CVE-2020-26299 in ftp-srv
Summary
by MITRE • 02/10/2021
ftp-srv is an open-source FTP server designed to be simple yet configurable. In ftp-srv before version 4.4.0 there is a path-traversal vulnerability. Clients of FTP servers utilizing ftp-srv hosted on Windows machines can escape the FTP user's defined root folder using the expected FTP commands, for example, CWD and UPDR. When windows separators exist within the path (`\`), `path.resolve` leaves the upper pointers intact and allows the user to move beyond the root folder defined for that user. We did not take that into account when creating the path resolve function. The issue is patched in version 4.4.0 (commit 457b859450a37cba10ff3c431eb4aa67771122e3).
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Analysis
by VulDB Data Team • 02/27/2021
The vulnerability identified as CVE-2020-26299 affects ftp-srv, an open-source FTP server implementation that prioritizes simplicity and configurability. This particular flaw represents a path traversal vulnerability that specifically impacts Windows-based deployments where the ftp-srv server operates. The vulnerability stems from improper handling of path resolution logic within the server's core functionality, creating a security gap that allows authenticated users to escape their designated root directories. The issue manifests through standard FTP commands such as CWD (change working directory) and UPDR (upload directory), which are commonly used by FTP clients to navigate server file systems. When Windows-style path separators are present in user requests, the path resolution mechanism fails to properly sanitize or validate the paths, leaving upper directory pointers (`..`) intact and enabling unauthorized navigation beyond the intended user boundaries. This vulnerability directly violates the principle of least privilege by allowing users to potentially access files and directories outside their designated scope, creating a significant risk for data exposure and unauthorized access.
The technical exploitation of this vulnerability occurs at the path resolution level where the `path.resolve` function in the ftp-srv implementation fails to properly handle Windows directory separators. When users submit directory change requests containing backslashes, the system does not adequately process these paths to prevent traversal beyond the user's root folder. This flaw essentially creates a directory traversal condition that allows attackers to move upward in the directory structure regardless of the defined root restrictions. The vulnerability is particularly dangerous in multi-user environments where different users should be isolated within their respective directories. The implementation issue resides in how the software handles path normalization for Windows systems, where the standard path resolution logic does not account for the specific behavior of backslash handling in Windows file systems. This type of vulnerability maps directly to CWE-22 Path Traversal and falls under the ATT&CK technique T1078 Valid Accounts, as it exploits legitimate user accounts to gain unauthorized access to system resources.
The operational impact of CVE-2020-26299 extends beyond simple directory traversal, potentially enabling attackers to access sensitive system files, configuration data, or other users' files if the server is configured with shared directories or if the user's root folder has broader permissions. On Windows systems, this vulnerability becomes particularly problematic because Windows path handling differs significantly from Unix-like systems, and the ftp-srv implementation fails to properly account for these differences. The vulnerability affects any deployment where ftp-srv is used as an FTP server on Windows machines with users having restricted root directories. Organizations relying on this FTP server for file transfers, user access control, or data sharing may experience unauthorized data access, potential data exfiltration, and violation of access control policies. The patch implemented in version 4.4.0 addresses this by correcting the path resolution logic to properly handle Windows path separators and ensure that directory traversal attempts are properly rejected, maintaining the integrity of user-defined root folders. This remediation specifically targets the commit referenced in the advisory, which modified the internal path resolution mechanism to prevent the escape of user-defined directory boundaries.