CVE-2025-8939 in AC20
Summary
by MITRE • 08/14/2025
A vulnerability was determined in Tenda AC20 up to 16.03.08.12. Affected is an unknown function of the file /goform/WifiGuestSet. The manipulation of the argument shareSpeed leads to buffer overflow. It is possible to launch the attack remotely. The exploit has been disclosed to the public and may be used.
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Analysis
by VulDB Data Team • 08/15/2025
The vulnerability identified as CVE-2025-8939 affects the Tenda AC20 wireless router firmware version 16.03.08.12 and earlier, representing a critical buffer overflow flaw within the device's web interface management system. This vulnerability resides in the /goform/WifiGuestSet endpoint, which serves as a communication interface for configuring guest network settings. The specific function affected handles the shareSpeed parameter, which is designed to control the bandwidth sharing configuration for guest network users. The flaw manifests when the shareSpeed argument is manipulated beyond its intended buffer size limits, creating an exploitable condition that allows arbitrary code execution. This issue represents a classic buffer overflow vulnerability that falls under CWE-121, which describes stack-based buffer overflow conditions where insufficient bounds checking permits data to overwrite adjacent memory locations. The remote exploitation capability of this vulnerability means that attackers can leverage the web interface without requiring physical access to the device, making it particularly dangerous for enterprise and residential deployments.
The technical implementation of this vulnerability stems from inadequate input validation within the firmware's form processing logic. When the web interface receives a malformed shareSpeed parameter through the /goform/WifiGuestSet endpoint, the application fails to properly validate the input length against the allocated buffer space. This allows an attacker to inject data that exceeds the predetermined buffer boundaries, potentially overwriting critical memory segments including return addresses, function pointers, or other control data structures. The attack vector operates through standard HTTP requests that target the router's web management interface, typically accessible via ports 80 or 443. Given that the exploit has been publicly disclosed, threat actors can readily develop automated tools to exploit this condition across vulnerable Tenda AC20 devices, potentially enabling full system compromise including persistent backdoor installation, data exfiltration, or lateral movement within network environments.
The operational impact of CVE-2025-8939 extends beyond simple privilege escalation to encompass complete system takeover capabilities that align with ATT&CK technique T1059.007 for command and script interpreter usage. An exploited device could serve as a persistent foothold for attackers to conduct reconnaissance, establish command and control channels, or launch further attacks against connected network segments. The vulnerability affects not only individual users but also enterprise networks where multiple Tenda AC20 devices may be deployed, potentially creating widespread compromise opportunities. Network administrators face significant challenges in identifying and remediating this issue since the attack surface includes publicly accessible web interfaces that may not be properly secured or monitored. The remote nature of the exploit means that devices could be compromised from anywhere on the internet, making traditional network perimeter security insufficient to prevent exploitation. This vulnerability particularly impacts organizations relying on consumer-grade networking equipment for critical infrastructure components, as these devices often lack proper security monitoring and update mechanisms.
Mitigation strategies for CVE-2025-8939 should prioritize immediate firmware updates from Tenda's official sources, as this represents the most effective remediation approach to address the root cause of the buffer overflow. Network segmentation techniques should be implemented to isolate affected devices from critical network segments, reducing potential lateral movement opportunities for attackers who successfully exploit the vulnerability. Security monitoring should include detection of anomalous requests to the /goform/WifiGuestSet endpoint, particularly those containing unusually long shareSpeed parameters. Access control measures such as restricting web interface access to trusted IP ranges or implementing multi-factor authentication can provide additional layers of protection. Organizations should consider disabling guest network functionality entirely if it is not required, as this reduces the attack surface for exploitation. Regular vulnerability scanning and penetration testing of network infrastructure can help identify other potentially vulnerable devices that may share similar firmware vulnerabilities. The implementation of network intrusion detection systems capable of identifying and blocking malicious traffic patterns associated with this specific exploit can provide real-time protection against active exploitation attempts. Additionally, maintaining comprehensive network documentation including firmware versions and security configurations enables rapid response and remediation when similar vulnerabilities are discovered in other network components.