XStream upgrade to 1.4.18

h3. Problem

XStream is vulnerable to security exploits such as highlighted in the image attached.
(i) The list of CVEs can be found in [https://x-stream.github.io/security.html]

This ticket tracks its upgrade to 1.4.18.
h3. Environment

Confluence v7.13
h3. Workaround

Set {{xstream.allowlist.enable}} sysprop to true. This is equivalent to XStream 1.4.18 behaviour and it exist in Confluence 7.10 and up. But it comes with a risk of broken third-party plugins which have not yet configured [xstream-security|https://confluence.atlassian.com/doc/xstream-1-4-upgrade-1026045605.html] module with their classes. Confirm with Third-party plugin vendors before toggling it if your Confluence instance uses a third-party plugin and it relies on XStream.