How to Fix CVE-2026-23651: Permissive regular expression in Microsoft ACI Confidential Containers

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

CVE-2026-23651 is a permissive regular expression in Microsoft Microsoft ACI Confidential Containers. The fix is to apply the vendor patch noted below.

What is CVE-2026-23651?

Permissive regular expression in Azure Compute Gallery allows an authorized attacker to elevate privileges locally. The CVSS base score is 6.7 (Medium). The official advisory is at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-23651.

Am I affected?

Check the version of Microsoft ACI Confidential Containers you are running and compare it against the Affected row above (Microsoft ACI Confidential Containers -). If your build sits inside the affected range, you must patch.

Run the version check that fits your platform:

# Linux package check
dpkg -s microsoft-aci-confidential-containers 2>/dev/null | grep -i ^Version
rpm -q microsoft-aci-confidential-containers 2>/dev/null
command -v microsoft-aci-confidential-containers >/dev/null && microsoft-aci-confidential-containers --version 2>/dev/null

# Windows (PowerShell)
Get-Package -Name "*Microsoft ACI Confidential Containers*" -ErrorAction SilentlyContinue | Select-Object Name, Version
winget list --name "Microsoft ACI Confidential Containers" 2>$null

How to fix CVE-2026-23651

Upgrade Microsoft ACI Confidential Containers to a patched build: See vendor advisory. The vendor advisory is the source of truth for the exact fixed version.

Ubuntu / Debian

sudo apt-get update
sudo apt-get install --only-upgrade microsoft-aci-confidential-containers
dpkg -s microsoft-aci-confidential-containers | grep -i ^Version

RHEL / CentOS / Rocky / AlmaLinux

sudo dnf upgrade --refresh microsoft-aci-confidential-containers -y
# or for older releases:
sudo yum update microsoft-aci-confidential-containers -y
rpm -q microsoft-aci-confidential-containers

SUSE / openSUSE

sudo zypper refresh
sudo zypper update microsoft-aci-confidential-containers
rpm -q microsoft-aci-confidential-containers

Container image

docker pull microsoft-aci-confidential-containers:latest
# In Kubernetes:
kubectl set image deploy/microsoft-aci-confidential-containers microsoft-aci-confidential-containers=microsoft-aci-confidential-containers:latest

Complete PowerShell remediation script (Windows)

# Fix script for CVE-2026-23651 affecting Microsoft ACI Confidential Containers
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-23651-fix-$(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] Detect installed version"
    $pkg = Get-Package -Name "*Microsoft ACI Confidential Containers*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

    Write-Host "[2/4] Backup configuration"
    $backup = "C:\Backup\microsoft-aci-confidential-containers-$(Get-Date -Format yyyyMMdd)"
    New-Item -ItemType Directory -Force $backup | Out-Null
    Get-ChildItem "C:\ProgramData\Microsoft ACI Confidential Containers" -ErrorAction SilentlyContinue |
        Copy-Item -Destination $backup -Recurse -Force -ErrorAction SilentlyContinue

    Write-Host "[3/4] Apply the upgrade to See vendor advisory"
    winget upgrade --name "Microsoft ACI Confidential Containers" --silent --accept-source-agreements --accept-package-agreements
    if ($LASTEXITCODE -ne 0) {
        # Fallback: pull latest via OS update channel
        Install-Module -Name PSWindowsUpdate -Force -SkipPublisherCheck -ErrorAction SilentlyContinue
        Import-Module PSWindowsUpdate
        Install-WindowsUpdate -MicrosoftUpdate -AcceptAll -IgnoreReboot
    }

    Write-Host "[4/4] Verify the patched build"
    winget list --name "Microsoft ACI Confidential Containers"
    Write-Host "Patch 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-2026-23651 affecting Microsoft ACI Confidential Containers
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-23651-fix-$(date +%Y%m%d-%H%M%S).log"
exec > >(tee -a "$LOG") 2>&1

echo "[1/4] Detect installed version"
if command -v dpkg >/dev/null; then
    dpkg -s microsoft-aci-confidential-containers 2>/dev/null | grep -i ^Version || echo "microsoft-aci-confidential-containers not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q microsoft-aci-confidential-containers || echo "microsoft-aci-confidential-containers not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-23651-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/microsoft-aci-confidential-containers /etc/microsoft-aci-confidential-containers.d /etc/microsoft-aci-confidential-containers.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Apply the upgrade (target: See vendor advisory)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y microsoft-aci-confidential-containers
elif command -v dnf >/dev/null; then
    dnf upgrade --refresh -y microsoft-aci-confidential-containers
elif command -v yum >/dev/null; then
    yum update -y microsoft-aci-confidential-containers
elif command -v zypper >/dev/null; then
    zypper --non-interactive update microsoft-aci-confidential-containers
fi

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s microsoft-aci-confidential-containers 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q microsoft-aci-confidential-containers
fi
echo "Done. Restart any service that loaded the old library."

If you can't patch immediately

Apply at least one of the following inline controls until you can deploy the patched build. None replace the upgrade.

Restrict exposure with nftables (Linux)

# Allow only trusted CIDR to reach the affected service ports
sudo nft add table inet filter 2>/dev/null || true
sudo nft 'add chain inet filter input { type filter hook input priority 0 ; }' 2>/dev/null || true
sudo nft 'add rule inet filter input tcp dport {80, 443} ip saddr != 10.0.0.0/8 drop'
sudo nft list ruleset

Block at the host firewall (Windows)

New-NetFirewallRule -DisplayName "Block-CVE-2026-23651" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-23651"

Disable the affected service (Linux)

sudo systemctl stop microsoft-aci-confidential-containers 2>/dev/null || true
sudo systemctl disable microsoft-aci-confidential-containers 2>/dev/null || true

If the vendor advisory lists an official workaround, prefer that wording verbatim. If no workaround is published, the only safe remediation is the patch.

How to verify the fix worked

After upgrading, confirm the installed version matches the patched build and that no old library is still loaded by a long-running process.

# Linux
dpkg -s microsoft-aci-confidential-containers 2>/dev/null | grep -i ^Version
rpm -q microsoft-aci-confidential-containers 2>/dev/null || true
# Pid map check for old library handles
sudo lsof +c0 2>/dev/null | grep -i "DEL.*lib" || true

# Windows
Get-Package -Name "*Microsoft ACI Confidential Containers*" | Select-Object Name, Version
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Expected: the reported version is at or above See vendor advisory. Restart the affected service (systemctl restart <service> on Linux, or restart the Windows service) so it loads the patched binary.

Frequently asked questions

Related fixes

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

• How to Fix CVE-2026-20811: Critical Vulnerability in Windows 11 version 22H3 — Critical Vulnerability in Windows 11 version 22H3
• How to Fix CVE-2026-26124: Path traversal: '.../...//' in Microsoft ACI Confidential Containers — Path traversal: '.../...//' in Microsoft ACI Confidential Containers
• How to Fix CVE-2026-35429: Critical Vulnerability in Microsoft Edge for Android , Critical Vulnerability in Microsoft Edge for Android
• How to Fix CVE-2026-21237: Critical Vulnerability in Windows 10 Version 21H2 , Critical Vulnerability in Windows 10 Version 21H2
• How to Fix CVE-2026-20819: Path Traversal in Windows 11 version 22H3 , Path Traversal in Windows 11 version 22H3

Is CVE-2026-23651 actively exploited?

There is no public confirmation of exploitation in the wild listed in CISA KEV at the time of this writing. Patch anyway. Public exploits commonly follow disclosure within weeks.

Do I need to reboot after patching CVE-2026-23651?

For kernel and OS-level updates, yes. For most userland packages a systemctl restart <service> is enough on Linux, and restarting the Windows service or app on Windows. Any process that loaded the old library keeps using it until restarted.

What is the CVSS score for CVE-2026-23651?

6.7 (Medium). Vector: CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:H/I:H/A:H/E:P/RL:O/RC:C.

Where is the official advisory for CVE-2026-23651?

The vendor advisory is at https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-23651. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-23651.

References

• Official vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-23651
• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-23651
• CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog

*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.*