CVE-2021-29921 in Communications Cloud Native Core Network Slice Selection Function
Summary
by MITRE • 05/06/2021
In Python before 3,9,5, the ipaddress library mishandles leading zero characters in the octets of an IP address string. This (in some situations) allows attackers to bypass access control that is based on IP addresses.
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Analysis
by VulDB Data Team • 11/04/2025
The vulnerability identified as CVE-2021-29921 represents a critical flaw in Python's ipaddress library that stems from improper handling of leading zero characters in IP address octets. This issue affects Python versions prior to 3.9.5 and creates a significant security risk through improper input validation. The flaw manifests when parsing IP address strings that contain leading zeros in their octet values, which should be interpreted according to standard network protocols but are instead processed inconsistently by the library's parsing mechanism.
The technical root cause of this vulnerability lies in the ipaddress library's failure to properly normalize IP address representations during parsing operations. When an IP address string contains octets with leading zeros such as 192.168.001.001, the library does not correctly interpret these as standard decimal values but rather treats them as octal representations in certain contexts. This inconsistent behavior creates opportunities for attackers to exploit access control mechanisms that rely on IP address validation, as they can craft malicious IP address strings that bypass intended security restrictions.
This vulnerability directly impacts access control systems that depend on IP address validation for authentication and authorization purposes. The operational impact extends beyond simple network access restrictions to encompass broader security controls that utilize IP-based filtering. Attackers can leverage this flaw to bypass security measures such as firewall rules, network access controls, and IP-based authentication systems by constructing IP addresses with leading zeros that are interpreted differently than expected. The vulnerability operates at the protocol level, making it particularly dangerous for applications that implement IP-based security controls without proper input sanitization.
The security implications of CVE-2021-29921 align with CWE-129, which addresses improper handling of input validation, and can be mapped to ATT&CK technique T1071.004 for application layer protocol manipulation. Organizations utilizing Python applications that implement IP-based access controls are particularly vulnerable, as the flaw can be exploited across various network services and security systems. The vulnerability's exploitation requires minimal technical expertise and can be automated, making it attractive to threat actors seeking to bypass network security controls. Proper mitigation involves upgrading to Python 3.9.5 or later where the ipaddress library correctly handles leading zero characters in IP address octets, ensuring consistent interpretation according to standard network protocols.
The flaw demonstrates the importance of proper input validation and normalization in security-critical libraries. Network security systems that rely on IP address parsing for access control decisions become fundamentally compromised when underlying libraries fail to handle edge cases correctly. This vulnerability serves as a reminder of the critical nature of standard compliance in security-sensitive components and the potential for seemingly minor parsing issues to create significant security weaknesses. Organizations should implement comprehensive testing procedures that include edge case validation for all network-related input processing to prevent similar vulnerabilities from being exploited in their systems.