CVE-1999-0856 in Linux
Summary
by MITRE
login in Slackware 7.0 allows remote attackers to identify valid users on the system by reporting an encryption error when an account is locked or does not exist.
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Analysis
by VulDB Data Team • 04/19/2026
This vulnerability exists in the login functionality of Slackware 7.0 systems where remote attackers can enumerate valid user accounts through differential response behavior during authentication attempts. The flaw stems from the system's handling of authentication errors where legitimate accounts that are locked or non-existent produce identical encryption error messages, allowing attackers to distinguish between valid and invalid accounts based on the nature of the error response. This type of information disclosure vulnerability falls under the category of user enumeration attacks that can be classified as CWE-204 according to the Common Weakness Enumeration standards. The vulnerability represents a significant security weakness in the authentication mechanism as it provides attackers with crucial information needed for subsequent attacks such as brute force or credential stuffing attempts.
The technical implementation of this vulnerability occurs at the authentication layer where the login service fails to normalize error responses for different account states. When an attacker attempts to authenticate with a valid username but incorrect password, or with a non-existent username, the system returns encryption-related error messages that differ in subtle ways between the two scenarios. This inconsistent error handling creates a timing or response pattern that can be analyzed to determine account validity. The vulnerability is particularly dangerous because it operates at the protocol level where authentication responses are processed, making it difficult to detect through standard network monitoring tools. Attackers can leverage this information to build targeted attack strategies against specific accounts, significantly reducing the complexity of credential guessing attacks.
The operational impact of this vulnerability extends beyond simple user enumeration as it fundamentally weakens the security posture of the entire system. Once attackers have identified valid user accounts, they can focus their efforts on those specific accounts rather than conducting broad brute force attacks against the entire user base. This vulnerability directly impacts the principle of least privilege and can lead to account compromise through various attack vectors including password spraying, credential reuse, or more sophisticated social engineering campaigns. The attack can be executed remotely without requiring any special privileges or access to the system, making it particularly dangerous for systems that are exposed to the internet. This type of attack aligns with techniques described in the MITRE ATT&CK framework under the credential access category, specifically targeting the enumeration of valid accounts as a reconnaissance step.
Mitigation strategies for this vulnerability require implementing consistent error handling throughout the authentication process where all authentication failures return identical responses regardless of whether the account exists or is locked. System administrators should configure the login service to normalize error messages and eliminate any differential responses that could reveal account status information. This approach aligns with security best practices outlined in various compliance frameworks including pci dss and iso 27001 standards that require proper handling of authentication failures to prevent information disclosure. Additional protective measures include implementing account lockout mechanisms with randomized delays, rate limiting for authentication attempts, and monitoring for unusual authentication patterns that could indicate automated enumeration attempts. The vulnerability also highlights the importance of regular security assessments and penetration testing to identify similar issues in authentication systems across different platforms and versions.