CVE-2026-21917 is a vulnerability classified 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. The vulnerability specifically affects the handling of SSL packets when UTM Web-Filtering is enabled. An attacker can craft a malformed SSL packet that triggers a failure in the Forwarding Plane Card (FPC), causing it to crash and subsequently restart. This results in a denial-of-service (DoS) condition, disrupting network traffic and security services. The vulnerability is exploitable remotely by an unauthenticated attacker without any user interaction, making it highly accessible. The affected Junos OS versions span multiple recent releases: 23.2 versions from 23.2R2-S2 up to but not including 23.2R2-S5, 23.4 versions 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 CVSS v3.1 score is 7.5 (high), reflecting the network attack vector, low attack complexity, no privileges or user interaction required, and impact limited to availability (no confidentiality or integrity impact). No known exploits have been reported in the wild yet, but the potential for disruption to critical network security infrastructure is significant. The vulnerability stems from insufficient input validation in the SSL packet processing logic within the Web-Filtering module, a critical component for enforcing security policies on web traffic.

For European organizations, the impact of CVE-2026-21917 can be severe, especially for those relying on Juniper SRX Series devices for perimeter defense, Unified Threat Management (UTM), and web filtering. A successful exploit results in denial-of-service by crashing the FPC, which can disrupt network traffic inspection and filtering, potentially exposing the network to other threats or causing service outages. Critical infrastructure sectors such as finance, telecommunications, government, and energy that depend on continuous network availability and security enforcement could face operational disruptions. The lack of authentication and user interaction requirements means attackers can launch attacks remotely and anonymously, increasing the risk of widespread exploitation. Additionally, the inability to patch older versions leaves some organizations exposed indefinitely. This vulnerability could be leveraged as part of a broader attack campaign to degrade network defenses or cause targeted disruption. The availability impact could also affect compliance with regulatory requirements around network security and uptime.

Organizations should prioritize upgrading affected Junos OS versions to the fixed releases as soon as they become available from Juniper Networks. For versions without available patches, consider disabling UTM Web-Filtering if feasible or limiting exposure by restricting access to SRX management and filtering interfaces via network segmentation and firewall rules. Implement strict ingress filtering to block malformed or suspicious SSL traffic from untrusted sources. Monitor SRX device logs and network traffic for signs of malformed SSL packets or FPC crashes. Employ redundancy and failover mechanisms to maintain network availability in case of device restarts. Engage with Juniper support for guidance on interim mitigations and updates. Regularly review and update incident response plans to address potential DoS scenarios targeting network security appliances. Finally, maintain an inventory of Juniper SRX devices and their firmware versions to ensure timely patch management.