lib/http2/connection.c in H2O before 1.7.3 and 2.x before 2.0.0-beta5 mishandles HTTP/2 disconnection, which allows remote attackers to cause a denial of service (use-after-free and application crash) or possibly execute arbitrary code via a crafted packet.
https://github.com/h2o/h2o/pull/920
https://github.com/h2o/h2o/commit/1c0808d580da09fdec5a9a74ff09e103ea058dd4