In versions of helm prior to 3.6.1, a vulnerability exists where the username and password credentials associated with a Helm repository could be passed on to another domain referenced by that Helm repository.
The index.yaml within a Helm chart repository contains a reference where to get the chart archive for each version of a chart. The reference can be relative to the index.yaml file or a URL to location. The URL can point to any domain and this is a feature leveraged by Helm users. For example, an index.yaml file can be hosted on GitHub pages while the chart archives are hosted as GitHub releases. These are on different domain names and the index.yaml file points to the other domain.
When a username and password were associated with a Helm repository the username and password were also passed on to other domains referenced in the index.yaml file. This occurred when Helm went to retrieve a specific chart archive on the other domain.