CVE-2026-21917: Juniper SRX Series DoS Vulnerability

CVE-2026-21917 Overview

An Improper Validation of Syntactic Correctness of Input vulnerability exists in the Web-Filtering module of Juniper Networks Junos OS on SRX Series devices. This flaw allows an unauthenticated, network-based attacker to cause a Denial-of-Service (DoS) condition by sending specifically malformed SSL packets to an SRX device configured for UTM Web-Filtering, resulting in an FPC (Flexible PIC Concentrator) crash and restart.

Critical Impact

Unauthenticated attackers can remotely crash SRX Series firewalls configured for UTM Web-Filtering by sending malformed SSL packets, causing network security infrastructure to become unavailable.

Affected Products

• Junos OS 23.2 versions from 23.2R2-S2 before 23.2R2-S5
• Junos OS 23.4 versions from 23.4R2-S1 before 23.4R2-S5
• Junos OS 24.2 versions before 24.2R2-S2
• Junos OS 24.4 versions before 24.4R1-S3 and 24.4R2
• Earlier versions of Junos OS (no fix available)

Discovery Timeline

• 2026-01-15 - CVE-2026-21917 published to NVD
• 2026-01-16 - Last updated in NVD database

Technical Details for CVE-2026-21917

Vulnerability Analysis

This vulnerability stems from improper validation of syntactic correctness of input (CWE-1286) within the Web-Filtering module of Junos OS running on SRX Series security appliances. When UTM (Unified Threat Management) Web-Filtering is enabled, the system processes SSL/TLS traffic to perform content inspection. The vulnerability occurs when the Web-Filtering module encounters a specially crafted SSL packet that it fails to properly validate before processing.

The impact is particularly significant for network security infrastructure, as SRX Series devices typically serve as perimeter firewalls and security gateways. A successful exploitation causes the FPC to crash and restart, temporarily disrupting all traffic flowing through the affected device and leaving the network without firewall protection during the recovery period.

Root Cause

The root cause is insufficient input validation in the Web-Filtering module's SSL packet parsing logic. When processing SSL handshake or encrypted traffic for UTM inspection, the module does not adequately verify the syntactic correctness of packet structures before attempting to process them. This allows malformed packets to trigger an unhandled exception or memory corruption condition that crashes the FPC.

Attack Vector

The attack can be executed remotely over the network without authentication. An attacker needs to:

1. Identify an SRX Series device with UTM Web-Filtering enabled
2. Craft a malformed SSL packet designed to trigger the parsing flaw
3. Send the packet to the target device through normal network communication
4. The FPC crashes and restarts, causing service interruption

The vulnerability can be triggered through any traffic path that passes through the Web-Filtering inspection, making it exploitable from both internal and external network segments depending on the firewall configuration.

Detection Methods for CVE-2026-21917

Indicators of Compromise

• Unexpected FPC crashes and restarts on SRX Series devices with UTM Web-Filtering enabled
• System log entries indicating FPC exceptions or memory faults related to the Web-Filtering module
• Repeated service interruptions correlating with increased SSL/TLS traffic from specific sources
• Core dumps containing references to Web-Filtering or SSL processing functions

Detection Strategies

• Monitor system logs for FPC crash events using show chassis fpc and show system core-dumps commands
• Implement SNMP monitoring for chassis FPC status alerts and unexpected restart notifications
• Deploy network traffic analysis to identify anomalous SSL packet patterns or high volumes of malformed traffic
• Configure syslog forwarding to a SIEM for centralized detection of repeated FPC failure events

Monitoring Recommendations

• Enable enhanced logging for the Web-Filtering subsystem to capture detailed processing errors
• Set up automated alerts for FPC restart events in your network monitoring infrastructure
• Implement baseline traffic analysis to detect unusual SSL traffic patterns targeting SRX devices
• Review show log messages regularly for UTM-related errors or exceptions

How to Mitigate CVE-2026-21917

Immediate Actions Required

• Upgrade Junos OS to a fixed version: 23.2R2-S5, 23.4R2-S5, 24.2R2-S2, 24.4R1-S3, or 24.4R2
• Review the Juniper Security Advisory JSA105996 for detailed patch information
• Assess all SRX Series devices in your environment for UTM Web-Filtering configuration
• Consider temporarily disabling UTM Web-Filtering on critical devices if patching cannot be performed immediately

Patch Information

Juniper Networks has released security updates to address this vulnerability. Fixed versions include Junos OS 23.2R2-S5, 23.4R2-S5, 24.2R2-S2, 24.4R1-S3, and 24.4R2. Organizations should consult the Juniper Support Portal for download links and detailed upgrade instructions. Note that earlier versions of Junos OS are also affected but no fixes are available for those releases.

Workarounds

• Disable UTM Web-Filtering if not business-critical until patches can be applied
• Implement upstream traffic filtering to block malformed SSL packets before they reach SRX devices
• Deploy redundant SRX devices in high-availability configurations to minimize service impact from potential crashes
• Restrict network access to SRX management interfaces and limit exposure to untrusted networks where possible

# Check current Junos OS version
show version

# Verify UTM Web-Filtering configuration status
show security utm feature-profile web-filtering

# Review FPC status for signs of instability
show chassis fpc detail

# Check for recent core dumps
show system core-dumps