Two security issues were identified in nginx HTTP/2 implementation, which might cause excessive memory consumption (CVE-2018-16843) and CPU usage (CVE-2018-16844).
The issues affect nginx compiled with the ngx_http_v2_module (not compiled by default) if the "http2" option of the "listen" directive is used in a configuration file.
The issues affect nginx 1.9.5 - 1.15.5. The issues are fixed in nginx 1.15.6, 1.14.1.
Thanks to Gal Goldshtein from F5 Networks for initial report of the CPU usage issue.