CVE-2026-21907 identifies a cryptographic vulnerability in the TLS/SSL server implementation of Juniper Networks Junos Space, a network management platform widely used in enterprise and service provider environments. The issue arises from the support and use of static key ciphers (ssl-static-key-ciphers), which rely on fixed cryptographic keys rather than ephemeral keys. This design flaw violates modern cryptographic best practices by disabling Perfect Forward Secrecy (PFS), a property that ensures session keys cannot be retroactively compromised even if long-term keys are exposed. Consequently, an attacker positioned on the network path (man-in-the-middle) can capture encrypted traffic and potentially decrypt it later if they obtain the static keys, severely undermining confidentiality. The vulnerability affects all versions of Junos Space before 24.1R5, with no authentication or user interaction required for exploitation, although the attack complexity is high due to the need for on-path access. The CVSS v3.1 base score is 5.9 (medium severity), reflecting the high confidentiality impact but no effect on integrity or availability. No public exploits have been reported yet, but the presence of broken cryptographic algorithms in a critical network management tool poses a latent risk. Remediation involves upgrading to Junos Space 24.1R5 or later and disabling static key cipher suites to enforce the use of secure ephemeral key exchanges that support PFS.

For European organizations, this vulnerability threatens the confidentiality of sensitive network management communications, potentially exposing configuration data, credentials, or operational details if intercepted. The lack of Perfect Forward Secrecy means that even if attackers cannot decrypt traffic immediately, they may do so in the future if static keys are compromised. This risk is particularly critical for sectors with stringent data protection requirements such as finance, telecommunications, energy, and government. Compromise of Junos Space communications could facilitate further attacks on network infrastructure, leading to broader security breaches. Although the vulnerability does not affect integrity or availability directly, the confidentiality breach alone can have severe regulatory and operational consequences under GDPR and other European cybersecurity frameworks. The medium severity rating suggests a moderate but non-negligible risk, warranting timely mitigation to prevent exploitation in environments where Junos Space is deployed.

European organizations should immediately assess their Junos Space deployments to identify affected versions prior to 24.1R5. The primary mitigation is to upgrade Junos Space to version 24.1R5 or later, where this vulnerability is addressed. Until upgrades can be performed, administrators should disable the use of static key cipher suites (ssl-static-key-ciphers) in the TLS/SSL configuration to enforce the use of cipher suites that support Perfect Forward Secrecy, such as those based on ephemeral Diffie-Hellman (DHE) or Elliptic Curve Diffie-Hellman (ECDHE). Network monitoring should be enhanced to detect anomalous on-path interception attempts. Additionally, organizations should review their cryptographic policies to ensure compliance with modern standards and avoid deprecated algorithms. Regular vulnerability scanning and penetration testing focused on TLS configurations can help verify the absence of risky cipher suites. Finally, maintaining strict network segmentation and limiting administrative access to Junos Space management interfaces will reduce the attack surface.