Possible path traversal by use of the `doc` module

Icinga Web 2 is an open source monitoring web interface, framework and command-line interface. Between versions 2.3.0 and 2.8.2, the `doc` module of Icinga Web 2 allows to view documentation directly in the UI. It must be enabled manually by an administrator and users need explicit access permission to use it. Then, by visiting a certain route, it is possible to gain access to arbitrary files readable by the web-server user. The issue has been fixed in the 2.9.0, 2.8.3, and 2.7.5 releases. As a workaround, an administrator may disable the `doc` module or revoke permission to use it from all users.