This affects the package nconf before 0.11.4. When using the memory engine, it is possible to store a nested JSON representation of the configuration. The .set() function, that is responsible for setting the configuration properties, is vulnerable to Prototype Pollution. By providing a crafted property, it is possible to modify the properties on the Object.prototype.
https://github.com/indexzero/nconf/pull/397
https://github.com/indexzero/nconf/releases/tag/v0.11.4