Affected versions of i18next may fail to sanitize user input when certain configuration options are used. When using the .init method, passing interpolation options without passing an escapeValue will default to undefined rather than the assumed true.

Proof of Concept

var init = i18n.init({
  interpolation: {
    prefix: "__",
    suffix: "__",
    escapeValue: true
  }
}, function(){
  var test = i18n.t('__firstName__ __lastName__', {
        firstName: 'Bob',
        lastName: '["foo","bar"]',
  });
  console.log(test);
});

When escapeValue is explicitly passed, the result of test is:

<script>alert(1)</script> Johnson

This is supposed to be the default. However, if escapeValue is not included, the result is the unescaped string:

<script>alert(1)</script> Johnson

Recommendation

Update to version 3.4.4 or later.