<s>
FTP	B-Protocol
bounce	I-Protocol
attack	I-Protocol
is	O
an	O
exploit	O
of	O
the	O
FTP	B-Protocol
protocol	O
whereby	O
an	O
attacker	O
is	O
able	O
to	O
use	O
the	O
command	O
to	O
request	O
access	O
to	O
ports	B-Protocol
indirectly	O
through	O
the	O
use	O
of	O
the	O
victim	O
machine	O
,	O
which	O
serves	O
as	O
a	O
proxy	O
for	O
the	O
request	O
,	O
similar	O
to	O
an	O
Open	O
mail	O
relay	O
using	O
SMTP	O
.	O
</s>
<s>
This	O
technique	O
can	O
be	O
used	O
to	O
port	O
scan	O
hosts	O
discreetly	O
,	O
and	O
to	O
potentially	O
bypass	O
a	O
network	O
Access-control	O
list	O
to	O
access	O
specific	O
ports	B-Protocol
that	O
the	O
attacker	O
cannot	O
access	O
through	O
a	O
direct	O
connection	O
,	O
for	O
example	O
with	O
the	O
nmap	B-Language
port	O
scanner	O
.	O
</s>
<s>
Nearly	O
all	O
modern	O
FTP	B-Protocol
server	O
programs	O
are	O
configured	O
by	O
default	O
to	O
refuse	O
commands	O
that	O
would	O
connect	O
to	O
any	O
host	O
but	O
the	O
originating	O
host	O
,	O
thwarting	O
FTP	B-Protocol
bounce	I-Protocol
attacks	I-Protocol
.	O
</s>
