See https://github.com/digitalbazaar/forge/blob/v1.3.1/lib/jsbn.js#L120-L133

The JSBN library uses the global navigator object to make optimisation assumptions about the runtime. It is currently assumed that node has no navigator global object and, thus, will follow the typeof navigator === 'undefined' path. This is no longer the case in v21+ and so now a different path is taken.

For now this can be avoided by running node with the --no-experimental-global-navigator option, but presumably this will eventually become permanent and the optimisation logic will need to change.