CVE-2026-21918 is a double free vulnerability categorized under CWE-415 found in the flow processing daemon (flowd) component of Juniper Networks Junos OS on SRX and MX Series platforms. The flaw occurs during TCP session establishment when a crafted sequence of packets triggers a double free condition in flowd's memory management. This leads to a crash of the flowd process and subsequent restart of the Flexible PIC Concentrator (FPC), which handles packet forwarding and processing. The vulnerability affects all Junos OS versions before 22.4R3-S7, 23.2 versions before 23.2R2-S3, 23.4 versions before 23.4R2-S4, and 24.2 versions before 24.2R2. The attack vector is network-based and requires no authentication or user interaction, making it remotely exploitable by unauthenticated attackers. The impact is a denial-of-service condition that disrupts network traffic processing and can degrade or interrupt service availability on affected devices. While no exploits have been observed in the wild, the vulnerability's nature and ease of exploitation make it a significant risk for network operators. Juniper has not yet published patches or mitigation details in the provided data, but affected organizations should monitor vendor advisories closely. The vulnerability does not affect confidentiality or integrity but poses a high risk to availability of critical network infrastructure components.

For European organizations, this vulnerability poses a significant risk to network availability, particularly for enterprises and service providers relying on Juniper SRX and MX Series devices for firewalling, routing, and traffic management. A successful attack can cause flowd crashes and FPC restarts, leading to temporary loss of network connectivity or degraded performance. This can disrupt business operations, critical communications, and services dependent on these network devices. Telecommunications providers, data centers, and large enterprises with extensive Juniper deployments are especially vulnerable, as disruptions could cascade to affect end users and customers. The lack of authentication requirement and remote exploitability increase the threat level, as attackers can launch denial-of-service attacks from anywhere on the network or internet. While no data breach or integrity compromise is indicated, the availability impact alone can cause significant operational and financial damage. Organizations in sectors such as finance, healthcare, government, and critical infrastructure in Europe must consider this vulnerability a high priority for remediation to maintain service continuity and compliance with operational security standards.

1. Immediately identify all Juniper SRX and MX Series devices running affected Junos OS versions within the network. 2. Monitor Juniper Networks' official security advisories for the release of patches or updated firmware addressing CVE-2026-21918 and apply them promptly. 3. Until patches are available, implement network-level protections such as rate limiting or filtering of suspicious TCP session establishment packets that could trigger the double free condition. 4. Employ intrusion detection/prevention systems (IDS/IPS) with updated signatures to detect anomalous packet sequences targeting flowd. 5. Segment critical network infrastructure to limit exposure of vulnerable devices to untrusted networks, especially the internet. 6. Conduct regular network traffic analysis to identify unusual patterns indicative of exploitation attempts. 7. Prepare incident response plans to quickly recover from potential DoS events caused by this vulnerability, including device restarts and failover procedures. 8. Engage with Juniper support for guidance and potential workarounds if immediate patching is not feasible. 9. Maintain up-to-date backups of device configurations to facilitate rapid restoration post-incident. 10. Educate network operations teams about the vulnerability specifics and signs of exploitation to enhance detection and response capabilities.