CVE-2020-5891 in BIG-IP
Summary
by MITRE
On BIG-IP 15.1.0-15.1.0.1, 15.0.0-15.0.1.2, and 14.1.0-14.1.2.3, undisclosed HTTP/2 requests can lead to a denial of service when sent to a virtual server configured with the Fallback Host setting and a server-side HTTP/2 profile.
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Analysis
by VulDB Data Team • 10/14/2020
The vulnerability described in CVE-2020-5891 represents a significant denial of service weakness affecting F5 BIG-IP appliances running specific software versions. This issue manifests when the system processes certain HTTP/2 requests that are not properly disclosed or handled within the context of virtual servers configured with both Fallback Host settings and server-side HTTP/2 profiles. The flaw resides in the processing logic that governs how the BIG-IP system manages incoming HTTP/2 traffic under these specific configuration conditions.
The technical implementation of this vulnerability stems from inadequate request handling within the HTTP/2 processing pipeline of the BIG-IP system. When an undisclosed HTTP/2 request reaches a virtual server configured with a Fallback Host setting and a server-side HTTP/2 profile, the system fails to properly process or reject these requests, leading to resource exhaustion or system instability. This behavior creates a condition where legitimate service availability is compromised, effectively rendering the affected virtual server unavailable to authorized users. The vulnerability operates at the application layer and specifically targets the HTTP/2 protocol implementation within the BIG-IP configuration management system.
The operational impact of CVE-2020-5891 extends beyond simple service disruption to potentially compromise the entire availability posture of affected BIG-IP deployments. Organizations relying on these appliances for load balancing, SSL termination, or application delivery face significant risk of service interruptions that could affect business continuity and customer access to critical applications. The vulnerability affects multiple major versions of the BIG-IP software, indicating a widespread exposure across the installed base of F5 appliances. Network administrators may experience unexplained service degradation or complete outages that are difficult to diagnose due to the nature of the undisclosed request patterns that trigger the condition.
Mitigation strategies for this vulnerability require immediate attention from system administrators and security teams responsible for BIG-IP deployments. The primary recommended approach involves applying the official F5 security patches and updates that address the specific HTTP/2 processing flaw. Organizations should also consider implementing temporary network-level controls to filter or block suspicious HTTP/2 traffic patterns until permanent patches are deployed. Configuration reviews should focus on identifying virtual servers with Fallback Host settings combined with HTTP/2 profiles, as these represent the primary attack surface for this vulnerability. Security monitoring should be enhanced to detect unusual traffic patterns that might indicate exploitation attempts, while maintaining compliance with industry standards such as those outlined in the CWE taxonomy for HTTP protocol handling weaknesses. The ATT&CK framework categorizes this vulnerability under the denial of service tactic, specifically within the network service disruption category, making it a critical target for both preventive and reactive security measures.