CVE-2021-31376 in Junos OS
Summary
by MITRE • 10/19/2021
An Improper Input Validation vulnerability in Packet Forwarding Engine manager (FXPC) process of Juniper Networks Junos OS allows an attacker to cause a Denial of Service (DoS) by sending specific DHCPv6 packets to the device and crashing the FXPC service. Continued receipt and processing of this specific packet will create a sustained Denial of Service (DoS) condition. This issue affects only the following platforms in ACX Series: ACX500, ACX1000, ACX1100, ACX2100, ACX2200, ACX4000, ACX5048, ACX5096 devices. Other ACX platforms are not affected from this issue. This issue affects Juniper Networks Junos OS on ACX500, ACX1000, ACX1100, ACX2100, ACX2200, ACX4000, ACX5048, ACX5096: 18.4 version 18.4R3-S7 and later versions prior to 18.4R3-S8. This issue does not affect: Juniper Networks Junos OS 18.4 versions prior to 18.4R3-S7 on ACX500, ACX1000, ACX1100, ACX2100, ACX2200, ACX4000, ACX5048, ACX5096.
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Analysis
by VulDB Data Team • 10/27/2021
The vulnerability described in CVE-2021-31376 represents a critical improper input validation flaw within the Packet Forwarding Engine manager component of Juniper Networks Junos OS operating on specific ACX Series devices. This issue manifests as a denial of service condition that can be triggered by sending specially crafted DHCPv6 packets to affected systems. The vulnerability specifically targets the FXPC process which handles packet forwarding operations, making it particularly dangerous as it directly impacts the core networking functionality of these devices. The flaw allows an unauthenticated attacker to disrupt network operations by exploiting insufficient validation mechanisms in the DHCPv6 processing pipeline.
The technical implementation of this vulnerability stems from inadequate input validation within the FXPC service when processing DHCPv6 packets. When the system receives malformed or specially constructed DHCPv6 packets, the validation routines fail to properly sanitize the input data before processing. This weakness creates a condition where the FXPC service becomes overwhelmed or enters an unstable state, ultimately leading to service termination and complete system disruption. The vulnerability is classified under CWE-20, Improper Input Validation, which is a fundamental security principle that emphasizes the necessity of validating all inputs to prevent unexpected behavior in software systems. The attack vector specifically targets the network infrastructure layer where DHCPv6 functionality is implemented, making it particularly effective against devices that rely heavily on dynamic IP address allocation.
The operational impact of this vulnerability extends beyond simple service disruption to potentially compromise network availability and business continuity for organizations relying on affected Juniper ACX Series equipment. The sustained denial of service condition means that once the attack is initiated, the affected devices remain in a crashed state until manual intervention or system reboot occurs. This creates significant operational challenges for network administrators who must monitor and maintain service availability across their infrastructure. The vulnerability affects a specific subset of Juniper ACX platforms including ACX500, ACX1000, ACX1100, ACX2100, ACX2200, ACX4000, ACX5048, and ACX5096 devices running Junos OS versions 18.4R3-S7 and later but before 18.4R3-S8. Organizations with these specific configurations face immediate risk and require urgent remediation to maintain network stability and security posture.
Mitigation strategies for CVE-2021-31376 should prioritize immediate deployment of the official Juniper security patches released for affected versions. Network administrators should implement network segmentation and access control measures to limit exposure of affected devices to untrusted networks. The recommended approach includes applying the specific software updates provided by Juniper that address the input validation flaw in the FXPC process. Organizations should also consider implementing network monitoring solutions to detect anomalous DHCPv6 traffic patterns that may indicate exploitation attempts. From an ATT&CK framework perspective, this vulnerability maps to T1499.004 - Endpoint Denial of Service, where the attacker leverages malformed input to cause service disruption. Additional defensive measures include implementing rate limiting on DHCPv6 traffic and establishing incident response procedures to quickly identify and remediate similar vulnerabilities. The vulnerability highlights the importance of maintaining up-to-date security patches and conducting regular vulnerability assessments to identify potential attack vectors within network infrastructure components.