CVE-2021-4456: Net::CIDR Auth Bypass Vulnerability

CVE-2021-4456 Overview

CVE-2021-4456 is an Input Validation Error vulnerability affecting the Net::CIDR Perl module in versions prior to 0.24. The vulnerability stems from improper handling of leading zeros in IP CIDR addresses, which can lead to unintended interpretation of IP address components as octal numbers rather than decimal values.

The affected functions addr2cidr and cidrlookup may return CIDR strings containing leading zeros. When these strings are subsequently parsed by other components, the leading zeros can cause IP address octets to be interpreted as octal values. This parsing discrepancy creates a security gap that attackers can exploit to bypass IP-based access controls.

Critical Impact

Attackers may leverage this vulnerability to bypass IP-based access controls by crafting malicious input that exploits the octal parsing behavior, potentially gaining unauthorized access to protected resources.

Affected Products

• Net::CIDR versions before 0.24 for Perl
• Applications using addr2cidr function with untrusted input
• Applications using cidrlookup function without cidrvalidate validation

Discovery Timeline

• 2026-02-27 - CVE CVE-2021-4456 published to NVD
• 2026-03-03 - Last updated in NVD database

Technical Details for CVE-2021-4456

Vulnerability Analysis

This vulnerability is classified under CWE-704 (Incorrect Type Conversion or Cast). The root issue lies in how the Net::CIDR module handles IP address representations containing leading zeros. When IP addresses or CIDR blocks are processed by addr2cidr or cidrlookup, the module may preserve or introduce leading zeros in the output.

The danger emerges when downstream applications or libraries parse these CIDR strings. Many programming languages and systems interpret numbers with leading zeros as octal (base-8) values. For example, the IP address segment 010 would be interpreted as decimal 8 rather than decimal 10.

This semantic mismatch between the expected decimal interpretation and the actual octal interpretation creates opportunities for security bypass. An attacker could craft an IP address that appears to be one value when validated but resolves to a different value when actually used for access control decisions.

Root Cause

The vulnerability originates from insufficient input sanitization and output normalization within the Net::CIDR module. The addr2cidr and cidrlookup functions do not strip or reject leading zeros from IP address components, allowing ambiguous representations to propagate through the system.

While the module's documentation recommends using the cidrvalidate function to validate untrusted CIDR strings, this mitigation is optional and not enforced by default. Many developers incorrectly assume that calling addr2cidr or cidrlookup with untrusted input is inherently safe, leading to vulnerable implementations.

Attack Vector

The attack exploits the network-accessible nature of the vulnerability, requiring no authentication or user interaction. An attacker can craft specially formatted IP addresses containing leading zeros and submit them to applications using the vulnerable Net::CIDR functions.

The exploitation process involves:

1. Identifying an application that uses Net::CIDR for IP-based access control
2. Crafting an IP address with leading zeros that passes initial validation but resolves differently when parsed by downstream components
3. Submitting the malicious IP address to bypass access restrictions

For example, an attacker might use 192.168.010.1 expecting it to be treated as 192.168.8.1 (octal interpretation) while the access control system validates it as 192.168.10.1. This discrepancy allows the attacker to masquerade as a different IP address or subnet.

For detailed technical analysis and exploitation techniques, see the Urth Blog Security Issues article discussing security issues in Perl IP address distributions.

Detection Methods for CVE-2021-4456

Indicators of Compromise

• Unusual IP address formats containing leading zeros in application logs (e.g., 192.168.010.001)
• Access control bypass attempts from IP addresses that appear legitimate but resolve unexpectedly
• Inconsistent IP address representations between different log sources or system components

Detection Strategies

• Implement log analysis rules to detect IP addresses with leading zeros in CIDR notation
• Monitor for access control violations that correlate with ambiguous IP address representations
• Audit Perl applications for usage of Net::CIDR module versions prior to 0.24
• Review access control decision logs for discrepancies between claimed and resolved IP addresses

Monitoring Recommendations

• Enable verbose logging on applications using IP-based access controls
• Set up alerts for IP address patterns matching regex \b0[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+\b or similar octal-ambiguous formats
• Monitor for failed access attempts followed by successful access from similar IP ranges
• Implement centralized logging to correlate IP address representations across multiple systems

How to Mitigate CVE-2021-4456

Immediate Actions Required

• Upgrade Net::CIDR to version 0.24 or later immediately
• Audit all applications using Net::CIDR for potential exposure to untrusted IP input
• Implement the cidrvalidate function for all untrusted CIDR string inputs as an interim measure
• Review access control logs for evidence of exploitation attempts

Patch Information

The vulnerability has been addressed in Net::CIDR version 0.24. The fix is available in the GitHub commit e3648c6bc6bdd018f90cca4149c467017d42bd10. The patch ensures proper handling of leading zeros in IP address components, preventing octal interpretation ambiguities.

For complete changelog details, refer to the MetaCPAN Net-CIDR changes documentation.

Workarounds

• Always use the cidrvalidate function before processing untrusted CIDR strings with addr2cidr or cidrlookup
• Implement input sanitization to strip leading zeros from IP address components before processing
• Add validation layers that reject IP addresses containing leading zeros in any octet
• Consider implementing a secondary validation using alternative IP parsing libraries that handle octal notation consistently

# Configuration example - Validate CIDR strings before processing
# Add to your Perl application before calling addr2cidr or cidrlookup

# Example validation pattern in shell for log analysis
grep -E '\b0[0-9]+\.[0-9]+\.[0-9]+\.[0-9]+' /var/log/application.log

# CPAN upgrade command
cpan Net::CIDR
# Or specify minimum version in your requirements
# requires 'Net::CIDR', '>= 0.24';