meteor/packages/accounts-github/github_server.js

Accounts.oauth.registerService('github', 2, function(query) {

  var accessToken = getAccessToken(query);
  var identity = getIdentity(accessToken);

  return {
    serviceData: {
      id: identity.id,
      accessToken: accessToken,
      email: identity.email,
      username: identity.login
    },
    options: {profile: {name: identity.name}}
  };
});

var getAccessToken = function (query) {
  var config = Accounts.loginServiceConfiguration.findOne({service: 'github'});
  if (!config)
    throw new Accounts.ConfigError("Service not configured");

  var result = Meteor.http.post(
    "https://github.com/login/oauth/access_token", {headers: {Accept: 'application/json'}, params: {
      code: query.code,
      client_id: config.clientId,
      client_secret: config.secret,
      redirect_uri: Meteor.absoluteUrl("_oauth/github?close"),
      state: query.state
    }});
  if (result.error) // if the http response was an error
    throw result.error;
  if (result.data.error) // if the http response was a json object with an error attribute
    throw result.data;
  return result.data.access_token;
};

var getIdentity = function (accessToken) {
  var result = Meteor.http.get(
    "https://api.github.com/user",
    {params: {access_token: accessToken}});
  if (result.error)
    throw result.error;
  return result.data;
};