CVE-1999-1559 in OmniSwitch
Summary
by MITRE
Xylan OmniSwitch before 3.2.6 allows remote attackers to bypass the login prompt via a CTRL-D (control d) character, which locks other users out of the switch because it only supports one session at a time.
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Analysis
by VulDB Data Team • 04/19/2026
The vulnerability identified as CVE-1999-1559 represents a critical authentication bypass flaw in Xylan OmniSwitch network equipment running versions prior to 3.2.6. This issue stems from a fundamental design weakness in the switch's session management and input handling mechanisms, creating a pathway for unauthorized remote access that fundamentally undermines the security model of the device. The vulnerability specifically exploits the system's response to control characters, particularly the CTRL-D (control d) character, which serves as an unintended escape mechanism that bypasses normal authentication procedures. This flaw operates at the core of the switch's user interface layer, where the system fails to properly validate or sanitize input sequences that could alter the normal operational flow of user sessions.
The technical implementation of this vulnerability demonstrates a classic case of insufficient input validation and improper session handling, which can be categorized under CWE-20, "Improper Input Validation," and CWE-305, "Authentication Bypass Through User-Controlled Key." When a remote attacker sends a CTRL-D character to the switch during the login process, the system interprets this as a session termination command rather than an authentication bypass attempt. This misinterpretation occurs because the switch's input processing logic does not adequately distinguish between legitimate session control commands and malicious input designed to exploit the single-session limitation. The vulnerability essentially creates a race condition where the system's response to the control character results in immediate session termination, allowing the attacker to gain access while simultaneously locking out legitimate users who are already authenticated or attempting to log in.
The operational impact of this vulnerability extends beyond simple unauthorized access, creating a denial of service scenario that affects legitimate users and potentially compromising network security. Since the switch only supports one session at a time, the vulnerability creates a situation where any successful exploitation results in immediate lockout of other users, effectively creating a privilege escalation scenario where an attacker can not only gain access but also prevent legitimate administrators from accessing the device. This vulnerability directly aligns with ATT&CK technique T1078.004, "Valid Accounts: SSH," as it exploits the authentication mechanism through an unintended input sequence rather than through credential theft or brute force attacks. The impact is particularly severe in network environments where switch management is critical for network operations, as unauthorized access to switch configuration can lead to complete network compromise, traffic interception, or disruption of network services.
Mitigation strategies for this vulnerability require immediate implementation of firmware updates to version 3.2.6 or later, which address the flawed input handling and session management logic. Network administrators should also implement additional security controls such as restricting remote access to switch management interfaces through network segmentation and implementing strict access control lists that limit which IP addresses can reach management ports. The vulnerability highlights the importance of proper input sanitization and session management in network infrastructure devices, emphasizing that even seemingly benign control characters can represent security threats when not properly handled. Organizations should also consider implementing monitoring solutions that can detect unusual input patterns or session termination events that might indicate exploitation attempts. This vulnerability serves as a reminder that network security must consider not just traditional attack vectors but also the potential for exploitation through unexpected input sequences, particularly in legacy network equipment where security updates may be limited or delayed.