CVE-2001-1013 in Linux
Summary
by MITRE
Apache on Red Hat Linux with with the UserDir directive enabled generates different error codes when a username exists and there is no public_html directory and when the username does not exist, which could allow remote attackers to determine valid usernames on the server.
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Analysis
by VulDB Data Team • 09/30/2025
This vulnerability exists in Apache web servers running on Red Hat Linux systems where the UserDir directive is enabled. The UserDir directive allows users to access their personal web directories through URLs like http://server/~username format. The flaw stems from Apache's inconsistent error handling when processing requests for user directories. When a request is made for a username that exists on the system but lacks a public_html directory, Apache returns a different HTTP status code compared to when the username itself does not exist in the system. This behavioral difference creates a distinguishable pattern that attackers can exploit to enumerate valid usernames on the server.
The technical implementation of this vulnerability relies on the Apache server's response handling mechanism. When a valid username exists but the corresponding public_html directory is missing, Apache typically returns a 403 Forbidden error code. However, when an invalid username is requested, Apache may return a different error code such as 404 Not Found or handle the request differently altogether. This inconsistency in error responses provides attackers with a means to distinguish between valid and invalid usernames through simple HTTP response analysis. The vulnerability specifically affects systems where the UserDir directive is configured to allow user directory access while the server does not properly mask the underlying authentication mechanism.
The operational impact of this vulnerability extends beyond simple information disclosure. Attackers can leverage this weakness to build comprehensive lists of valid user accounts on the system, which can then be used for targeted attacks including password spraying, brute force attempts, or social engineering operations. This information disclosure vulnerability directly violates security principles by exposing system user enumeration details that should remain hidden from external parties. The vulnerability is particularly dangerous in environments where user accounts may correspond to actual individuals, as it enables attackers to identify legitimate users who may have access to sensitive resources or systems.
From a cybersecurity framework perspective, this vulnerability aligns with CWE-200 (Information Exposure) and relates to ATT&CK technique T1087.001 (Account Discovery: Local Account) and T1087.002 (Account Discovery: Domain Account). The issue demonstrates poor error handling practices that violate the principle of least privilege and information hiding. Organizations should implement proper input validation and consistent error responses to prevent information leakage. Mitigation strategies include disabling the UserDir directive when not required, implementing consistent HTTP error responses regardless of user account existence, and employing additional security measures such as rate limiting and access controls to prevent automated enumeration attempts. The vulnerability also highlights the importance of regular security audits and proper configuration management to prevent such exposure of system internals through web server responses.