CVE-2026-21917 is a vulnerability categorized under CWE-1286, indicating improper validation of syntactic correctness of input within the Web-Filtering module of Juniper Networks Junos OS running on SRX Series devices. Specifically, when UTM Web-Filtering is enabled, the device improperly validates SSL packets. An attacker can craft a malformed SSL packet that bypasses syntactic checks, causing the Forwarding Packet Card (FPC) to crash and subsequently restart. This leads to a denial-of-service condition, temporarily disrupting network traffic passing through the affected SRX device. The vulnerability affects Junos OS versions 23.2 from 23.2R2-S2 up to but not including 23.2R2-S5, 23.4 from 23.4R2-S1 up to but not including 23.4R2-S5, 24.2 versions before 24.2R2-S2, and 24.4 versions before 24.4R1-S3 and 24.4R2. Earlier versions are also vulnerable but lack available patches. The flaw requires no authentication or user interaction and can be exploited remotely over the network, increasing its risk profile. The CVSS v3.1 score is 7.5 (high), reflecting the network attack vector, low complexity, no privileges required, no user interaction, and impact limited to availability (DoS). No known exploits have been reported in the wild as of now, but the vulnerability poses a clear risk to network stability and availability for organizations using affected Juniper SRX devices with web filtering enabled.

For European organizations, the primary impact of CVE-2026-21917 is the potential for denial-of-service attacks against critical network security infrastructure. Juniper SRX Series devices are widely deployed in enterprise and service provider environments across Europe for firewalling and unified threat management (UTM). A successful exploit can cause the FPC to crash and restart, leading to temporary loss of firewall and filtering capabilities, which can disrupt business operations, degrade network security posture, and potentially expose internal networks to further attacks during downtime. Organizations relying on these devices for perimeter defense, secure web filtering, or VPN termination may experience service outages, impacting availability and potentially causing regulatory compliance issues under frameworks like GDPR if service disruptions affect data processing or availability. The lack of authentication and user interaction requirements means attackers can launch these attacks remotely and anonymously, increasing the threat surface. Additionally, critical infrastructure operators and large enterprises with high dependency on Juniper SRX devices are at elevated risk of operational disruption.

To mitigate CVE-2026-21917, European organizations should immediately assess their Juniper SRX devices to identify affected Junos OS versions running UTM Web-Filtering. Applying vendor-provided patches or updates to fixed versions (e.g., 23.2R2-S5, 23.4R2-S5, 24.2R2-S2, 24.4R1-S3, or later) is the most effective mitigation. If patching is not immediately feasible, organizations should consider temporarily disabling UTM Web-Filtering on SRX devices to prevent exposure to malformed SSL packets. Network-level mitigations such as deploying intrusion prevention systems (IPS) or firewall rules to detect and block malformed SSL traffic patterns targeting SRX devices can reduce attack surface. Monitoring device logs and network traffic for signs of FPC crashes or unusual SSL packet anomalies is recommended to detect exploitation attempts. Additionally, segmenting critical SRX devices behind additional layers of network protection and restricting access to management interfaces can limit attacker reach. Organizations should also review incident response plans to handle potential DoS incidents impacting firewall availability. Coordination with Juniper Networks support and subscribing to security advisories will ensure timely updates on patches and mitigations.