Sonatype Nexus is a Java based artifact repository where developers can publish their code artifacts for reuse in other projects. The open source version of that product does this job phenomenally well, with some small exceptions. One of those exceptions is that the open source version does not support token authentication out of the box.
It does however have a nod to those who might like to implement this capability themselves. Built in to the OSS version is a plugin called the RUT Auth Plugin. This is a terse name for "Remote User Token" authentication plugin. What this plugin does is it allows an external service/application/proxy to set a header on requests to the Nexus server and this header specifies the username to be authenticated as. No other information is required. This means that if you enable this plugin and don't put some sort of filter in front of your Nexus server, anyone can get it just by setting a header on their requests.
What this application does is use the stand-alone Nexus OSS distribution. That archive includes an installation of Jetty which loads and runs Nexus. This application will ALSO load itself into that same Jetty container and run at a different endpoint (/nexusmanager). In addition, it will load a filter library into the Jetty configuration which will prevent externally set REMOTE_USER headers from being accepted. That filter will also check for a attached AUTH_TOKEN header. That AUTH_TOKEN header will be verified against this application's built-in database to either allow or deny access to the Nexus server by setting or unsetting the appropriate REMOTE_USER header to be passed on to the underlying servlets.
By default, the application uses a file-based instance of HSQLDB to store token information. Changing the web.xml and including the appropriate JDBC driver will allow you to override that behavior.
To configure where the application looks for a Nexus server to authenticate against, you would also modify some of the initParam values in the applications's web.xml file.
The web.xml file can be found at /src/main/webapp/WEB-INF/web.xml