CVE-2021-23401 in Flask-User
Summary
by MITRE • 07/05/2021
This affects all versions of package Flask-User. When using the make_safe_url function, it is possible to bypass URL validation and redirect a user to an arbitrary URL by providing multiple back slashes such as /////evil.com/path or \\\evil.com/path. This vulnerability is only exploitable if an alternative WSGI server other than Werkzeug is used, or the default behaviour of Werkzeug is modified using 'autocorrect_location_header=False.
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Analysis
by VulDB Data Team • 07/09/2021
The vulnerability identified as CVE-2021-23401 resides within the Flask-User package, a popular authentication and user management extension for Flask web applications. This flaw represents a critical security weakness that undermines the integrity of URL validation mechanisms designed to prevent open redirect attacks. The vulnerability specifically targets the make_safe_url function, which is intended to sanitize and validate URLs before redirection occurs. When attackers exploit this weakness, they can manipulate URL redirection behavior to direct users toward malicious destinations, potentially enabling phishing attacks, credential theft, or other malicious activities that compromise user security and application integrity.
The technical implementation of this vulnerability stems from insufficient input validation and handling of special characters within URL parsing. The flaw manifests when multiple backslashes are introduced into URL strings such as /////evil.com/path or \\evil.com/path, which the make_safe_url function fails to properly sanitize. This occurs because the function does not adequately normalize URL representations or properly handle escape sequences that could bypass validation checks. The vulnerability is classified under CWE-601 as an Open Redirect vulnerability, where the application's redirect functionality can be manipulated to point to attacker-controlled domains. The exploit requires specific environmental conditions to be effective, as the vulnerability is only exploitable when alternative WSGI servers are used instead of the default Werkzeug server, or when Werkzeug's default behavior is modified by setting 'autocorrect_location_header=False'. This dependency on specific server configurations means that applications using standard Flask-Werkzeug setups may not be immediately vulnerable, but the risk remains present if configuration changes are made.
The operational impact of this vulnerability extends beyond simple redirection attacks to encompass broader security implications for web applications relying on Flask-User for user authentication and session management. Attackers can leverage this weakness to create sophisticated social engineering campaigns where users are redirected to fraudulent websites that mimic legitimate services, potentially capturing credentials or sensitive information. The vulnerability affects all versions of the Flask-User package, meaning that any application deployed with this library could be at risk, regardless of the specific version installed. Organizations may face significant consequences including reputation damage, regulatory compliance violations, and potential data breaches when this vulnerability is exploited. The attack surface is particularly concerning for applications that implement user redirection after authentication or authorization events, as these are common entry points for attackers seeking to extend their access or harvest user credentials.
Mitigation strategies for CVE-2021-23401 require a multi-layered approach addressing both immediate remediation and long-term security hardening. The primary recommendation involves upgrading to the latest version of Flask-User where the vulnerability has been patched and the make_safe_url function properly handles special character sequences. Organizations should also implement comprehensive input validation and sanitization measures that normalize URL representations before processing, ensuring that multiple backslashes or other escape sequences are properly handled. Network-level protections such as web application firewalls can provide additional defense-in-depth by monitoring for suspicious URL patterns and blocking known malicious redirection attempts. Security teams should conduct thorough code reviews to identify any custom implementations of URL validation logic that might be susceptible to similar weaknesses. The ATT&CK framework categorizes this vulnerability under T1566 as "Phishing" and T1071 as "Application Layer Protocol", emphasizing the social engineering aspects and network protocol manipulation involved in exploitation. Organizations must also consider implementing proper security configuration management practices to prevent unintended modifications to WSGI server behavior that could expose applications to this vulnerability. Regular security assessments and vulnerability scanning should include checks for outdated Flask-User installations to prevent exploitation attempts targeting this specific weakness.