CVE-2024-51251 is a command injection vulnerability identified in the Draytek Vigor3900 router firmware version 1.5.1.3. The flaw exists in the mainfunction.cgi endpoint, specifically when handling the backup function. An attacker with low privileges (PR:L) and no need for user interaction (UI:N) can inject malicious commands that the system executes with the privileges of the affected process. The vulnerability is remotely exploitable over an adjacent network (AV:A), meaning the attacker must have network access but does not require local access. The vulnerability falls under CWE-78, indicating improper neutralization of OS command elements, which allows injection of arbitrary commands. The CVSS v3.1 vector (AV:A/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H) reflects high impact on confidentiality, integrity, and availability, with low attack complexity and no user interaction needed. Although no public exploits have been reported yet, the vulnerability's characteristics suggest it could be weaponized quickly. The lack of available patches at the time of publication increases the urgency for organizations to implement interim mitigations. Draytek Vigor3900 routers are commonly used in enterprise and ISP environments, making this vulnerability a significant threat to network infrastructure.

Successful exploitation of CVE-2024-51251 allows attackers to execute arbitrary commands on the affected Draytek Vigor3900 devices, leading to full compromise of the router. This can result in unauthorized access to sensitive network traffic, disruption of network services, and potential pivoting into internal networks. The confidentiality of data passing through the device can be breached, integrity of network configurations can be altered, and availability can be disrupted by executing destructive commands or causing denial of service. Organizations relying on these routers for critical network functions face risks including data exfiltration, network downtime, and further compromise of connected systems. The vulnerability's low complexity and lack of user interaction requirements increase the likelihood of exploitation, especially in environments where network access controls are weak or management interfaces are exposed.

1. Immediately restrict access to the Draytek Vigor3900 management interface to trusted IP addresses only, preferably via VPN or secure management networks. 2. Implement strict network segmentation to isolate management interfaces from general user networks and the internet. 3. Monitor network traffic for unusual requests to mainfunction.cgi, especially those invoking the backup function, and set up alerts for suspicious command injection patterns. 4. Disable or limit the backup function if possible until a vendor patch is available. 5. Regularly audit router configurations and logs for signs of compromise or unauthorized changes. 6. Engage with Draytek support to obtain firmware updates or patches addressing this vulnerability as soon as they are released. 7. Employ intrusion detection/prevention systems (IDS/IPS) with signatures targeting command injection attempts on Draytek devices. 8. Educate network administrators on this vulnerability and enforce strong authentication and access policies for device management.