replacing unsafe `sprintf` calls with `snprintf`, enforcing input validation, or increasing buffer sizes.'}, {'type': 'paragraph', 'content': 'If no patch is available, consider replacing the affected device with a more secure alternative.'}] [1, 2]

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