CVE-2026-27199 in Werkzeug
Summary
by MITRE • 02/21/2026
Werkzeug is a comprehensive WSGI web application library. Versions 3.1.5 and below, the safe_join function allows Windows device names as filenames if preceded by other path segments. This was previously reported as GHSA-hgf8-39gv-g3f2, but the added filtering failed to account for the fact that safe_join accepts paths with multiple segments, such as example/NUL. The function send_from_directory uses safe_join to safely serve files at user-specified paths under a directory. If the application is running on Windows, and the requested path ends with a special device name, the file will be opened successfully, but reading will hang indefinitely. This issue has been fixed in version 3.1.6.
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Analysis
by VulDB Data Team • 03/03/2026
The vulnerability identified as CVE-2026-27199 affects the Werkzeug web application library, specifically impacting versions 3.1.5 and earlier. This security flaw resides within the safe_join function which is designed to prevent directory traversal attacks by safely joining path components. The issue manifests when Windows device names such as NUL, CON, PRN, AUX, COM1, LPT1, and others are used as filenames in paths that contain multiple segments. While the function previously attempted to address similar issues, the implementation failed to properly handle cases where these device names appear as the final component of multi-segment paths, creating a bypass mechanism that allows malicious actors to exploit the system.
The technical flaw in the safe_join function stems from inadequate filtering logic that does not properly validate Windows device names when they appear in the context of multi-segment paths. The function accepts paths like example/NUL where NUL represents a Windows device name, and while the function may successfully open the file, attempting to read from it causes the application to hang indefinitely. This behavior occurs because Windows treats these device names specially and when accessed through the file system, they create a blocking read operation that never completes. The vulnerability is particularly dangerous because it can be exploited through the send_from_directory function which relies on safe_join to serve files from user-specified paths under a directory, making it possible for attackers to cause denial of service conditions on vulnerable applications.
The operational impact of this vulnerability extends beyond simple denial of service, as it can be leveraged to disrupt application availability and potentially mask other attacks. Applications running on Windows systems that utilize Werkzeug's send_from_directory functionality become vulnerable to this attack vector, where an attacker can craft malicious requests that cause the application to hang indefinitely. This creates a persistent availability issue that can be difficult to detect and remediate, as the hanging behavior may not be immediately apparent to system administrators. The vulnerability aligns with CWE-22 (Improper Limitation of a Pathname to a Restricted Directory) and CWE-73 (External Control of File Name or Path) which govern path traversal and file path manipulation issues. From an attacker's perspective, this vulnerability maps to ATT&CK technique T1499.004 (Endpoint Denial of Service) and potentially T1059.001 (Command and Scripting Interpreter) when combined with other exploitation techniques.
Mitigation strategies for CVE-2026-27199 require immediate application updates to version 3.1.6 or later, which contains the necessary fixes to properly filter Windows device names in multi-segment paths. Organizations should also implement additional defensive measures including input validation at multiple layers, monitoring for unusual file access patterns, and implementing timeouts for file operations to prevent indefinite hanging. Network-level protections can include implementing rate limiting and content filtering to detect and block suspicious path patterns that contain Windows device names. Security teams should also conduct thorough application audits to identify all instances where send_from_directory or similar functions are used, ensuring that proper path validation is implemented throughout the application stack. The fix implemented in version 3.1.6 addresses the root cause by ensuring that Windows device names are properly rejected regardless of their position within multi-segment paths, preventing the bypass that previously allowed malicious requests to succeed while causing the application to hang during file reads.