Cordova doesn't work with private npm registry

We have a private npm registry using Sinopia, which requires authentication.

When I try to use "cordova plugin add foobar", I get an error

Fetching plugin "foobar" via npm
Fetching from npm failed: Cannot insert data into the registry without auth
Error: Cannot insert data into the registry without auth
at RegClient.regRequest [as request] (/home/patrick/.nvm/versions/node/v0.12.4/lib/node_modules/cordova/node_modules/cordova-lib/node_modules/npm/node_modules/npm-registry-client/lib/request.js:96:15)
at RegClient.get_ (/home/patrick/.nvm/versions/node/v0.12.4/lib/node_modules/cordova/node_modules/cordova-lib/node_modules/npm/node_modules/npm-registry-client/lib/get.js:116:8)
at RegClient.<anonymous> (/home/patrick/.nvm/versions/node/v0.12.4/lib/node_modules/cordova/node_modules/cordova-lib/node_modules/npm/node_modules/npm-registry-client/lib/get.js:43:15)
at FSReqWrap.oncomplete (fs.js:95:15)

I suspect this might be a problem with the older version of npm, 1.3.4, included with Cordova. It seems like npm-registry-client/lib/request.js isn't properly handling the auth settings in npmrc.