<s>
A	O
framekiller	B-General_Concept
(	O
or	O
framebuster	B-General_Concept
or	O
framebreaker	B-General_Concept
)	O
is	O
a	O
technique	O
used	O
by	O
websites	O
and	O
web	B-Application
applications	I-Application
to	O
prevent	O
their	O
web	O
pages	O
from	O
being	O
displayed	O
within	O
a	O
frame	O
.	O
</s>
<s>
A	O
framekiller	B-General_Concept
is	O
usually	O
used	O
to	O
prevent	O
a	O
website	O
from	O
being	O
loaded	O
from	O
within	O
a	O
frameset	O
without	O
permission	O
or	O
as	O
an	O
attack	O
,	O
as	O
with	O
clickjacking	O
.	O
</s>
<s>
Framekillers	B-General_Concept
are	O
implemented	O
using	O
JavaScript	B-Language
that	O
validates	O
if	O
the	O
current	O
window	O
is	O
the	O
main	O
window	O
.	O
</s>
<s>
An	O
alternative	O
choice	O
is	O
to	O
allow	O
the	O
user	O
to	O
determine	O
whether	O
to	O
let	O
the	O
framekiller	B-General_Concept
work	O
.	O
</s>
<s>
Historically	O
,	O
the	O
first	O
framekiller	B-General_Concept
scripts	O
were	O
as	O
simple	O
as	O
this	O
:	O
</s>
<s>
This	O
method	O
however	O
can	O
be	O
easily	O
bypassed	O
by	O
blocking	O
execution	O
of	O
the	O
framebuster	B-General_Concept
script	O
from	O
the	O
outer	O
frame	O
.	O
</s>
<s>
Client-side	O
JavaScript	B-Language
solution	O
relies	O
on	O
the	O
end-user	O
'	O
s	O
browser	O
enforcing	O
their	O
own	O
security	O
.	O
</s>
<s>
The	O
user	O
agent	O
does	O
not	O
support	O
JavaScript	B-Language
.	O
</s>
<s>
The	O
user	O
agent	O
supports	O
JavaScript	B-Language
but	O
the	O
user	O
has	O
turned	O
support	O
off	O
.	O
</s>
<s>
The	O
user	O
agent	O
's	O
JavaScript	B-Language
support	O
is	O
flawed	O
or	O
partially	O
implemented	O
.	O
</s>
<s>
If	O
the	O
value	O
is	O
empty	O
or	O
not	O
set	O
,	O
the	O
iframe	O
's	O
content	O
will	O
not	O
execute	O
JavaScript	B-Language
,	O
and	O
wo	O
n't	O
allow	O
top-level	O
navigation	O
.	O
</s>
<s>
By	O
specifying	O
in	O
the	O
space	O
separated	O
set	O
of	O
exceptions	O
in	O
the	O
value	O
,	O
the	O
iframe	O
will	O
allow	O
JavaScript	B-Language
,	O
but	O
will	O
still	O
disallow	O
top-level	O
navigation	O
,	O
rendering	O
framekillers	B-General_Concept
in	O
the	O
iframe	O
impotent	O
.	O
</s>
