CVE-2026-21903

6.5 MEDIUM

Denial of Service Buffer Overflow

📋 TL;DR

A stack-based buffer overflow vulnerability in Juniper Junos OS Packet Forwarding Engine allows authenticated low-privilege attackers to cause denial-of-service by crashing FPCs through telemetry sensor subscription. This affects Junos OS versions before specific patched releases. Network administrators with Juniper devices in affected versions are at risk.

💻 Affected Systems

Products:

• Juniper Networks Junos OS

Versions: All versions before 22.4R3-S7, 23.2 before 23.2R2-S4, 23.4 before 23.4R2

Operating Systems: Junos OS

Default Config Vulnerable: ⚠️ Yes

Notes: Vulnerability manifests when subscribing to telemetry sensors at scale. Not seen when YANG packages for specific sensors are installed.

📦 What is this software?

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

Junos by Juniper

Junos OS is Juniper Networks' flagship network operating system running on enterprise routers, switches, security appliances, and data center infrastructure worldwide. Deployed across telecommunications providers, ISPs, cloud service providers, financial institutions, and large enterprises, Junos po...

Learn more about Junos →

⚠️ Risk & Real-World Impact

🔴

Worst Case

Complete network disruption as multiple FPCs crash and restart, causing extended service outages across affected devices.

🟠

Likely Case

Intermittent network instability and service degradation as FPCs crash and restart, requiring manual intervention.

🟢

If Mitigated

Minimal impact if proper access controls prevent low-privilege users from subscribing to telemetry sensors at scale.

🌐 Internet-Facing: MEDIUM - Requires authenticated access but low privileges, and network-based attackers could exploit if they gain credentials.

🏢 Internal Only: HIGH - Internal users with legitimate low-privilege access could accidentally or intentionally trigger the vulnerability.

🎯 Exploit Status

Public PoC: ✅ No

Weaponized: UNKNOWN

Unauthenticated Exploit: ✅ No

Complexity: MEDIUM

Requires authenticated access with low privileges and knowledge of telemetry sensor subscription mechanisms.

🛠️ Fix & Mitigation

✅ Official Fix

Patch Version: 22.4R3-S7, 23.2R2-S4, 23.4R2 or later

Vendor Advisory: https://kb.juniper.net/JSA106022

Restart Required: Yes

Instructions:

1. Check current Junos OS version. 2. Download appropriate patched version from Juniper support portal. 3. Apply upgrade following Juniper's standard upgrade procedures. 4. Reboot device to complete installation.

🔧 Temporary Workarounds

Install YANG packages

all

Install YANG packages for specific telemetry sensors to prevent the vulnerability from manifesting

Copy

request system software add yang-package.tgz

Restrict telemetry access

all

Limit low-privilege user access to telemetry subscription capabilities

Copy

set system services telemetry access-control deny-all

🧯 If You Can't Patch

• Implement strict access controls to prevent low-privilege users from accessing telemetry subscription features
• Monitor for abnormal telemetry subscription activity and FPC crash/restart events

🔍 How to Verify

Check if Vulnerable:

Copy

Check Junos OS version with 'show version' and compare against affected versions list

Check Version:

Copy

show version

Verify Fix Applied:

Copy

Verify version is 22.4R3-S7 or later, 23.2R2-S4 or later, or 23.4R2 or later using 'show version'

📡 Detection & Monitoring

Log Indicators:

• FPC crash messages
• Packet Forwarding Engine restart logs
• Abnormal telemetry subscription activity

Network Indicators:

• Sudden loss of FPC connections
• Increased packet loss on affected interfaces

SIEM Query:

Copy

source="junos" AND ("FPC crash" OR "pfe restart" OR "telemetry subscription")

📊 Metadata

CVE ID: CVE-2026-21903

CVSS v3 Score: 6.5 (MEDIUM)

CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

CWE: CWE-121

EPSS Score: 0.04% exploit probability (12.4th percentile)

Published: January 15, 2026

Last Updated: January 23, 2026

🔗 References

• https://kb.juniper.net/JSA106022 Vendor Advisory
• https://supportportal.juniper.net/JSA106022 Vendor Advisory

📤 Share & Export

Twitter LinkedIn Reddit Copy Link

📄 Export Markdown 📋 Export JSON

🔗 Related Vulnerabilities

If you're affected by CVE-2026-21903, you might also want to check these:

CVE-2024-39791 10.0

CVE-2024-39791 is a critical stack-based buffer overflow vulnerability in Vonets industrial WiFi bri...

Same vulnerability type (CWE-121)

CVE-2022-20699 10.0

This critical vulnerability in Cisco Small Business routers allows unauthenticated remote attackers ...

Same vulnerability type (CWE-121)

CVE-2022-20701 10.0

This CVE describes multiple critical vulnerabilities in Cisco Small Business RV series routers that ...

Same vulnerability type (CWE-121)