CVE-2021-0299 in Junos OS
Summary
by MITRE • 10/19/2021
An Improper Handling of Exceptional Conditions vulnerability in the processing of a transit or directly received malformed IPv6 packet in Juniper Networks Junos OS results in a kernel crash, causing the device to restart, leading to a Denial of Service (DoS). Continued receipt and processing of this packet will create a sustained Denial of Service (DoS) condition. This issue only affects systems with IPv6 configured. Devices with only IPv4 configured are not vulnerable to this issue. This issue affects Juniper Networks Junos OS: 19.4 versions prior to 19.4R3; 20.1 versions prior to 20.1R2; 20.2 versions prior to 20.2R1-S1, 20.2R2. This issue does not affect Juniper Networks Junos OS versions prior to 19.4R1.
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Analysis
by VulDB Data Team • 10/22/2021
This vulnerability represents a critical improper handling of exceptional conditions within the Juniper Networks Junos OS kernel processing stack specifically when handling IPv6 packets. The flaw manifests as a failure to properly validate and process malformed IPv6 packets that are either transit traffic or directly received by the device. When such malformed packets are processed, the kernel experiences an unhandled exception that results in an immediate system crash and subsequent automatic restart of the device. This behavior constitutes a classic denial of service condition that can be easily exploited by remote attackers who have access to the network path to the vulnerable device.
The technical implementation of this vulnerability stems from inadequate input validation mechanisms within the IPv6 packet processing pipeline of the Junos OS kernel. When the system encounters a malformed IPv6 packet that violates expected packet structure conventions, the kernel's exception handling routines fail to properly manage the error condition. This failure creates a scenario where the system cannot gracefully recover from the malformed packet processing, instead triggering a complete kernel panic that necessitates a full device restart. The vulnerability specifically impacts systems where IPv6 is configured and actively enabled, making it a protocol-specific issue rather than a general system flaw.
The operational impact of this vulnerability extends beyond simple service disruption to create sustained denial of service conditions that can be maintained indefinitely. An attacker who successfully exploits this vulnerability can repeatedly send malformed IPv6 packets to the target device, causing continuous restart cycles that effectively render the device unavailable for legitimate network operations. This sustained attack capability makes the vulnerability particularly dangerous in production environments where network availability is critical. The DoS condition can persist until the device is manually rebooted or until the malformed packet processing is mitigated through firmware updates or network configuration changes.
From a cybersecurity perspective, this vulnerability aligns with CWE-704 (Improper Handling of Exceptional Conditions) and maps to ATT&CK technique T1499.004 (Endpoint Denial of Service) within the adversary tactics and techniques framework. The vulnerability affects specific Junos OS versions including 19.4 prior to 19.4R3, 20.1 prior to 20.1R2, and 20.2 prior to 20.2R1-S1 and 20.2R2, while excluding systems running versions prior to 19.4R1 which were not affected by this particular kernel processing flaw. The attack surface is limited to IPv6-enabled devices, which provides some mitigation but also creates a clear attack vector for adversaries targeting networks with IPv6 configurations. Organizations should immediately implement firmware updates to address this vulnerability and consider network segmentation strategies to limit exposure to potential attackers who might exploit this flaw for sustained disruption attacks.
The remediation approach requires immediate deployment of patched Junos OS versions across all affected devices, with particular attention to network infrastructure components that have IPv6 enabled. System administrators should also implement network monitoring solutions to detect unusual restart patterns that might indicate exploitation attempts. Additionally, temporary network configuration changes such as disabling IPv6 on non-essential devices or implementing packet filtering rules that can identify and drop malformed IPv6 traffic can provide interim protection while full patch deployment is underway. This vulnerability underscores the importance of robust exception handling in network operating systems and demonstrates how seemingly minor input validation flaws can result in significant operational disruptions in critical network infrastructure.