CVE-2026-21920 is a vulnerability in the DNS module of Juniper Networks Junos OS running on SRX Series devices. The root cause is an unchecked return value (CWE-252) in the DNS processing code, which leads to a crash of the flowd process when the device receives a specially crafted DNS request. The flowd process is responsible for flow management and stateful inspection in the SRX firewall, so its crash results in a denial-of-service (DoS) condition, causing service interruption until the process automatically restarts. The vulnerability affects Junos OS versions 23.4 (before 23.4R2-S5), 24.2 (before 24.2R2-S1), and 24.4 (before 24.4R2). Versions prior to 23.4R1 are not affected. Exploitation requires no authentication or user interaction and can be performed remotely over the network by sending malicious DNS packets to the device. The CVSS v3.1 base score is 7.5 (high), reflecting the network attack vector, low attack complexity, no privileges required, no user interaction, and impact limited to availability (no confidentiality or integrity impact). No public exploits or active exploitation have been reported yet. The vulnerability poses a significant risk to network availability, especially in environments where Juniper SRX devices handle DNS traffic and are critical for network security and connectivity.

For European organizations, the primary impact of CVE-2026-21920 is the potential for denial-of-service attacks against Juniper SRX Series firewalls, which are widely deployed in enterprise and service provider networks. A successful exploit can disrupt DNS processing and flow management, leading to temporary loss of firewall functionality and network interruptions. This can affect business continuity, especially for organizations relying on these devices for perimeter defense, VPN termination, and DNS security. Critical infrastructure sectors such as finance, telecommunications, energy, and government agencies in Europe could face operational disruptions and increased exposure to secondary attacks during downtime. The lack of authentication and ease of exploitation increase the risk of opportunistic attacks from external threat actors. Although no data confidentiality or integrity is directly compromised, the availability impact can have cascading effects on dependent systems and services. The vulnerability also raises concerns for managed security service providers (MSSPs) and cloud providers using Juniper SRX devices in their infrastructure.

European organizations should prioritize upgrading affected Junos OS versions to the fixed releases: 23.4R2-S5 or later for 23.4, 24.2R2-S1 or later for 24.2, and 24.4R2 or later for 24.4. Until patches are applied, organizations should consider implementing network-level mitigations such as filtering or rate-limiting DNS traffic to SRX devices, especially from untrusted sources. Monitoring for unusual DNS traffic patterns and flowd process crashes can provide early detection of exploitation attempts. Deploying intrusion detection/prevention systems (IDS/IPS) with signatures targeting malformed DNS packets may help block exploit attempts. Network segmentation to isolate critical SRX devices and restricting management access can reduce exposure. Regularly reviewing Juniper security advisories and subscribing to vendor notifications will ensure timely awareness of updates. Finally, organizations should test patches in controlled environments before deployment to avoid service disruptions.