CVE-1999-1559 is a vulnerability affecting Xylan OmniSwitch devices prior to version 3.2.6, a network switch product by Alcatel. The vulnerability allows remote attackers to bypass the login prompt by sending a CTRL-D (control-D) character to the device. This character causes the switch to bypass authentication and grants access without requiring valid credentials. Additionally, because the OmniSwitch supports only a single session at a time, this exploit can lock out legitimate users from accessing the device, effectively causing a denial of service (DoS). The vulnerability does not impact confidentiality or integrity directly, as it does not provide unauthorized access to sensitive data or allow modification of configurations, but it does impact availability by preventing legitimate administrative access. The CVSS score is 5.0 (medium severity), reflecting the network vector (remote exploit), low attack complexity, no authentication required, no confidentiality or integrity impact, but partial availability impact. No patches are available for this vulnerability, and there are no known exploits in the wild. Given the age of the vulnerability (published in 1999) and the product version affected, this issue primarily concerns legacy systems still in operation.

For European organizations, the primary impact of this vulnerability is the potential denial of service on network management interfaces of affected OmniSwitch devices. Organizations relying on these switches for critical network infrastructure could experience administrative lockout, preventing timely configuration changes or incident response. This could lead to prolonged network outages or degraded network performance if the switch cannot be managed or rebooted remotely. While the vulnerability does not allow data exfiltration or direct compromise of network traffic, the loss of administrative control can indirectly affect network security posture and operational continuity. European enterprises in sectors with legacy network equipment, such as industrial, telecommunications, or government agencies, may be particularly vulnerable if they have not upgraded or replaced affected OmniSwitch models. The lack of a patch means organizations must rely on compensating controls or device replacement to mitigate risk.

Since no patch is available for this vulnerability, European organizations should consider the following specific mitigation steps: 1) Identify and inventory all Xylan OmniSwitch devices in their network environment, focusing on versions prior to 3.2.6. 2) Limit network access to the management interfaces of these switches by implementing strict access control lists (ACLs) or firewall rules that restrict access to trusted administrative hosts only. 3) Use network segmentation to isolate legacy switches from general user networks to reduce exposure to remote attackers. 4) Monitor network traffic for unusual patterns, such as unexpected CTRL-D characters or repeated login attempts that could indicate exploitation attempts. 5) Where possible, replace or upgrade affected OmniSwitch devices to versions that are not vulnerable or migrate to modern switch hardware with supported security features. 6) Implement out-of-band management channels for critical network devices to ensure administrative access remains available even if the primary management interface is compromised or locked out. 7) Train network administrators to recognize and respond to signs of this vulnerability exploitation, including lockouts and unexpected session terminations.