CVE-1999-1257 in Terminal Server
Summary
by MITRE
Xyplex terminal server 6.0.1S1, and possibly other versions, allows remote attackers to bypass the password prompt by entering (1) a CTRL-Z character, or (2) a ? (question mark).
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Analysis
by VulDB Data Team • 04/17/2026
The vulnerability identified as CVE-1999-1257 affects Xyplex terminal servers running version 6.0.1S1 and potentially other versions within the same release line. This represents a critical authentication bypass flaw that fundamentally undermines the security model of the terminal server by allowing unauthorized access through seemingly innocuous input sequences. The vulnerability resides in the terminal server's input processing mechanism where specific control characters are not properly validated or handled during the authentication process, creating a pathway for remote attackers to circumvent the password protection entirely.
The technical exploitation of this vulnerability occurs through two distinct methods that leverage the terminal server's handling of special characters. The first method involves sending a CTRL-Z character, which is a control character traditionally used to signal end-of-file in many terminal environments. When this character is entered during the authentication process, the terminal server fails to properly process it and instead accepts it as valid input, allowing the attacker to proceed past the password prompt without authentication. The second method utilizes a question mark character, which similarly bypasses the authentication checks through improper input validation. Both approaches exploit a fundamental flaw in the input sanitization process where the terminal server does not adequately filter or reject these specific characters during authentication, treating them as legitimate input rather than potential attack vectors.
The operational impact of this vulnerability extends far beyond simple unauthorized access, as it represents a complete breakdown in the authentication security model of the affected terminal servers. Attackers can gain administrative access to network infrastructure without requiring valid credentials, potentially leading to complete network compromise, data exfiltration, and unauthorized system modifications. The remote nature of the attack means that threat actors can exploit this vulnerability from anywhere on the network, making it particularly dangerous in enterprise environments where terminal servers are often exposed to external networks or used for remote access. This vulnerability directly violates the principle of least privilege and undermines the confidentiality, integrity, and availability of the affected systems, as unauthorized parties can access sensitive network resources and potentially escalate their privileges to gain full administrative control.
The underlying technical flaw in this vulnerability aligns with CWE-254, which addresses security weaknesses related to improper input validation and inadequate handling of special characters in authentication mechanisms. This vulnerability also maps to several ATT&CK techniques including T1110.001 for password guessing and credential dumping, as well as T1078 for valid accounts and T1566 for social engineering attacks that exploit authentication bypasses. Organizations should implement immediate mitigations including patching to the latest available versions of the Xyplex terminal server software, implementing network segmentation to limit access to these critical systems, and deploying additional authentication layers such as two-factor authentication or network access control lists. The vulnerability highlights the importance of proper input validation and the need for comprehensive security testing of authentication mechanisms, particularly in legacy systems where such flaws may persist for extended periods due to the difficulty of upgrading older network infrastructure components.