CVE-2021-31755

The stack buffer overflow vulnerability identified in Tenda AC11 devices presents a significant risk to the integrity and security of affected systems. This flaw arises from improper handling of input data in the /goform/setmac function, which processes MAC address settings. When a crafted POST request is sent to the device, it can overflow the buffer allocated for input, allowing attackers to overwrite adjacent memory. This can lead to arbitrary code execution, enabling an attacker to gain control over the device, alter its configuration, or deploy malicious payloads. The severity of this vulnerability is underscored by its high CVSS score of 9.8, indicating critical risk.

Exploitation of this vulnerability can occur through various attack vectors. An attacker could leverage remote access to the device, potentially targeting users on the same network or exploiting unsecured internet-facing interfaces. For instance, a malicious actor could craft a specially formatted request that exceeds the buffer size, leading to code execution. This could be executed by sending the crafted request via a simple HTTP POST method, making it accessible to individuals with minimal technical expertise. Furthermore, the lack of authentication mechanisms in the affected firmware exacerbates the risk, as attackers do not need to bypass any security controls to exploit the vulnerability.

The real-world impact of this vulnerability can be profound, particularly for businesses relying on Tenda AC11 devices for network connectivity. Successful exploitation could lead to unauthorized access to sensitive data, disruption of network services, or even the establishment of a foothold for further attacks within an organization’s infrastructure. The potential for data breaches and the subsequent financial and reputational damage cannot be overstated. Organizations may face regulatory scrutiny, loss of customer trust, and significant recovery costs, all of which contribute to a heightened business risk profile.

To effectively detect and mitigate the risks associated with this vulnerability, organizations should implement a multi-layered security strategy. Regularly updating firmware to the latest versions is crucial, as manufacturers often release patches to address known vulnerabilities. Additionally, network segmentation can help isolate vulnerable devices from critical systems, reducing the potential impact of an exploit. Intrusion detection systems (IDS) can be employed to monitor for unusual traffic patterns or unauthorized access attempts, providing an additional layer of defense. Organizations should also conduct regular security assessments and penetration testing to identify and remediate vulnerabilities proactively.

In conclusion, the stack buffer overflow vulnerability in Tenda AC11 devices poses a serious threat to network security. Its ease of exploitation and potential for significant impact necessitate immediate attention from organizations utilizing these devices. By adopting robust detection and mitigation strategies, businesses can better protect themselves against the risks associated with this vulnerability, ensuring the integrity and security of their networks.

CSURFACE threat intelligence has detected a marked escalation in activity related to CVE-2021-31755, with new indications of exploitation attempts emerging after a period of dormancy. While no novel exploit techniques or ransomware affiliations have been identified, the resurgence of targeting against Tenda AC11 devices underscores a renewed adversary interest. This uptick in telemetry suggests that threat actors may be actively probing for vulnerable systems or testing weaponization methods, increasing the likelihood of successful compromise. Consequently, the risk level associated with this vulnerability has shifted from theoretical to more imminent, warranting heightened vigilance in monitoring network traffic and device behavior for signs of exploitation.

Update 2 — June 07, 2026

CSURFACE threat intelligence has identified a marked escalation in activity targeting the CVE-2021-31755 vulnerability on Tenda AC11 routers. Our telemetry indicates that adversaries are increasingly engaging in reconnaissance and exploitation attempts, suggesting a shift from opportunistic scanning to more deliberate intrusion efforts. Although no new exploit variants or ransomware affiliations have been detected, the intensified probing heightens the probability of successful compromise, especially in environments where patching remains incomplete. This development elevates the threat posture from a latent risk to an active concern, underscoring the necessity for continuous monitoring of network traffic and device logs to detect anomalous behavior indicative of exploitation attempts.

Update 3 — June 16, 2026

CSURFACE threat intelligence has detected a modest uptick in activity targeting the CVE-2021-31755 vulnerability on Tenda AC11 devices, reflected by a slight increase in exploitation attempts observed across our sensors. Although the EPSS score has marginally declined, indicating a subtle reduction in predicted exploit probability, the increased detection frequency suggests adversaries are maintaining or slightly intensifying their focus on this vector. This divergence underscores the persistence of threat actors leveraging this critical stack buffer overflow, despite no new exploit variants or ransomware affiliations emerging. For defenders, this signals that the vulnerability remains an active attack surface with ongoing reconnaissance and exploitation attempts, particularly in environments where patch deployment is incomplete or delayed. Consequently, the threat level should be considered elevated from latent to actively targeted, warranting sustained vigilance in monitoring network and device behaviors to promptly identify and respond to exploitation indicators.

Update 4 — July 06, 2026

CSURFACE threat intelligence has identified a marked escalation in exploitation attempts targeting the CVE-2021-31755 vulnerability on Tenda AC11 devices. Our telemetry indicates that adversaries are increasingly leveraging this stack buffer overflow flaw, as evidenced by a significant uptick in detection activity across multiple monitored environments. Although no novel exploit variants or ransomware affiliations have surfaced, the sustained and growing exploitation attempts underscore that threat actors continue to prioritize this vulnerability as a viable attack vector. This heightened activity suggests that unpatched or inadequately secured devices remain at considerable risk, amplifying the urgency for defenders to maintain robust monitoring and incident response protocols. Consequently, the threat level associated with CVE-2021-31755 should be elevated from a latent concern to an actively exploited risk, reflecting the evolving adversary focus and persistent targeting observed in recent intelligence.