CVE-2021-0243 in Junos OS
Summary
by MITRE • 04/23/2021
Improper Handling of Unexpected Data in the firewall policer of Juniper Networks Junos OS on EX4300 switches allows matching traffic to exceed set policer limits, possibly leading to a limited Denial of Service (DoS) condition. When the firewall policer discard action fails on a Layer 2 port, it will allow traffic to pass even though it exceeds set policer limits. Traffic will not get discarded, and will be forwarded even though a policer discard action is configured. When the issue occurs, traffic is not discarded as desired, which can be observed by comparing the Input bytes with the Output bytes using the following command: user@junos> monitor interface traffic Interface Link Input bytes (bps) Output bytes (bps) ge-0/0/0 Up 37425422 (82616) 37425354 (82616)
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Analysis
by VulDB Data Team • 04/29/2021
The vulnerability CVE-2021-0243 represents a critical flaw in the firewall policer implementation of Juniper Networks Junos OS running on EX4300 switches. This issue stems from improper handling of unexpected data within the policer mechanism, specifically affecting Layer 2 port operations where the discard action fails to function correctly. The flaw manifests when traffic exceeds configured policer limits yet continues to pass through the network infrastructure despite the intended discard action. This improper behavior creates a security and operational risk where network administrators believe traffic is being properly controlled and discarded according to policy, while in reality the traffic bypasses these controls entirely.
The technical implementation of this vulnerability involves the policer component within the Junos OS firewall system that is designed to control traffic rates and enforce bandwidth limits. When configured with discard actions on Layer 2 interfaces, the policer should prevent traffic exceeding specified limits from being forwarded. However, the flaw causes the discard mechanism to fail silently, allowing matching traffic to continue flowing through the switch even when it surpasses the configured policer thresholds. This behavior directly violates the fundamental security principle of traffic control and can be detected through monitoring interface statistics where input bytes significantly exceed output bytes, indicating traffic that should have been discarded but was not.
The operational impact of CVE-2021-0243 extends beyond simple traffic control failures to potentially enable limited Denial of Service conditions. Network administrators may observe that their traffic shaping and rate limiting policies are ineffective, leading to potential network congestion and performance degradation. The specific scenario occurs when Layer 2 port policer discard actions fail, allowing excess traffic to bypass controls and consume network resources that should be reserved for legitimate traffic. This vulnerability affects the integrity of network security policies and can be exploited by malicious actors to overwhelm network resources or bypass traffic control mechanisms, making it particularly concerning for environments where network security and bandwidth management are critical.
The flaw aligns with CWE-129, Improper Validation of Array Index, and CWE-131, Incorrect Calculation of Buffer Size, as it involves improper handling of expected versus unexpected data within the policer implementation. From an ATT&CK perspective, this vulnerability maps to T1498.001, Subvert Trust Controls, and T1562.001, Impairing Security Tools, as it undermines the trust in network security controls and impairs the effectiveness of traffic management mechanisms. Organizations should immediately implement mitigations including applying the latest Juniper security patches, monitoring interface traffic statistics for anomalous input versus output byte ratios, and temporarily disabling problematic policer configurations until proper updates are deployed. The vulnerability demonstrates the critical importance of proper input validation and error handling in network security implementations, particularly in control plane components that manage traffic flow and security enforcement policies.