Insertion of sensitive information into a log file in the SMTP server of Progressive Robot hMailServer 6.0.0 through 6.3.6 allows a local user who can read the server's log, or anyone given a copy of it, to obtain credentials. With SMTP logging enabled, the server hid the credential of an AUTH PLAIN command only when it was sent after EHLO in one exact form, and wrote the rest of the SASL exchange as the client sent it: a PLAIN password sent before EHLO or with extra spacing, the OAuth 2.0 bearer token of AUTH XOAUTH2 and AUTH OAUTHBEARER (from 6.2.0), the SCRAM-SHA-256 client proof (from 6.2.0), which with the logged salt and iteration count allows offline password guessing, and the GSSAPI authenticator (from 6.3.2). SMTP logging is off by default; on Windows before 6.3.6 the log folder was readable by every local user.
Upgrade to hMailServer 6.3.7, which masks every line of a SASL exchange and every AUTH initial response in the SMTP log, but the user name of AUTH LOGIN and the authentication identity of AUTH PLAIN, as before. Until then, turn SMTP logging off where it is not needed or keep the logs readable by administrators only, remove the AUTH exchanges from a log before sharing it, and where a log may have been read by others revoke the OAuth 2.0 grants and change the passwords of the accounts that signed in (after upgrading as well: an upgrade does not take credentials out of logs already written, so delete or redact those logs).