CVE-2019-11879 in WEBrick Gem
Summary
by MITRE
** DISPUTED ** The WEBrick gem 1.4.2 for Ruby allows directory traversal if the attacker once had local access to create a symlink to a location outside of the web root directory. NOTE: The vendor states that this is analogous to Options FollowSymlinks in the Apache HTTP Server, and therefore it is "not a problem."
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Analysis
by VulDB Data Team • 08/05/2024
The vulnerability identified as CVE-2019-11879 affects the WEBrick gem version 1.4.2 in Ruby applications, presenting a directory traversal risk through symbolic link manipulation. This issue arises when an attacker with local access to the system can create symbolic links that point outside the designated web root directory, potentially enabling unauthorized access to sensitive files and directories beyond the intended web serving scope.
The technical flaw stems from WEBrick's handling of symbolic links within its file serving mechanism, where the server does not properly validate or restrict access to symbolic links that reference locations outside the configured document root. When a symbolic link is created pointing to a directory outside the web root, the web server may follow this link and serve content from unauthorized locations, effectively bypassing the intended directory restrictions. This behavior aligns with the Apache HTTP Server's Options FollowSymlinks directive, which similarly allows following symbolic links outside the document root when configured to do so.
From an operational impact perspective, this vulnerability represents a significant security risk for Ruby applications using WEBrick as their web server, particularly in environments where local system access might be compromised or where untrusted users have the ability to create files on the system. The vulnerability is particularly concerning because it requires only local access to create the malicious symbolic links, making it exploitable in scenarios where attackers have shell access or can otherwise manipulate the file system. The security implications extend beyond simple file access, potentially allowing attackers to read sensitive configuration files, application source code, or other system files that should remain protected.
The vendor's position that this is analogous to Apache's FollowSymlinks behavior reflects a philosophical approach to web server security that prioritizes explicit configuration over automatic protection. However, this stance contradicts common security best practices that emphasize defense in depth and automatic protection against known attack vectors. The vulnerability demonstrates the importance of proper access controls and the principle of least privilege in web server configurations, as well as the need for administrators to carefully consider how symbolic links are handled within their web serving environments. This issue is categorized under CWE-22, which describes improper limitation of a pathname to a restricted directory, and relates to ATT&CK technique T1083, which involves discovering files and directories, and T1566, which covers credential access through various means including path traversal attacks.
Organizations should implement mitigations that include disabling symbolic link following in web server configurations, implementing proper file system access controls, and ensuring that web server processes run with minimal necessary privileges. Additionally, administrators should monitor for unauthorized symbolic link creation and consider using more secure web server alternatives that do not exhibit this behavior by default. The vulnerability underscores the importance of understanding how different web server implementations handle symbolic links and the need for comprehensive security testing that includes file system traversal scenarios.