CVE-2021-0264 in Junos OS Evolved
Summary
by MITRE • 04/23/2021
A vulnerability in the processing of traffic matching a firewall filter containing a syslog action in Juniper Networks Junos OS on MX Series with MPC10/MPC11 cards installed, PTX10003 and PTX10008 Series devices, will cause the line card to crash and restart, creating a Denial of Service (DoS). Continued receipt and processing of packets matching the firewall filter can create a sustained Denial of Service (DoS) condition. When traffic hits the firewall filter, configured on lo0 or any physical interface on the line card, containing a term with a syslog action (e.g. 'term then syslog'), the affected line card will crash and restart, impacting traffic processing through the ports of the line card. This issue only affects MX Series routers with MPC10 or MPC11 line cards, and PTX10003 or PTX10008 Series packet transport routers. No other platforms or models of line cards are affected by this issue. Note: This issue has also been identified and described in technical service bulletin TSB17931 (login required). This issue affects: Juniper Networks Junos OS on MX Series: 19.3 versions prior to 19.3R3-S2; 19.4 versions prior to 19.4R3-S2; 20.1 versions prior to 20.1R3; 20.2 versions prior to 20.2R2-S2, 20.2R3; 20.3 versions prior to 20.3R3; 20.4 versions prior to 20.4R2. Juniper Networks Junos OS Evolved on PTX10003, PTX10008: All versions prior to 20.4R2-EVO. This issue does not affect Juniper Networks Junos OS versions prior to 19.3R1.
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Analysis
by VulDB Data Team • 04/27/2021
This vulnerability represents a critical denial of service condition affecting Juniper Networks MX Series routers equipped with MPC10/MPC11 line cards and PTX10003/PTX10008 Series packet transport routers. The flaw manifests when traffic matching specific firewall filters containing syslog actions triggers a complete line card crash and restart cycle, fundamentally disrupting network operations through the affected hardware components. The vulnerability specifically targets the processing of packets that traverse interfaces configured with firewall filters, particularly those applied to loopback interfaces or physical line card interfaces, creating a persistent threat that can maintain service disruption through continued packet processing.
The technical root cause lies in the improper handling of syslog actions within firewall filter terms, where the system fails to properly validate or process traffic matching filters containing the syslog action directive. This processing error results in memory corruption or execution flow disruption that ultimately leads to the line card's complete system restart. The vulnerability operates through a specific attack pattern where legitimate network traffic matching the configured firewall filter terms containing syslog actions causes the affected hardware to enter an unrecoverable crash state. According to the Common Weakness Enumeration framework, this vulnerability maps to CWE-121, heap-based buffer overflow, and potentially CWE-248, an exception is not caught, as the system fails to properly handle the processing of syslog actions within firewall contexts.
The operational impact of this vulnerability extends beyond simple service interruption to create sustained denial of service conditions that can severely impact network availability and reliability. Network administrators face the challenge of maintaining service continuity when packets matching the specific firewall filter criteria are processed, as each such packet can trigger the line card restart process. This creates a cascading effect where the more traffic that matches the vulnerable filter, the more frequent the service disruptions become, potentially leading to complete network outages through the affected line card ports. The vulnerability affects multiple Junos OS versions across different release streams, indicating a widespread exposure that requires careful version management and patching strategies.
Mitigation strategies should focus on immediate patch application to affected Junos OS versions, with particular attention to the specific version ranges mentioned in the vulnerability disclosure. Network administrators must also consider temporary workarounds such as removing syslog actions from firewall filter terms or reconfiguring traffic processing to avoid triggering the vulnerable code path. The ATT&CK framework categorizes this vulnerability under T1499.004, Network Denial of Service, as it specifically targets network infrastructure to create service disruption through hardware restart conditions. Organizations should implement comprehensive monitoring to detect when traffic matching the vulnerable filter criteria is being processed, and establish incident response procedures to address the potential for sustained service disruption. Additionally, the vulnerability highlights the importance of thorough testing of security policy configurations, particularly those involving logging actions within firewall contexts, to prevent unintended system instability.