Google::Auth versions 0.12 for Perl allow server side request forgery via unvalidated credential_source URLs in AWS external account credentials. An external_account configuration with the AWS subject token type selects Google::Auth::ExternalAccountCredentials::Aws. Its retrieve_subject_token sends requests to the imdsv2_session_token_url, url and region_url fields of credential_source without the host check that the other credential classes apply. Any caller that builds credentials from a configuration it does not fully control issues GET and PUT requests to hosts the configuration names, including internal services and link-local metadata endpoints.