CVE-2021-4456 in Net::CIDR
Summary
by MITRE • 02/27/2026
Net::CIDR versions before 0.24 for Perl mishandle leading zeros in IP CIDR addresses, which may have unspecified impact.
The functions `addr2cidr` and `cidrlookup` may return leading zeros in a CIDR string, which may in turn be parsed as octal numbers by subsequent users. In some cases an attacker may be able to leverage this to bypass access controls based on IP addresses.
The documentation advises validating untrusted CIDR strings with the `cidrvalidate` function. However, this mitigation is optional and not enforced by default. In practice, users may call `addr2cidr` or `cidrlookup` with untrusted input and without validation, incorrectly assuming that this is safe.
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Analysis
by VulDB Data Team • 03/12/2026
The vulnerability identified as CVE-2021-4456 affects the Net::CIDR Perl module version 0.24 and earlier, presenting a significant security risk through improper handling of leading zeros in IP CIDR addresses. This flaw exists in the core functionality of the module where the addr2cidr and cidrlookup functions fail to normalize IP address representations, allowing leading zeros to persist in the output. The technical implementation of these functions does not adequately sanitize input or normalize output formats, creating a potential pathway for attackers to exploit the inconsistency in how octal numbers are interpreted by different systems and applications. The vulnerability stems from a fundamental design flaw in the module's handling of IP address representations where the presence of leading zeros in IP addresses can be interpreted as octal notation by subsequent parsing functions, a behavior that contradicts standard IP address formatting conventions.
The operational impact of this vulnerability extends beyond simple parsing errors to potentially enable privilege escalation and access control bypass scenarios. When applications rely on the Net::CIDR module for IP address validation and access control decisions, the presence of leading zeros in CIDR strings can be interpreted as octal values, allowing attackers to craft malicious IP address ranges that may bypass intended security restrictions. This behavior particularly affects systems that perform access control based on IP address ranges, where an attacker could potentially exploit the octal interpretation to gain unauthorized access to restricted resources. The vulnerability creates a pathway for attackers to manipulate IP address representations in ways that traditional security controls might not anticipate, as the module's default behavior does not enforce proper normalization of IP address formats. The risk is exacerbated by the fact that the module's documentation suggests using the cidrvalidate function as a mitigation, but this is presented as optional rather than mandatory, leading to widespread potential exploitation across systems that do not implement this validation step.
Security professionals should recognize this vulnerability as a variant of CWE-704, which covers incorrect type conversion or parsing, and it aligns with ATT&CK techniques related to privilege escalation and access control bypass. The vulnerability demonstrates poor input validation and sanitization practices, where the module fails to properly normalize IP address representations before returning them to calling applications. Organizations using the Net::CIDR module must implement mandatory input validation and output sanitization measures, ensuring that all IP address representations are normalized to standard decimal notation before being processed by downstream applications. The recommended mitigations include enforcing the use of cidrvalidate for all untrusted CIDR input, implementing additional validation layers in applications that consume these functions, and potentially upgrading to versions of the module that properly address the leading zero handling issue. The vulnerability also highlights the importance of defensive programming practices where security controls are not dependent on optional validation steps, but rather enforced through mandatory processing requirements that ensure consistent and secure handling of network address representations.