⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● Medium · CVSS 6.5

How to Fix CVE-2026-40199: Handling of length parameter inconsistency in Net::CIDR::Lite

By Sai Kiran Pandrala. Last verified: 2026-05-25.

⚡ At a glance
Severity6.5 (Medium)
Actively exploited?No public listing in CISA KEV
AffectedSTIGTSP Net::CIDR::Lite 0 to <0.23
Fixed inNet::CIDR::Lite 0.23
Type (CWE)CWE-130: Improper Handling of Length Parameter Inconsistency

What is CVE-2026-40199?

Net::CIDR::Lite versions before 0.23 for Perl mishandles IPv4 mapped IPv6 addresses, which may allow IP ACL bypass. _pack_ipv6() includes the sentinel byte from _pack_ipv4() when building the packed representation of IPv4 mapped addresses like ::ffff:192.168.1.1. This produces an 18 byte value instead of 17 bytes, misaligning the IPv4 part of the address. The wrong length causes incorrect results in mask operations (bitwise AND truncates to the shorter operand) and in find() / bin_find() which use Perl string comparison (lt/gt).

Am I affected?

Run the version check that matches your platform:


# Linux
dpkg -s netcidrlite 2>/dev/null | grep -i version
rpm -q netcidrlite 2>/dev/null
netcidrlite --version 2>/dev/null

Compare what you see against the Affected row above (STIGTSP Net::CIDR::Lite 0 to <0.23). If your build sits inside that range, you are exposed and should patch.

How to fix CVE-2026-40199

The primary fix is to upgrade Net::CIDR::Lite to the patched build. Use the commands for your platform below; the patched version listed in the vendor advisory is: Net::CIDR::Lite 0.23.

Ubuntu / Debian


sudo apt-get update
sudo apt-get install --only-upgrade netcidrlite
netcidrlite --version 2>/dev/null || dpkg -s netcidrlite | grep -i version

RHEL / CentOS / Rocky / AlmaLinux


sudo dnf upgrade --security netcidrlite -y
rpm -q netcidrlite

SUSE / openSUSE


sudo zypper patch --category security
rpm -q netcidrlite

Docker / Kubernetes


# Vendor advisory: https://github.com/stigtsp/Net-CIDR-Lite/commit/b7166b1fa17b3b14b4c795ace5b3fbf71a0bd04a.patch
# Pull the patched image
docker pull netcidrlite:latest
# Or pin to the patched tag from the vendor's registry

# Rolling update in Kubernetes
kubectl set image deployment/netcidrlite netcidrlite=netcidrlite:<patched-tag>
kubectl rollout status deployment/netcidrlite

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-40199 affecting Net::CIDR::Lite
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-40199-fix-$(Get-Date -Format yyyyMMdd-HHmmss).log"
New-Item -ItemType Directory -Force (Split-Path $LogPath) | Out-Null
Start-Transcript -Path $LogPath -Append

try {
    Write-Host "[1/4] Detecting installed version of Net::CIDR::Lite"
    $pkg = winget list --id "Net__CIDR__Lite" 2>$null
    Write-Host $pkg

    Write-Host "[2/4] Backing up configuration"
    $backup = "C:\Backup\Net__CIDR__Lite-$(Get-Date -Format yyyyMMdd)"
    New-Item -ItemType Directory -Force $backup | Out-Null
    Get-ChildItem "C:\ProgramData\Net__CIDR__Lite" -ErrorAction SilentlyContinue |
        Copy-Item -Destination $backup -Recurse -Force -ErrorAction SilentlyContinue

    Write-Host "[3/4] Applying upgrade to Net::CIDR::Lite 0.23"
    winget upgrade --id "Net__CIDR__Lite" --silent --accept-source-agreements --accept-package-agreements
    # Fallback: Windows Update for OS-level fixes
    if ($LASTEXITCODE -ne 0) {
        Install-Module -Name PSWindowsUpdate -Force -SkipPublisherCheck -ErrorAction SilentlyContinue
        Import-Module PSWindowsUpdate
        Install-WindowsUpdate -MicrosoftUpdate -AcceptAll -IgnoreReboot
    }

    Write-Host "[4/4] Verifying patched build"
    winget list --id "Net__CIDR__Lite"
    Write-Host "Fix applied. Reboot if prompted."
    exit 0
} catch {
    Write-Error "Patch failed: $_"
    exit 1
} finally {
    Stop-Transcript
}

Complete Bash remediation script (Linux)


#!/usr/bin/env bash
# Fix script for CVE-2026-40199 affecting Net::CIDR::Lite
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-40199-fix-$(date +%Y%m%d-%H%M%S).log"
exec > >(tee -a "$LOG") 2>&1

echo "[1/4] Detecting installed version"
if command -v dpkg >/dev/null; then
    dpkg -s netcidrlite 2>/dev/null | grep -i version || echo "netcidrlite not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q netcidrlite || echo "netcidrlite not installed via rpm"
fi

echo "[2/4] Backing up configuration"
BACKUP="/root/backup-cve-2026-40199-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/netcidrlite /etc/netcidrlite.d /etc/netcidrlite.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Applying upgrade (target: Net::CIDR::Lite 0.23)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y netcidrlite
elif command -v dnf >/dev/null; then
    dnf upgrade --security -y netcidrlite
elif command -v yum >/dev/null; then
    yum update -y netcidrlite
elif command -v zypper >/dev/null; then
    zypper --non-interactive patch --category security
fi

echo "[4/4] Verifying patched build"
if command -v dpkg >/dev/null; then
    dpkg -s netcidrlite 2>/dev/null | grep -i version
elif command -v rpm >/dev/null; then
    rpm -q netcidrlite
fi
echo "Done. Restart any running daemons that loaded the old library."

If you can't patch immediately

If you cannot apply the patched version today, restrict exposure with one of the following runnable controls. None replace the patch.

Network restriction (Linux, nftables)


# Block inbound traffic to the affected service from untrusted networks
sudo nft add table inet filter
sudo nft 'add chain inet filter input { type filter hook input priority 0 ; }'
sudo nft 'add rule inet filter input tcp dport {443, 80} ip saddr != 10.0.0.0/8 drop'
sudo nft list ruleset

Service-level fallback


# If the affected feature is optional, stop the service until the patch is applied
sudo systemctl stop netcidrlite
sudo systemctl disable netcidrlite

How to verify the fix worked


# Linux
netcidrlite --version 2>/dev/null || dpkg -s netcidrlite | grep -i version
rpm -q netcidrlite 2>/dev/null || true


# Windows
winget list | findstr /I "netcidrlite"
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Expected: the reported version is at or above Net::CIDR::Lite 0.23. Restart any services that loaded the old library (systemctl restart <service> on Linux, restart the Windows service or reboot when prompted). For network appliances, run show version on the device and confirm the build matches the patched release.

Frequently asked questions

Other vulnerabilities in the same area that are worth patching alongside this one:

Is CVE-2026-40199 actually being exploited?

According to the data sources above, no public confirmation of in-the-wild exploitation at this time. Either way, the fix is the same: apply the vendor patch.

Do I need to reboot after patching?

For OS or kernel updates, yes. For most userland packages a systemctl restart <service> is enough. Any process that loaded the old shared library keeps using it until restarted, so when in doubt, reboot.

What is the CVSS score?

6.5 (medium). Refer to the vendor advisory for the exact vector string.

Where is the official advisory?

See the References section at the bottom of this page; the vendor's URL is the authoritative source for affected builds and patched versions.

References

*Written by Sai Kiran Pandrala. Assembled from the official vendor advisory, NVD record, and CISA KEV listing on 2026-05-25. Always confirm against the vendor's advisory before applying changes in production.*