CVE-2017-7418 in ProFTPD
Summary
by MITRE
ProFTPD before 1.3.5e and 1.3.6 before 1.3.6rc5 controls whether the home directory of a user could contain a symbolic link through the AllowChrootSymlinks configuration option, but checks only the last path component when enforcing AllowChrootSymlinks. Attackers with local access could bypass the AllowChrootSymlinks control by replacing a path component (other than the last one) with a symbolic link. The threat model includes an attacker who is not granted full filesystem access by a hosting provider, but can reconfigure the home directory of an FTP user.
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Analysis
by VulDB Data Team • 08/24/2024
The vulnerability identified as CVE-2017-7418 affects ProFTPD versions prior to 1.3.5e and 1.3.6rc5, representing a critical security flaw in the handling of symbolic links within user home directories. This issue stems from an incomplete implementation of the AllowChrootSymlinks configuration option, which was designed to control whether users could have symbolic links in their home directories while maintaining chroot security boundaries. The flaw manifests when the software performs symlink validation checks, specifically examining only the final path component of a user's home directory rather than the complete path structure. This partial validation creates a security bypass opportunity that undermines the intended protection mechanisms.
The technical implementation of this vulnerability resides in the path validation logic where ProFTPD fails to recursively check all components of a user's home directory path when determining whether symbolic links are permitted. When a user's home directory contains multiple path components, such as /home/user/documents, the system only validates the final component 'documents' for symbolic link presence rather than examining the entire path from root to the final directory. This incomplete validation allows attackers to circumvent the intended security controls by strategically placing symbolic links in intermediate path components, effectively creating a bypass mechanism that defeats the purpose of the AllowChrootSymlinks configuration.
From an operational perspective, this vulnerability presents significant risks for hosting environments where multiple users share the same system, particularly in scenarios where hosting providers grant users limited filesystem access but allow configuration of their home directory paths. The threat model encompasses attackers who may not possess full administrative privileges or complete filesystem access but can manipulate their user's home directory configuration. Such attackers can exploit this vulnerability by replacing any non-terminal path component with a symbolic link that points to a directory outside the intended chroot jail, thereby gaining unauthorized access to restricted filesystem areas. This represents a privilege escalation vulnerability that directly compromises the security isolation typically provided by chroot environments.
The security implications of CVE-2017-7418 align with CWE-22 (Improper Limitation of a Pathname to a Restricted Directory) and CWE-23 (Relative Path Traversal), as the vulnerability demonstrates improper path validation and traversal mechanisms. This flaw can be categorized under ATT&CK technique T1059.001 (Command and Scripting Interpreter: PowerShell) and T1548.002 (Abuse Elevation Control Mechanisms: Bypass User Access Control) when exploited in multi-user environments. The vulnerability essentially undermines the fundamental security principle of chroot jails and privilege separation, allowing attackers to escape confined filesystem boundaries through clever manipulation of symbolic link placement within directory structures.
Mitigation strategies for this vulnerability require immediate patching of ProFTPD installations to versions 1.3.5e or 1.3.6rc5 and later, which contain the corrected path validation logic. System administrators should also implement strict monitoring of symbolic link usage within user home directories and consider implementing additional security controls such as mandatory access controls or extended filesystem permissions. Organizations should conduct comprehensive audits of their FTP configurations to ensure that AllowChrootSymlinks is properly set according to their security requirements, and that all users with access to home directory configuration cannot manipulate path structures to exploit this vulnerability. Additionally, implementing network segmentation and limiting local access privileges for FTP users can reduce the attack surface and limit the potential impact of successful exploitation attempts.