Ask HN: How do you manage server side credentials?

At work we maintain passwords, keys and secrets for a few dozens internal and 3rd party services such as S3, Sendgrid, Xero and more. For most services we actually have at least two accounts, one being used exclusively on production. So far we have been been storing most credentials in plain text config files under version control but we are looking for something more secure in case the source code is compromised. Any suggestions?