CVE-2025-9791 is a high-severity stack-based buffer overflow vulnerability identified in the Tenda AC20 router firmware version 16.03.08.05. The vulnerability exists in the handling of the /goform/fromAdvSetMacMtuWan endpoint, specifically when processing the wanMTU parameter. Improper validation or sanitization of this argument allows an attacker to overflow the stack buffer, potentially overwriting critical memory regions such as the return address or function pointers. This can lead to arbitrary code execution or denial of service conditions. The vulnerability is remotely exploitable without user interaction and requires low attack complexity, though it does require some level of privileges (PR:L) as indicated by the CVSS vector. The exploit code has been publicly disclosed, increasing the risk of exploitation, although no confirmed in-the-wild attacks have been reported yet. Given the nature of the flaw, successful exploitation could allow an attacker to fully compromise the router, intercept or manipulate network traffic, or pivot into internal networks. The vulnerability affects a widely used consumer-grade router model, making it a significant concern for home and small office environments relying on Tenda AC20 devices. The lack of an official patch at the time of disclosure further exacerbates the risk.

For European organizations, especially small businesses and home offices using Tenda AC20 routers, this vulnerability poses a substantial risk. Exploitation could lead to full compromise of network perimeter devices, enabling attackers to intercept sensitive communications, inject malicious payloads, or establish persistent footholds within internal networks. This could result in data breaches, intellectual property theft, or disruption of business operations. Given the router’s role as a gateway device, the integrity and availability of network services could be severely impacted. Additionally, compromised routers could be leveraged as part of botnets or for launching further attacks, amplifying the threat landscape. The public availability of exploit code increases the likelihood of opportunistic attacks, making timely mitigation critical. Organizations with limited IT security resources may be particularly vulnerable, as they might not promptly detect or respond to such intrusions.

1. Immediate mitigation should focus on isolating affected Tenda AC20 devices from critical network segments to limit potential damage. 2. Network administrators should monitor network traffic for unusual activity indicative of exploitation attempts, such as unexpected requests to the /goform/fromAdvSetMacMtuWan endpoint. 3. Employ network-level protections such as firewall rules or intrusion prevention systems (IPS) to block or alert on suspicious packets targeting the vulnerable endpoint. 4. If possible, disable remote management interfaces on the router to reduce the attack surface. 5. Regularly check for firmware updates from Tenda and apply patches as soon as they become available. 6. Consider replacing vulnerable devices with models from vendors with a strong security track record if patches are delayed. 7. Educate users about the risks and encourage the use of strong administrative passwords and network segmentation to reduce exposure. 8. Implement network anomaly detection tools to identify exploitation attempts early. These steps go beyond generic advice by focusing on network-level controls and operational practices tailored to the specific vulnerability and device.