An overly large HTTP/2 SETTINGS frame payload causes denial of service. The proof of concept attack involves a malicious client constructing a SETTINGS frame with a length of 14,400 bytes (2400 individual settings entries) over and over again. The attack causes the CPU to spike at 100%.
For Debian 8 jessie, these problems have been fixed in version 0.6.4-2+deb8u1.
We recommend that you upgrade your nghttp2 packages.
Further information about Extended LTS security advisories can be found at: https://deb.freexian.com/extended-lts/