CVE-2025-22939 is a critical command injection vulnerability identified in the telnet service of the Adtran 411 Optical Network Terminal (ONT) running firmware version L80.00.0011.M2. This vulnerability allows unauthenticated remote attackers to execute arbitrary commands with root privileges on the affected device. The root cause is improper input validation in the telnet service, which enables attackers to inject shell commands (classified under CWE-77: Improper Neutralization of Special Elements used in a Command). Exploitation requires no authentication or user interaction and can be performed remotely over the network. The vulnerability has a CVSS v3.1 base score of 9.8, reflecting its critical severity with high impact on confidentiality, integrity, and availability. Successful exploitation can lead to full system compromise, enabling attackers to control the ONT device, intercept or manipulate network traffic, disrupt service, or use the device as a pivot point for further attacks within the network. Although no public exploits have been reported yet, the severity and ease of exploitation make it a significant threat, especially in environments where these ONT devices are deployed as part of broadband infrastructure.

For European organizations, this vulnerability poses a substantial risk, particularly to Internet Service Providers (ISPs), telecommunications companies, and enterprises relying on Adtran 411 ONT devices for broadband connectivity. A compromised ONT can lead to network outages, data interception, and unauthorized access to internal networks, impacting service availability and customer trust. Given the critical role of ONTs in last-mile connectivity, exploitation could disrupt large numbers of end-users and critical infrastructure services. Additionally, attackers gaining root access could manipulate firmware or configurations to establish persistent backdoors, complicating incident response. The potential for lateral movement within corporate or service provider networks increases the threat to broader organizational assets. The vulnerability also raises concerns for compliance with European data protection regulations (e.g., GDPR) due to the risk of data breaches stemming from compromised network devices.

Immediate mitigation steps include isolating affected Adtran 411 ONT devices from untrusted networks and disabling the telnet service if possible, replacing it with more secure management protocols such as SSH. Network segmentation should be enforced to limit access to management interfaces. Monitoring network traffic for unusual telnet activity and implementing intrusion detection systems tuned to detect command injection attempts can help identify exploitation attempts. Since no patches are currently available, organizations should engage with Adtran support for firmware updates or advisories. Applying strict access control lists (ACLs) to restrict telnet access to trusted management hosts and employing multi-factor authentication where possible will reduce risk. Additionally, organizations should conduct thorough audits of ONT configurations and logs to detect signs of compromise and prepare incident response plans tailored to potential ONT exploitation scenarios.