CVE-2020-5877 in BIG-IP
Summary
by MITRE
On BIG-IP 15.0.0-15.1.0.1, 14.1.0-14.1.2.3, 13.1.0-13.1.3.3, 12.1.0-12.1.5.1, and 11.6.1-11.6.5.1, malformed input to the DATAGRAM::tcp iRules command within a FLOW_INIT event may lead to a denial of service.
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Analysis
by VulDB Data Team • 10/14/2020
The vulnerability identified as CVE-2020-5877 represents a critical denial of service weakness affecting F5 BIG-IP appliances across multiple version ranges including 15.0.0 through 15.1.0.1, 14.1.0 through 14.1.2.3, 13.1.0 through 13.1.3.3, 12.1.0 through 12.1.5.1, and 11.6.1 through 11.6.5.1. This flaw specifically targets the DATAGRAM::tcp iRules command when executed within a FLOW_INIT event context, creating a condition where malformed input can trigger system instability. The vulnerability operates at the application layer and leverages the iRules scripting framework that allows administrators to customize traffic handling behaviors on the BIG-IP platform, making it particularly dangerous as it can be exploited through legitimate configuration mechanisms.
The technical implementation of this vulnerability stems from insufficient input validation within the iRules processing engine when handling TCP datagram operations during flow initialization. When a malformed packet or malformed iRules command is processed through the DATAGRAM::tcp command within a FLOW_INIT event, the system fails to properly sanitize the input data, leading to potential memory corruption or unexpected behavior that ultimately results in service disruption. This weakness manifests as a denial of service condition where legitimate traffic processing becomes impossible, effectively rendering the affected BIG-IP appliance unable to function normally. The vulnerability is classified under CWE-129 as an improper validation of the length of a field, specifically in how the system handles the length parameter during TCP datagram processing within iRules.
From an operational impact perspective, this vulnerability presents a severe threat to network infrastructure reliability and business continuity. Organizations relying on BIG-IP appliances for load balancing, application delivery, and traffic management face potential service outages that could affect hundreds or thousands of users depending on the scale of deployment. The exploitability of this vulnerability requires minimal prerequisites since it operates through legitimate iRules functionality, making it particularly dangerous as it could be triggered by malformed network traffic or through configuration changes. Attackers could potentially craft specific traffic patterns or manipulate iRules configurations to induce the denial of service condition, creating sustained disruption to critical services. The vulnerability also represents a potential vector for persistent attacks where repeated triggering could maintain service disruption without requiring additional authentication or privileges.
Mitigation strategies for CVE-2020-5877 should prioritize immediate patch application from F5 as the primary defense mechanism, with the vendor releasing security updates specifically addressing the input validation weakness in the iRules processing engine. Network administrators should implement strict iRules validation procedures, particularly for any custom rules involving the DATAGRAM::tcp command within FLOW_INIT events, ensuring that all input parameters are properly sanitized and validated before execution. Monitoring systems should be enhanced to detect anomalous iRules execution patterns or unusual traffic processing behavior that might indicate exploitation attempts. Organizations should also consider implementing network segmentation to limit exposure and reduce the potential impact of successful exploitation. Additionally, the implementation of intrusion detection systems capable of identifying malformed iRules traffic patterns and the establishment of incident response procedures specifically addressing this vulnerability are recommended. The ATT&CK framework categorizes this vulnerability under T1499.004 for network denial of service attacks, emphasizing the need for defensive measures that prevent unauthorized access to system resources through malformed input manipulation.