CVE-2020-5871 in BIG-IP
Summary
by MITRE
On BIG-IP 14.1.0-14.1.2.3, undisclosed requests can lead to a denial of service (DoS) when sent to BIG-IP HTTP/2 virtual servers. The problem can occur when ciphers, which have been blacklisted by the HTTP/2 RFC, are used on backend servers. This is a data-plane issue. There is no control-plane exposure.
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Analysis
by VulDB Data Team • 05/01/2020
The vulnerability identified as CVE-2020-5871 represents a significant denial of service weakness affecting F5 BIG-IP systems running specific software versions. This issue manifests within the data-plane processing of HTTP/2 virtual servers, where certain cryptographic configurations can trigger system instability. The vulnerability specifically impacts BIG-IP versions 14.1.0 through 14.1.2.3, creating a window of exposure for organizations utilizing these particular releases. The root cause stems from the system's handling of requests that contain blacklisted ciphers as defined by the HTTP/2 RFC standards, which creates an unexpected processing path that can result in system resource exhaustion or complete service interruption.
The technical flaw operates at the HTTP/2 protocol layer where the BIG-IP system processes incoming requests that contain cryptographic ciphers that have been explicitly prohibited by RFC specifications. When these blacklisted ciphers are present in backend server configurations, the BIG-IP system's HTTP/2 virtual server implementation fails to properly handle the request processing, leading to resource consumption anomalies that ultimately result in denial of service conditions. This behavior constitutes a protocol processing error that falls under CWE-129, which addresses issues related to improper handling of input boundaries and protocol violations. The vulnerability's classification as a data-plane issue indicates that it operates within the forwarding plane of the system rather than affecting management or control functions, though this also means that legitimate traffic can be disrupted through carefully crafted malicious requests.
The operational impact of this vulnerability extends beyond simple service interruption to potentially affect business continuity and customer experience for organizations relying on F5 BIG-IP load balancing services. When exploited, the vulnerability can cause complete service unavailability for HTTP/2 virtual servers, forcing organizations to either implement emergency workarounds or apply time-sensitive patches. The nature of the vulnerability means that it can be triggered by seemingly legitimate requests that happen to contain the specific blacklisted cipher combinations, making detection and prevention challenging. This vulnerability aligns with ATT&CK technique T1499.004 which describes network denial of service attacks that target specific protocols or services, and represents a critical weakness in the system's resilience against protocol-based attack vectors.
Organizations affected by this vulnerability should prioritize immediate remediation through official F5 security patches or hotfixes that address the HTTP/2 protocol handling for blacklisted ciphers. Network administrators should also implement monitoring solutions to detect unusual traffic patterns that might indicate exploitation attempts, particularly focusing on HTTP/2 virtual server traffic. The mitigation strategy should include reviewing backend server cipher configurations to eliminate the use of blacklisted ciphers as defined by HTTP/2 RFC specifications. Additionally, implementing rate limiting and traffic filtering mechanisms at the network perimeter can provide additional protection layers while permanent patches are deployed. Organizations should also consider maintaining detailed logs of HTTP/2 traffic patterns to aid in forensic analysis and prevent future incidents, ensuring that their security posture remains robust against similar protocol-based vulnerabilities.