CVE-2020-11080 is a denial of service (DoS) vulnerability in the nghttp2 library, an implementation of the HTTP/2 protocol. The vulnerability exists in versions prior to 1.41.0, where the library does not properly enforce limits on the size of the HTTP/2 SETTINGS frame payload. Specifically, a malicious client can send a SETTINGS frame with an excessively large payload—up to 14,400 bytes containing 2,400 individual settings entries—repeatedly. This causes the CPU usage of the affected system to spike to 100%, effectively leading to a denial of service by exhausting processing resources. The root cause is improper enforcement of message or data structure constraints (CWE-707), allowing the attacker to overwhelm the system with oversized SETTINGS frames. The vulnerability does not affect confidentiality or integrity but impacts availability. The issue was fixed in nghttp2 version 1.41.0. A workaround involves implementing the nghttp2_on_frame_recv_callback callback to detect SETTINGS frames with an unusually high number of entries (e.g., more than 32) and dropping the connection to prevent resource exhaustion. The CVSS v3.1 base score is 3.7 (low severity), reflecting that the attack requires network access but has high attack complexity and no privileges or user interaction needed. No known exploits in the wild have been reported to date.

For European organizations using nghttp2 versions prior to 1.41.0, this vulnerability poses a risk of denial of service through resource exhaustion. Systems acting as HTTP/2 servers or intermediaries that rely on nghttp2 could be targeted by attackers sending malicious SETTINGS frames, causing service degradation or outages. This can impact web services, APIs, or any applications using nghttp2 for HTTP/2 communication, potentially disrupting business operations and user access. While the vulnerability does not compromise data confidentiality or integrity, the availability impact can affect customer trust and operational continuity, especially for critical infrastructure or high-traffic web services. Organizations in sectors such as finance, telecommunications, and government, which often rely on robust HTTP/2 implementations, may experience service interruptions if unpatched. The lack of known exploits reduces immediate risk, but the simplicity of the attack vector (network access only) means opportunistic attackers could exploit unpatched systems.

European organizations should prioritize upgrading nghttp2 to version 1.41.0 or later to fully remediate this vulnerability. If immediate upgrade is not feasible, implement the recommended workaround by using the nghttp2_on_frame_recv_callback callback to monitor incoming SETTINGS frames and drop connections with an excessive number of settings entries (e.g., more than 32). Network-level protections such as rate limiting and anomaly detection on HTTP/2 traffic can help mitigate attack attempts by identifying and blocking abnormal SETTINGS frame sizes. Additionally, organizations should audit their software dependencies to identify all instances of nghttp2 usage, including embedded or third-party applications, to ensure comprehensive coverage. Regular monitoring of CPU usage and HTTP/2 traffic patterns can provide early detection of potential exploitation attempts. Finally, maintain up-to-date intrusion detection and prevention systems (IDS/IPS) with signatures or heuristics for abnormal HTTP/2 frames.