CVE-2025-21594 Overview
CVE-2025-21594 is an Improper Check for Unusual or Exceptional Conditions vulnerability (CWE-754) affecting the packet forwarding engine (pfe) of Juniper Networks Junos OS on MX Series routers. This vulnerability allows remote attackers to cause a Denial of Service (DoS) condition by sending crafted IPv6 traffic in DS-Lite (Dual-Stack Lite) and NAT (Network Address Translation) scenarios.
When crafted IPv6 traffic is received and the prefix-length is set to 56, the ports assigned to users within a NAT source port block are not properly freed. This resource exhaustion condition leads to port pool depletion, preventing users from establishing new connections. Recovery requires a manual restart of the affected FPC/PIC components.
Critical Impact
Network-based attackers can remotely exhaust NAT port resources on Juniper MX Series routers, causing service disruption for all users relying on DS-Lite or NAT services. Manual intervention is required to restore service.
Affected Products
- Juniper Junos OS 21.2 (before 21.2R3-S8)
- Juniper Junos OS 21.4 (before 21.4R3-S7)
- Juniper Junos OS 22.1 (before 22.1R3-S6)
- Juniper Junos OS 22.2 (before 22.2R3-S4)
- Juniper Junos OS 22.3 (before 22.3R3-S3)
- Juniper Junos OS 22.4 (before 22.4R3-S2)
- Juniper Junos OS 23.2 (before 23.2R2-S1)
- Juniper Junos OS 23.4 (before 23.4R1-S2 or 23.4R2)
- Juniper MX Series routers: MX204, MX240, MX304, MX480, MX960, MX2008, MX2010, MX2020, MX10004, MX10008
Discovery Timeline
- April 9, 2025 - CVE-2025-21594 published to NVD
- January 26, 2026 - Last updated in NVD database
Technical Details for CVE-2025-21594
Vulnerability Analysis
This vulnerability exists in the packet forwarding engine's handling of IPv6 traffic within DS-Lite and NAT configurations. The core issue lies in improper exception handling when processing specifically crafted IPv6 packets with a prefix-length of 56 bits. Under normal operation, when a NAT session ends, the allocated ports should be returned to the available pool. However, due to this flaw, the port deallocation mechanism fails to execute properly for affected traffic patterns.
The vulnerability can be triggered remotely without authentication, requiring only network access to the affected device. Once exploited, the condition persists until manual administrative intervention, as the system does not automatically recover the leaked port resources.
Root Cause
The root cause is an Improper Check for Unusual or Exceptional Conditions (CWE-754) in the packet forwarding engine. When the system processes IPv6 traffic with specific characteristics—particularly when prefix-length is configured as 56—the PFE fails to properly handle edge cases in the port allocation and deallocation logic. This oversight results in ports remaining in a "used" state indefinitely, even after the associated connections have terminated.
Attack Vector
The attack can be executed remotely over the network by sending crafted IPv6 packets to an affected MX Series router configured for DS-Lite or NAT services. The attacker does not require any privileges or user interaction to trigger the vulnerability. By repeatedly sending malicious traffic, an attacker can gradually exhaust the entire port pool, preventing legitimate users from establishing new connections.
Administrators can identify whether the vulnerability has been exploited using the following Junos OS command:
user@host> show services nat source port-block
Host_IP External_IP Port_Block Ports_Used/ Block_State/
Range Ports_Total Left_Time(s)
2001:: x.x.x.x 58880-59391 256/256 Active/-
When Ports_Used equals Ports_Total (e.g., 256/256) and these ports are not being released despite connection termination, the system is experiencing the port exhaustion condition described in this vulnerability.
Detection Methods for CVE-2025-21594
Indicators of Compromise
- NAT source port blocks showing 100% utilization (Ports_Used equals Ports_Total) that does not decrease over time
- User complaints about inability to establish new connections through DS-Lite or NAT services
- Unusual patterns of IPv6 traffic with prefix-length 56 targeting NAT services
- Persistent port block states showing "Active" status without normal port turnover
Detection Strategies
- Monitor show services nat source port-block output for persistent high port utilization patterns
- Implement SNMP or telemetry monitoring for NAT port pool utilization metrics
- Configure threshold-based alerts when NAT port utilization exceeds normal operational levels (e.g., above 80%)
- Analyze network traffic for anomalous IPv6 packet patterns targeting NAT services
Monitoring Recommendations
- Establish baseline metrics for normal NAT port pool utilization and configure alerts for deviations
- Implement periodic automated checks of NAT port block status across MX Series infrastructure
- Deploy network flow analysis to detect unusual IPv6 traffic patterns with suspicious prefix-length configurations
- Integrate Junos OS syslog events with SIEM platforms for correlation with other security indicators
How to Mitigate CVE-2025-21594
Immediate Actions Required
- Identify all Juniper MX Series routers running vulnerable Junos OS versions with DS-Lite or NAT configurations
- Plan and schedule upgrades to fixed Junos OS versions during the next available maintenance window
- Monitor NAT port block utilization on affected devices for signs of exploitation
- Prepare procedures for manual FPC/PIC restart as a recovery measure if exploitation is detected
Patch Information
Juniper Networks has released security patches addressing this vulnerability. Upgrade to the following fixed versions based on your current release train:
- Junos OS 21.2R3-S8 or later
- Junos OS 21.4R3-S7 or later
- Junos OS 22.1R3-S6 or later
- Junos OS 22.2R3-S4 or later
- Junos OS 22.3R3-S3 or later
- Junos OS 22.4R3-S2 or later
- Junos OS 23.2R2-S1 or later
- Junos OS 23.4R1-S2 or 23.4R2 or later
For complete details, refer to the Juniper Security Advisory JSA96449.
Note: Versions before 20.2R1 are not affected by this vulnerability.
Workarounds
- If exploitation is detected and immediate patching is not possible, manually restart the affected FPC/PIC to restore port availability
- Consider implementing rate limiting on IPv6 traffic to reduce the speed at which port exhaustion can occur
- Monitor and alert on NAT port pool utilization to enable rapid detection and response
- Evaluate network segmentation to limit exposure of vulnerable NAT services to untrusted networks
# Check current NAT port block status
show services nat source port-block
# Identify affected FPC/PIC for restart if exploitation is detected
show chassis fpc
# Restart affected FPC (replace X with FPC slot number) - use during maintenance window
request chassis fpc slot X restart
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
