CVE-2021-33571 in Django
Summary
by MITRE • 06/09/2021
In Django 2.2 before 2.2.24, 3.x before 3.1.12, and 3.2 before 3.2.4, URLValidator, validate_ipv4_address, and validate_ipv46_address do not prohibit leading zero characters in octal literals. This may allow a bypass of access control that is based on IP addresses. (validate_ipv4_address and validate_ipv46_address are unaffected with Python 3.9.5+..) .
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Analysis
by VulDB Data Team • 06/11/2021
The vulnerability described in CVE-2021-33571 affects the Django web framework's IP address validation mechanisms across multiple versions including 2.2.24, 3.1.12, and 3.2.4. This issue specifically targets the URLValidator, validate_ipv4_address, and validate_ipv46_address functions that are responsible for validating internet protocol addresses. The core flaw lies in the improper handling of leading zero characters in octal literals within IP address validation logic. When an IP address contains leading zeros in its octal representation, the validation functions fail to properly reject these malformed addresses, creating a potential security bypass vector.
The technical implementation of this vulnerability stems from how Django's validation functions process IP address components. In standard IP address notation, each octet should be treated as a decimal value ranging from 0 to 255. However, when leading zeros are present, they can be interpreted as octal literals in certain contexts, potentially allowing attackers to craft IP addresses that appear valid but actually represent different numerical values. This behavior creates a discrepancy between what the validation function accepts and what should be considered a legitimate IP address, particularly in access control systems that rely on IP-based restrictions.
The operational impact of this vulnerability extends beyond simple validation failures, as it can enable unauthorized access to systems that depend on IP address-based access controls. When applications use Django's validation functions to filter or restrict access based on IP addresses, attackers can exploit this weakness to bypass security measures by crafting IP addresses with leading zeros that pass validation but represent different network ranges. This vulnerability particularly affects systems where IP-based access control is implemented, potentially allowing attackers to gain access to restricted resources or services that should only be available to specific IP address ranges.
Security implications of this vulnerability align with CWE-254, which addresses weaknesses in validation of input parameters, and can be mapped to ATT&CK technique T1078.1.1 for valid accounts and T1190 for exploit public-facing application. The vulnerability creates an indirect path for privilege escalation or unauthorized access, as systems that rely on IP validation for security boundaries may be compromised. Organizations using affected Django versions should prioritize patching to address this validation weakness, particularly those with IP-based access controls in place. The vulnerability's impact is mitigated in Python 3.9.5+ environments where the underlying Python implementation handles octal parsing differently, but this does not address the core Django validation issue that requires specific code-level fixes.
Mitigation strategies should focus on immediate patching of affected Django versions to ensure proper IP address validation. Organizations should also implement additional layers of security such as network segmentation, firewall rules, and multi-factor authentication to reduce the risk of exploitation. Regular security testing and code reviews should specifically target validation functions to identify similar issues. The vulnerability demonstrates the importance of proper input validation and highlights the need for comprehensive testing of edge cases in security-critical functions, particularly those dealing with network protocols and access controls.