<s>
The	O
System	B-Device
Service	I-Device
Descriptor	I-Device
Table	I-Device
(	O
SSDT	O
)	O
is	O
an	O
internal	O
dispatch	O
table	O
within	O
Microsoft	B-Application
Windows	I-Application
.	O
</s>
<s>
When	O
a	O
syscall	O
is	O
issued	O
by	O
a	O
user	B-Operating_System
space	I-Operating_System
application	O
,	O
it	O
contains	O
the	O
service	O
index	O
as	O
parameter	O
to	O
indicate	O
which	O
syscall	O
is	O
called	O
.	O
</s>
<s>
A	O
parameter	O
passed	O
by	O
the	O
calling	O
userspace	B-Operating_System
application	O
determines	O
which	O
SSDT	O
shall	O
be	O
used	O
.	O
</s>
<s>
For	O
both	O
reasons	O
,	O
hooking	O
SSDT	O
calls	O
is	O
often	O
used	O
as	O
a	O
technique	O
in	O
both	O
Windows	O
kernel	O
mode	O
rootkits	O
and	O
antivirus	B-Application
software	I-Application
.	O
</s>
<s>
In	O
2010	O
,	O
many	O
computer	O
security	O
products	O
which	O
relied	O
on	O
hooking	O
SSDT	O
calls	O
were	O
shown	O
to	O
be	O
vulnerable	O
to	O
exploits	O
using	O
race	B-Operating_System
conditions	I-Operating_System
to	O
attack	O
the	O
products	O
 '	O
security	O
checks	O
.	O
</s>
