CVE-2021-29424
📋 TL;DR
This vulnerability in the Net::Netmask Perl module allows attackers to bypass IP-based access controls by using IP addresses with leading zeros. Systems using this module for IP address validation or access control are affected, potentially allowing unauthorized access to restricted resources.
💻 Affected Systems
- Perl applications using Net::Netmask module
- Systems with custom Perl scripts using Net::Netmask
📦 What is this software?
\ by Net\
Fedora by Fedoraproject
Fedora by Fedoraproject
Fedora by Fedoraproject
⚠️ Risk & Real-World Impact
Worst Case
Complete bypass of IP-based access controls, allowing attackers to access sensitive systems, data, or administrative interfaces from unauthorized IP addresses.
Likely Case
Partial access control bypass where attackers can reach resources intended to be restricted to specific IP ranges, potentially exposing internal services or data.
If Mitigated
Limited impact if additional authentication layers or network segmentation exist beyond IP-based controls.
🎯 Exploit Status
Exploitation requires sending IP addresses with leading zeros to bypass validation. The technique is well-documented in security advisories.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2.0000 and later
Vendor Advisory: https://metacpan.org/changes/distribution/Net-Netmask#L11-22
Restart Required: No
Instructions:
1. Update Net::Netmask module using CPAN: 'cpan Net::Netmask'
2. Or install specific version: 'cpan Net::Netmask@2.0000'
3. Verify installation with: 'perl -MNet::Netmask -e "print $Net::Netmask::VERSION"'
🔧 Temporary Workarounds
Input Sanitization
allStrip leading zeros from IP addresses before passing to Net::Netmask
$ip =~ s/^0+//g; # Remove leading zeros from IP string
Alternative Validation
allUse inet_aton or other IP validation methods instead of Net::Netmask
use Socket;
$ip = inet_aton($ip_string);
🧯 If You Can't Patch
- Implement additional authentication layers beyond IP-based controls
- Use network-level controls (firewalls, ACLs) instead of application-level IP validation
🔍 How to Verify
Check if Vulnerable:
Check Net::Netmask version: 'perl -MNet::Netmask -e "print $Net::Netmask::VERSION"' - if version is less than 2.0000, system is vulnerable.Check Version:
perl -MNet::Netmask -e "print $Net::Netmask::VERSION"Verify Fix Applied:
After update, verify version is 2.0000 or higher using same command. Test with IP addresses containing leading zeros to ensure proper rejection.📡 Detection & Monitoring
Log Indicators:
- Multiple failed access attempts from unusual IP formats
- Access logs showing IP addresses with leading zeros
- Successful access from IPs outside allowed ranges
Network Indicators:
- Traffic from IP addresses with non-standard formatting
- Unexpected access patterns to restricted resources
SIEM Query:
source="access.log" AND (ip="0*" OR ip MATCHES "^0+")🔗 References
- https://blog.urth.org/2021/03/29/security-issues-in-perl-ip-address-distros/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/CBJVLXJSWN6DKSF5ADUEERI6M23R3GGP/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/JF4CYIZELC3NISB3RMV4OCI4GYBC557B/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/Y7JIPQAY5OZ5D3DA7INQILU7SGHTHMWB/
- https://metacpan.org/changes/distribution/Net-Netmask#L11-22
- https://security.netapp.com/advisory/ntap-20210604-0007/
- https://blog.urth.org/2021/03/29/security-issues-in-perl-ip-address-distros/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/CBJVLXJSWN6DKSF5ADUEERI6M23R3GGP/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/JF4CYIZELC3NISB3RMV4OCI4GYBC557B/
- https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/Y7JIPQAY5OZ5D3DA7INQILU7SGHTHMWB/
- https://metacpan.org/changes/distribution/Net-Netmask#L11-22
- https://security.netapp.com/advisory/ntap-20210604-0007/
