The web mail service in Woppoware PostMaster 4.2.2 (build 3.2.5) generates different error messages depending on whether a user exists or not, which allows remote attackers to determine valid usernames.
Exploitability
AV:NAC:LAu:NImpact
C:PI:NA:N5/AV:N/AC:L/Au:N/C:P/I:N/A:N