CVE-2026-1637 is a stack-based buffer overflow vulnerability found in the Tenda AC21 router firmware version 16.03.08.16, specifically in the function fromAdvSetMacMtuWan located in the /goform/AdvSetMacMtuWan endpoint. This vulnerability occurs due to improper input validation or bounds checking when handling parameters related to MAC and MTU settings on the WAN interface, leading to a stack overflow condition. The flaw allows remote attackers to send crafted requests to the vulnerable endpoint, triggering the overflow and potentially enabling arbitrary code execution on the device. The vulnerability does not require authentication or user interaction, making it highly exploitable remotely over the network. The CVSS 4.0 vector indicates network attack vector (AV:N), low attack complexity (AC:L), no privileges required (PR:L), no user interaction (UI:N), and high impact on confidentiality, integrity, and availability (VC:H, VI:H, VA:H). Although no confirmed exploits are currently observed in the wild, proof-of-concept exploit code is publicly available, increasing the likelihood of exploitation by threat actors. The vulnerability affects a widely deployed consumer and small business router model, which is often used in home and office environments, making it a significant risk for network security. The lack of an official patch at the time of reporting necessitates immediate mitigation efforts to reduce exposure.

The impact of CVE-2026-1637 on European organizations can be severe. Successful exploitation allows remote attackers to execute arbitrary code on affected Tenda AC21 routers, potentially leading to full device compromise. This can result in interception or manipulation of network traffic, disruption of internet connectivity, and use of the compromised device as a foothold for lateral movement or launching further attacks within the network. Confidentiality is at risk as attackers may capture sensitive data passing through the router. Integrity can be compromised by altering routing or firewall rules, while availability may be affected by causing device crashes or denial of service. European organizations relying on Tenda AC21 devices for critical connectivity, especially small and medium enterprises or remote offices, face increased risk of operational disruption and data breaches. The presence of publicly available exploit code further elevates the threat level, making timely mitigation essential to prevent exploitation.

1. Immediately monitor Tenda’s official channels for firmware updates addressing CVE-2026-1637 and apply patches as soon as they are released. 2. Until patches are available, disable remote management interfaces (such as WAN-side web administration) to reduce exposure to remote attacks. 3. Segment networks to isolate vulnerable routers from critical infrastructure and sensitive data environments. 4. Implement strict firewall rules to restrict inbound traffic to router management ports only from trusted IP addresses. 5. Conduct network traffic monitoring and anomaly detection to identify suspicious requests targeting the /goform/AdvSetMacMtuWan endpoint. 6. Replace or upgrade devices that cannot be patched in a timely manner with more secure alternatives. 7. Educate IT staff about the vulnerability and ensure incident response plans include steps for handling potential exploitation. 8. Regularly audit router configurations to ensure no unnecessary services or open ports are exposed externally.