CVE-2026-40198 Overview
CVE-2026-40198 is an input validation vulnerability in Net::CIDR::Lite, a Perl module used for manipulating CIDR network address blocks. Versions before 0.23 fail to validate IPv6 group count in the _pack_ipv6() function, which may allow attackers to bypass IP-based Access Control Lists (ACLs).
The vulnerability exists because _pack_ipv6() does not check that uncompressed IPv6 addresses (without ::) have exactly 8 hex groups. Malformed inputs like "abcd", "1:2:3", or "1:2:3:4:5:6:7" are incorrectly accepted and produce packed values of wrong length (3, 7, or 15 bytes instead of 17). These incorrectly packed values are then used for mask and comparison operations, where find() and bin_find() use Perl string comparison (lt/gt) on values of different lengths, producing incorrect results.
Critical Impact
Applications relying on Net::CIDR::Lite for IP-based access control may incorrectly allow or deny network access due to improper IPv6 address validation, potentially bypassing security boundaries.
Affected Products
- Net::CIDR::Lite versions before 0.23 for Perl
- Applications using Net::CIDR::Lite for IP ACL validation
- Network security tools implementing CIDR-based access controls with this module
Discovery Timeline
- 2026-04-10 - CVE CVE-2026-40198 published to NVD
- 2026-04-13 - Last updated in NVD database
Technical Details for CVE-2026-40198
Vulnerability Analysis
This vulnerability falls under CWE-1286 (Improper Validation of Syntactically Incorrect Input). The core issue resides in the _pack_ipv6() function which processes IPv6 addresses for internal storage and comparison operations.
When an IPv6 address is provided without the compression notation (::), the function should enforce that exactly 8 hexadecimal groups are present. However, the validation logic fails to perform this check, allowing addresses with fewer groups to be processed. The resulting packed representation has an incorrect byte length, which corrupts subsequent range comparison operations.
The vulnerability is particularly dangerous in security-sensitive contexts where Net::CIDR::Lite is used to implement IP-based access controls. An attacker could craft malformed IPv6 addresses that pass or fail find() checks incorrectly, effectively bypassing network access restrictions.
This issue is classified as the same class of input validation vulnerability as CVE-2021-47154, which addressed IPv4 leading zeros in the same module. A related vulnerability, CVE-2026-40199, affects IPv4-mapped IPv6 addresses in the same function.
Root Cause
The root cause is missing validation in _pack_ipv6() that should verify uncompressed IPv6 addresses contain exactly 8 hex groups (or 6 groups when an IPv4 suffix is present). Without this check, malformed addresses produce packed values of incorrect length, causing string comparison operations to yield erroneous results when checking if an address falls within a CIDR range.
Attack Vector
The attack vector is network-based and requires no privileges or user interaction. An attacker can exploit this vulnerability by submitting malformed IPv6 addresses to applications that use Net::CIDR::Lite for access control decisions. The malformed address may incorrectly match or fail to match CIDR ranges, allowing unauthorized access or causing denial of legitimate requests.
Example exploitation scenario:
my $cidr = Net::CIDR::Lite->new("::/8");
$cidr->find("1:2:3"); # invalid input, incorrectly returns true
In this case, the invalid IPv6 address "1:2:3" is incorrectly reported as being within the ::/8 range.
Detection Methods for CVE-2026-40198
Indicators of Compromise
- Unusual IPv6 addresses in application logs with fewer than 8 colon-separated groups and no :: compression
- Access control bypass events where malformed IPv6 addresses gain unauthorized access
- Application errors related to IP address comparison or CIDR range lookups
Detection Strategies
- Audit application logs for IPv6 addresses that don't conform to RFC 5952 format
- Monitor for unexpected access grants or denials that may indicate ACL bypass
- Review applications using Net::CIDR::Lite and check module version numbers
Monitoring Recommendations
- Implement additional input validation for IPv6 addresses before passing to Net::CIDR::Lite
- Set up alerts for malformed IP address patterns in network logs
- Monitor Perl module dependencies for versions prior to 0.23
How to Mitigate CVE-2026-40198
Immediate Actions Required
- Upgrade Net::CIDR::Lite to version 0.23 or later immediately
- Audit applications that use this module for IP-based access control decisions
- Implement additional IPv6 validation at the application layer as defense-in-depth
- Review access logs for potential exploitation attempts using malformed addresses
Patch Information
The vulnerability has been fixed in Net::CIDR::Lite version 0.23. The patch adds validation to ensure uncompressed IPv6 addresses have the correct number of hex groups.
The fix is available via:
The security patch adds the following validation to _pack_ipv6():
return;
}
return if $ipv4 and @nums > 6;
+ return unless $empty or @nums == ($ipv4 ? 6 : 8);
$str =~ s/X/"0" x (($ipv4 ? 25 : 33)-length($str))/e if $empty;
pack("H*", "00" . $str).$ipv4;
}
Source: GitHub Patch Commit
This patch ensures that uncompressed IPv6 addresses must have exactly 8 hex groups (or 6 groups when an IPv4 suffix is present), rejecting malformed inputs that would produce incorrect packed values.
Workarounds
- Validate IPv6 address format before passing to Net::CIDR::Lite methods
- Implement regex-based pre-validation ensuring proper IPv6 structure
- Use alternative IP validation libraries as a preprocessing step
- Consider network-level IP validation before application-layer processing
# Upgrade Net::CIDR::Lite via CPAN
cpan install Net::CIDR::Lite
# Or using cpanminus
cpanm Net::CIDR::Lite@0.23
# Verify installed version
perl -MNet::CIDR::Lite -e 'print $Net::CIDR::Lite::VERSION'
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
