CVE-2026-21920 is a vulnerability classified under CWE-252 (Unchecked Return Value) found in the DNS processing module of Juniper Networks Junos OS on SRX Series devices. The flaw arises because the software fails to properly check the return value of a function handling DNS requests, leading to a crash of the flowd process when it receives a specially crafted DNS request. Flowd is a critical process responsible for flow monitoring and network traffic analysis on SRX devices. When flowd crashes and restarts, it causes a temporary denial-of-service (DoS) condition, interrupting network traffic monitoring and potentially impacting firewall and routing functions dependent on flow data. The vulnerability affects Junos OS versions 23.4 before 23.4R2-S5, 24.2 before 24.2R2-S1, and 24.4 before 24.4R2, but not versions prior to 23.4R1. Exploitation requires no authentication or user interaction and can be performed remotely by sending malicious DNS requests to the device. The CVSS v3.1 base score is 7.5, reflecting high severity due to network attack vector, no privileges required, no user interaction, and a high impact on availability. No known exploits have been reported in the wild yet, but the vulnerability presents a clear risk to network stability and security monitoring capabilities on affected devices.

For European organizations, this vulnerability poses a significant risk to network infrastructure stability and security monitoring. SRX Series devices are widely used in enterprise and service provider environments for firewalling, routing, and traffic analysis. A successful attack could cause intermittent or prolonged denial-of-service conditions, disrupting critical network services and potentially exposing organizations to further attacks due to loss of flow monitoring visibility. This could impact sectors with high dependency on network uptime such as finance, telecommunications, government, and critical infrastructure. The disruption could also affect compliance with regulatory requirements for network security and monitoring. Given the remote, unauthenticated nature of the exploit, attackers could leverage this vulnerability to cause widespread outages or as part of multi-stage attacks targeting European networks.

Organizations should prioritize upgrading Junos OS on SRX Series devices to fixed versions 23.4R2-S5 or later, 24.2R2-S1 or later, and 24.4R2 or later as soon as possible. In the interim, network administrators should implement strict ingress filtering to block unsolicited DNS traffic to SRX devices, especially from untrusted networks. Monitoring network traffic for unusual DNS request patterns targeting SRX devices can help detect exploitation attempts. Additionally, deploying rate limiting on DNS requests and isolating management interfaces from general network traffic can reduce exposure. Regularly reviewing device logs for flowd crashes or restarts will aid in early detection. Finally, maintaining an up-to-date inventory of Junos OS versions deployed and applying vendor security advisories promptly will minimize risk.