CVE-2026-21907: Junos Space Cryptographic Vulnerability

CVE-2026-21907 Overview

A Use of a Broken or Risky Cryptographic Algorithm vulnerability (CWE-327) has been identified in the TLS/SSL server of Juniper Networks Junos Space. This security flaw allows the use of static key ciphers (ssl-static-key-ciphers), which significantly reduces the confidentiality of on-path traffic communicated across encrypted connections.

The vulnerability is particularly concerning because these ciphers do not support Perfect Forward Secrecy (PFS), meaning that if a private key is compromised, all past encrypted communications could be decrypted. This affects the long-term confidentiality of sensitive data transmitted through Junos Space management interfaces.

Critical Impact

Network attackers can potentially intercept and decrypt sensitive management traffic. Lack of Perfect Forward Secrecy means historical encrypted communications are at risk if keys are later compromised.

Affected Products

• Juniper Networks Junos Space (all versions before 24.1R5)

Discovery Timeline

• 2026-01-15 - CVE CVE-2026-21907 published to NVD
• 2026-01-16 - Last updated in NVD database

Technical Details for CVE-2026-21907

Vulnerability Analysis

This vulnerability stems from the TLS/SSL server in Junos Space accepting connections that use static key cipher suites. Static key ciphers derive the session key directly from the server's private key rather than using ephemeral key exchange mechanisms like Diffie-Hellman Ephemeral (DHE) or Elliptic Curve Diffie-Hellman Ephemeral (ECDHE).

The absence of Perfect Forward Secrecy (PFS) represents a fundamental cryptographic weakness. In properly configured TLS implementations, even if an attacker later obtains the server's private key, they cannot decrypt previously captured traffic because each session used a unique, ephemeral key. Without PFS, all historical sessions encrypted with static key ciphers become vulnerable to retrospective decryption.

This vulnerability is exploitable over the network without requiring authentication or user interaction, though exploitation requires the attacker to be in a position to intercept network traffic (on-path attacker). The primary impact is to confidentiality, with potential secondary exposure to adjacent systems.

Root Cause

The root cause is the TLS/SSL server configuration in Junos Space that permits the use of deprecated static key cipher suites. These cipher suites, such as TLS_RSA_WITH_AES_128_CBC_SHA and similar RSA key exchange ciphers, do not implement ephemeral key exchange. The server should enforce the use of cipher suites that support PFS (DHE or ECDHE-based suites) and reject connections attempting to negotiate static key ciphers.

Attack Vector

An attacker positioned on the network path between a client and the Junos Space server can exploit this vulnerability through the following attack pattern:

1. Passive Interception: The attacker captures encrypted TLS traffic between legitimate users and the Junos Space management interface
2. Key Compromise: Through a separate attack vector (theft, compromise, or future cryptanalysis), the attacker obtains the server's private RSA key
3. Retrospective Decryption: Using the compromised private key, the attacker decrypts all previously captured sessions that used static key ciphers

This attack is particularly dangerous in environments where network management traffic contains sensitive configuration data, credentials, or proprietary network topology information. The vulnerability can be exploited by passive adversaries who record traffic now with the intent to decrypt it later when keys become available.

Detection Methods for CVE-2026-21907

Indicators of Compromise

• TLS/SSL connections to Junos Space using cipher suites without ECDHE or DHE key exchange
• Server negotiating deprecated cipher suites such as TLS_RSA_WITH_AES_* variants
• Absence of ECDHE or DHE in negotiated cipher suite names in connection logs
• Unusual network traffic patterns indicating potential passive interception or man-in-the-middle positioning

Detection Strategies

• Monitor TLS handshake logs for connections using static key cipher suites (RSA key exchange without ephemeral parameters)
• Deploy network security monitoring tools to identify weak cipher negotiations on ports used by Junos Space (typically HTTPS port 443)
• Conduct regular TLS configuration audits using tools such as testssl.sh or sslyze to identify weak cipher support
• Review Junos Space server logs for connections from unexpected sources or unusual connection patterns

Monitoring Recommendations

• Enable detailed TLS handshake logging on Junos Space servers and forward logs to a SIEM solution
• Configure alerts for any TLS connections negotiating non-PFS cipher suites
• Implement network traffic analysis to detect on-path adversary positioning or traffic interception attempts
• Establish baseline metrics for cipher suite usage and alert on deviations

How to Mitigate CVE-2026-21907

Immediate Actions Required

• Upgrade Juniper Networks Junos Space to version 24.1R5 or later to address this vulnerability
• Review current TLS cipher suite configurations on all Junos Space deployments
• Disable static key cipher suites and enforce PFS-enabled cipher suites (ECDHE/DHE) where possible through manual configuration
• Conduct an assessment of potentially exposed historical traffic and consider rotating sensitive credentials that may have been transmitted

Patch Information

Juniper Networks has addressed this vulnerability in Junos Space version 24.1R5. Organizations should upgrade to this version or later to remediate the issue. Detailed patch and upgrade information is available through the Juniper Knowledge Base Advisory and the Juniper Support Portal Advisory.

Workarounds

• Configure the TLS/SSL server to reject static key cipher suites and only accept ECDHE or DHE-based cipher suites
• Implement network segmentation to limit exposure of Junos Space management interfaces
• Deploy additional encryption layers (VPN tunnels, IPsec) for management traffic as defense-in-depth
• Consider deploying a reverse proxy with properly configured TLS in front of Junos Space to enforce strong cipher policies

Organizations should prioritize upgrading to the patched version as workarounds may not fully address all aspects of the vulnerability.