Viewport does bundle XStream itself, but because of SPI bootstrapping of the newer library, this update breaks also older implementations
Currently the only known workaround is to disable the Viewport app (and/or downgrade Confluence). To disable the app, you need to login through <baseUrl>/login.action to prevent the Viewport filter from picking up the request. After that you can use 'Manage apps' to disable the app.
An update to the XStream library in Confluence dependencies (actually scheduled for 7.13 https://confluence.atlassian.com/doc/preparing-for-confluence-7-13-1044114061.html) breaks the library for existing plugins.
Viewport does bundle XStream itself, but because of SPI bootstrapping of the newer library, this update breaks also older implementations
Currently the only known workaround is to disable the Viewport app (and/or downgrade Confluence). To disable the app, you need to login through <baseUrl>/login.action to prevent the Viewport filter from picking up the request. After that you can use 'Manage apps' to disable the app.