pasv FTP DoS attacks?

Transition

Banned
Sep 8, 2001
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Flipping through the logs on our Web/FTP server i noticed
we get about 20 of these errors a day...

"FTP Server could not create a client worker thread for
user at host 'IPAddress'. The connection to this user is
terminated. The data is the error."

If anyone has any information regarding how i can stop
this (or if it's even malicious at all) i'd appreciate the
help. There should be only 1 person with FTP access by
the way - and i'm getting a sleu of IP's ranging from AOL,
to Ripe.net, to Rogers@Home.

I'd appreciate any input anyone can provide me with.