● Critical · CVSS 9.8

How to Fix CVE-2026-21902: Incorrect permission assignment in Junos OS Evolved

Q: What is the CVSS score for CVE-2026-21902?

9.8 (Critical). Vector: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`.

Q: Where is the official advisory for CVE-2026-21902?

The vendor advisory is at [https://supportportal.juniper.net/JSA107128](https://supportportal.juniper.net/JSA107128). The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-21902.

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

CVE-2026-21902 is a incorrect permission assignment in Juniper Networks Junos OS Evolved. The fix is to upgrade to Junos OS Evolved 0.

⚡ At a glance


Severity	9.8 (Critical)
Actively exploited?	No public listing in CISA KEV
Affected	Junos OS Evolved 25.4 to <25.4R1-S1-EVO, 25.4R2-EVO
Fixed in	Junos OS Evolved 0
Type (CWE)	CWE-732 Incorrect Permission Assignment for Critical Resource

What is CVE-2026-21902?

An Incorrect Permission Assignment for Critical Resource vulnerability in the On-Box Anomaly detection framework of Juniper Networks Junos OS Evolved on PTX Series allows an unauthenticated, network-based attacker to execute code as root.

The On-Box Anomaly detection framework should only be reachable by other internal processes over the internal routing instance, but not over an externally exposed port. With the ability to access and manipulate the service to execute code as root a remote attacker can take complete control of the device.

Please note that this service is enabled by default as no specific configuration is required.

This issue affects Junos OS Evolved on PTX Series:

* 25.4 versions before 25.4R1-S1-EVO, 25.4R2-EVO.

This issue does not affect Junos OS Evolved versions before 25.4R1-EVO.

This issue does not affect Junos OS. The CVSS base score is 9.8 (Critical), which puts this in the upper risk band and warrants a fast patch cycle. The official advisory is at https://supportportal.juniper.net/JSA107128.

Am I affected?

Check the version of Junos OS Evolved you are running and compare it against the Affected row above (Junos OS Evolved 25.4 to <25.4R1-S1-EVO, 25.4R2-EVO). If your build sits inside the affected range, you must patch.

Run the version check that fits your platform:


# Linux package check
dpkg -s junos-os-evolved 2>/dev/null | grep -i ^Version
rpm -q junos-os-evolved 2>/dev/null
command -v junos-os-evolved >/dev/null && junos-os-evolved --version 2>/dev/null


# Windows (PowerShell)
Get-Package -Name "*Junos OS Evolved*" -ErrorAction SilentlyContinue | Select-Object Name, Version
winget list --name "Junos OS Evolved" 2>$null

How to fix CVE-2026-21902

Upgrade Junos OS Evolved to a patched build: Junos OS Evolved 0. 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 junos-os-evolved
dpkg -s junos-os-evolved | grep -i ^Version

RHEL / CentOS / Rocky / AlmaLinux


sudo dnf upgrade --refresh junos-os-evolved -y
# or for older releases:
sudo yum update junos-os-evolved -y
rpm -q junos-os-evolved

SUSE / openSUSE


sudo zypper refresh
sudo zypper update junos-os-evolved
rpm -q junos-os-evolved

Complete PowerShell remediation script (Windows)


# Fix script for CVE-2026-21902 affecting Junos OS Evolved
# Run as administrator. Detect -> backup -> upgrade -> verify -> log.

$ErrorActionPreference = "Stop"
$LogPath  = "C:\Logs\CVE-2026-21902-fix-$(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"
    $pkg = Get-Package -Name "*Junos OS Evolved*" -ErrorAction SilentlyContinue
    if ($pkg) { $pkg | Format-Table Name, Version }
    else { Write-Host "Not detected via Get-Package; try winget list" }

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

    Write-Host "[3/4] Apply the upgrade to Junos OS Evolved 0"
    winget upgrade --name "Junos OS Evolved" --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 "Junos OS Evolved"
    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-2026-21902 affecting Junos OS Evolved
# Detect -> backup -> upgrade -> verify -> log.

set -euo pipefail
LOG="/var/log/cve-2026-21902-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 junos-os-evolved 2>/dev/null | grep -i ^Version || echo "junos-os-evolved not installed via dpkg"
elif command -v rpm >/dev/null; then
    rpm -q junos-os-evolved || echo "junos-os-evolved not installed via rpm"
fi

echo "[2/4] Backup configuration"
BACKUP="/root/backup-cve-2026-21902-$(date +%Y%m%d)"
mkdir -p "$BACKUP"
for d in /etc/junos-os-evolved /etc/junos-os-evolved.d /etc/junos-os-evolved.conf; do
    [ -e "$d" ] && cp -a "$d" "$BACKUP/" || true
done

echo "[3/4] Apply the upgrade (target: Junos OS Evolved 0)"
if command -v apt-get >/dev/null; then
    apt-get update
    apt-get install --only-upgrade -y junos-os-evolved
elif command -v dnf >/dev/null; then
    dnf upgrade --refresh -y junos-os-evolved
elif command -v yum >/dev/null; then
    yum update -y junos-os-evolved
elif command -v zypper >/dev/null; then
    zypper --non-interactive update junos-os-evolved
fi

echo "[4/4] Verify the patched build"
if command -v dpkg >/dev/null; then
    dpkg -s junos-os-evolved 2>/dev/null | grep -i ^Version
elif command -v rpm >/dev/null; then
    rpm -q junos-os-evolved
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-2026-21902" -Direction Inbound -Action Block -Protocol TCP -LocalPort 80,443 -RemoteAddress Any -Enabled True
Get-NetFirewallRule -DisplayName "Block-CVE-2026-21902"

Disable the affected service (Linux)


sudo systemctl stop junos-os-evolved 2>/dev/null || true
sudo systemctl disable junos-os-evolved 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 junos-os-evolved 2>/dev/null | grep -i ^Version
rpm -q junos-os-evolved 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 "*Junos OS Evolved*" | Select-Object Name, Version
Get-HotFix | Sort-Object InstalledOn -Descending | Select-Object -First 5

Expected: the reported version is at or above Junos OS Evolved 0. Restart the affected service (systemctl restart <service> on Linux, or restart the Windows service) so it loads the patched binary.

Frequently asked questions

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

How to Fix CVE-2026-21918: Critical Vulnerability in Junos OS — Critical Vulnerability in Junos OS
How to Fix CVE-2026-21905: Denial of Service in Junos OS — Denial of Service in Junos OS
How to Fix CVE-2026-21908: Use-After-Free in Junos OS , Use-After-Free in Junos OS
How to Fix CVE-2026-33775: Memory leak in Junos OS , Memory leak in Junos OS
How to Fix CVE-2026-33783: Function call with incorrect argument type in Junos OS Evolved , Function call with incorrect argument type in Junos OS Evolved

Is CVE-2026-21902 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-2026-21902?

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-2026-21902?

9.8 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.

Where is the official advisory for CVE-2026-21902?

The vendor advisory is at https://supportportal.juniper.net/JSA107128. The NVD record is at https://nvd.nist.gov/vuln/detail/CVE-2026-21902.

References

Official vendor advisory: https://supportportal.juniper.net/JSA107128
NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-21902
CISA KEV catalog: https://www.cisa.gov/known-exploited-vulnerabilities-catalog
Additional reference: https://kb.juniper.net/JSA107128

*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.*