pgAdmin 4: LDAP or Kerberos login can open another account's session

In server mode pgAdmin 4 keeps an account per authentication source, so the same username can exist as, for example, an internal account and an LDAP account. After LDAP or Kerberos authenticated a user, pgAdmin chose the account to log in by username alone, or took the account Flask-Security's login form had looked up, without checking that it came from the source that authenticated the user. The session therefore belonged to another account, with its role, its servers and its saved database passwords:

• An LDAP or Kerberos user whose username matched an internal account was logged in as that internal account, including the administrator account created when pgAdmin is first set up.
• With internal authentication listed before LDAP or Kerberos in AUTHENTICATION_SOURCES, the login form's internal validation looks up the account whose email was typed and keeps it even though its password does not match. A Kerberos user could type another Kerberos user's principal and be logged in as that user, and an LDAP user could do the same with an LDAP account whose email is their username.

Exploitation requires valid credentials for an LDAP directory or Kerberos realm that pgAdmin is configured to use.

The fix looks the account up by the authenticated username and the source that authenticated it, and no longer uses the account the login form found for any source but internal authentication. Internal authentication logs in that account only if it is an internal account. Desktop mode is not affected.

This issue affects pgAdmin 4: before 9.19.