CVE-2026-4903 is a stack-based buffer overflow vulnerability identified in the Tenda AC5 router firmware version 15.03.06.47. The vulnerability resides in the formQuickIndex function of the /goform/QuickIndex endpoint, which processes POST requests. Specifically, the PPPOEPassword parameter is improperly handled, allowing an attacker to supply crafted input that overflows the stack buffer. This overflow can overwrite critical control data on the stack, enabling remote code execution with elevated privileges on the device. The vulnerability does not require authentication or user interaction, making it exploitable over the network by unauthenticated attackers. The CVSS 4.0 base score is 8.7, reflecting its high impact on confidentiality, integrity, and availability, combined with low attack complexity and no required privileges. While no confirmed exploits are currently active in the wild, proof-of-concept exploits have been published, increasing the likelihood of future attacks. The affected product, Tenda AC5, is a widely used consumer and small office router, often deployed in regions with growing internet infrastructure. The lack of available patches at the time of disclosure further elevates the risk. Attackers exploiting this vulnerability could gain full control of the router, enabling interception or manipulation of network traffic, disruption of services, or pivoting to internal networks.

The impact of CVE-2026-4903 is significant for organizations and individuals using the Tenda AC5 router with vulnerable firmware. Successful exploitation allows remote attackers to execute arbitrary code with elevated privileges, potentially leading to full device compromise. This can result in unauthorized access to internal networks, interception or modification of sensitive data, disruption of internet connectivity, and use of the compromised device as a foothold for further attacks. Enterprises relying on these routers for network perimeter security or VPN connectivity could face severe confidentiality and integrity breaches. Additionally, the availability of the network could be compromised, impacting business operations. The vulnerability's ease of exploitation and lack of authentication requirements increase the risk of widespread attacks, especially in regions where Tenda AC5 devices are prevalent. Without timely remediation, organizations may experience data breaches, service outages, and reputational damage.

To mitigate CVE-2026-4903, organizations should immediately verify if their network infrastructure includes Tenda AC5 routers running firmware version 15.03.06.47. Since no official patches are currently available, temporary mitigations include restricting remote access to the router's management interfaces by implementing strict firewall rules and disabling remote administration features. Network segmentation should be enforced to isolate vulnerable devices from critical assets. Monitoring network traffic for unusual POST requests to /goform/QuickIndex and anomalous authentication attempts can help detect exploitation attempts. Organizations should also consider replacing vulnerable devices with models from vendors providing timely security updates. Once a vendor patch is released, prompt firmware updates are essential. Additionally, applying network intrusion detection/prevention systems (IDS/IPS) with signatures targeting this vulnerability can reduce risk. Educating network administrators about this vulnerability and encouraging regular firmware audits will improve overall security posture.