<s>
Double	B-Protocol
encoding	I-Protocol
is	O
the	O
act	O
of	O
encoding	O
data	O
twice	O
in	O
a	O
row	O
using	O
the	O
same	O
encoding	O
scheme	O
.	O
</s>
<s>
In	O
double	B-Protocol
encoding	I-Protocol
attacks	O
against	O
security	O
filters	O
,	O
characters	O
of	O
the	O
payload	O
that	O
are	O
treated	O
as	O
illegal	O
by	O
those	O
filters	O
are	O
replaced	O
with	O
their	O
double-encoded	O
form	O
.	O
</s>
<s>
Double	O
URI-encoding	O
is	O
a	O
special	O
type	O
of	O
double	B-Protocol
encoding	I-Protocol
in	O
which	O
data	O
is	O
URI-encoded	O
twice	O
in	O
a	O
row	O
.	O
</s>
<s>
It	O
has	O
been	O
used	O
to	O
bypass	O
authorization	O
schemes	O
and	O
security	O
filters	O
against	O
code	O
injection	O
,	O
directory	B-Protocol
traversal	I-Protocol
,	O
cross-site	O
scripting	O
(	O
XSS	O
)	O
and	O
SQL	B-Language
injection	I-Language
.	O
</s>
<s>
In	O
double	B-Protocol
encoding	I-Protocol
,	O
data	O
is	O
encoded	O
twice	O
in	O
a	O
row	O
using	O
the	O
same	O
encoding	O
scheme	O
,	O
that	O
is	O
,	O
double-encoded	O
form	O
of	O
data	O
X	O
is	O
Encode(Encode(X )	O
)	O
where	O
Encode	O
is	O
an	O
encoding	O
function	O
.	O
</s>
<s>
Double	B-Protocol
encoding	I-Protocol
is	O
usually	O
used	O
as	O
an	O
attack	O
technique	O
to	O
bypass	O
authorization	O
schemes	O
or	O
security	O
filters	O
that	O
intercept	O
user	O
input	O
.	O
</s>
<s>
In	O
double	B-Protocol
encoding	I-Protocol
attacks	O
against	O
security	O
filters	O
,	O
characters	O
of	O
the	O
payload	O
that	O
are	O
treated	O
as	O
illegal	O
by	O
those	O
filters	O
are	O
replaced	O
with	O
their	O
double-encoded	O
form	O
.	O
</s>
<s>
Double	O
URI-encoding	O
,	O
also	O
referred	O
to	O
as	O
double	O
percent-encoding	O
,	O
is	O
a	O
special	O
type	O
of	O
double	B-Protocol
encoding	I-Protocol
in	O
which	O
data	O
is	O
URI-encoded	O
twice	O
in	O
a	O
row	O
.	O
</s>
<s>
and	O
its	O
URI-encoded	O
form	O
%2E	O
are	O
used	O
in	O
some	O
directory	B-Protocol
traversal	I-Protocol
attacks	I-Protocol
,	O
they	O
are	O
usually	O
treated	O
as	O
illegal	O
by	O
security	O
filters	O
.	O
</s>
<s>
However	O
,	O
it	O
is	O
still	O
possible	O
for	O
%252E	O
,	O
which	O
is	O
the	O
double-URI-encoded	O
form	O
of	O
.	O
,	O
to	O
not	O
to	O
be	O
treated	O
as	O
illegal	O
by	O
security	O
filters	O
and	O
hence	O
pass	O
through	O
them	O
,	O
but	O
later	O
on	O
,	O
when	O
the	O
target	O
system	O
is	O
building	O
the	O
path	O
related	O
to	O
the	O
directory	B-Protocol
traversal	I-Protocol
attack	I-Protocol
it	O
might	O
use	O
the	O
double-URI-decoded	O
form	O
of	O
%252E	O
,	O
which	O
is	O
.	O
,	O
something	O
that	O
the	O
filters	O
would	O
have	O
been	O
treated	O
as	O
illegal	O
.	O
</s>
<s>
Double	O
URI-encoding	O
attacks	O
have	O
been	O
used	O
to	O
bypass	O
authorization	O
schemes	O
and	O
security	O
filters	O
against	O
code	O
injection	O
,	O
directory	B-Protocol
traversal	I-Protocol
,	O
XSS	O
and	O
SQL	B-Language
injection	I-Language
.	O
</s>
<s>
Decoding	O
some	O
user	O
input	O
twice	O
using	O
the	O
same	O
decoding	O
scheme	O
,	O
once	O
before	O
a	O
security	O
measure	O
and	O
once	O
afterwards	O
,	O
may	O
allow	O
double	B-Protocol
encoding	I-Protocol
attacks	O
to	O
bypass	O
that	O
security	O
measure	O
.	O
</s>
<s>
Thus	O
,	O
to	O
prevent	O
double	B-Protocol
encoding	I-Protocol
attacks	O
,	O
all	O
decoding	O
operations	O
on	O
user	O
input	O
should	O
occur	O
before	O
authorization	O
schemes	O
and	O
security	O
filters	O
that	O
intercept	O
user	O
input	O
.	O
</s>
<s>
This	O
opens	O
the	O
possibility	O
for	O
directory	B-Protocol
traversal	I-Protocol
attacks	I-Protocol
that	O
incorporate	O
their	O
payload	O
into	O
the	O
HTTP	B-Protocol
GET	O
parameter	O
file	O
.	O
</s>
<s>
As	O
a	O
security	O
filter	O
against	O
directory	B-Protocol
traversal	I-Protocol
attacks	I-Protocol
,	O
this	O
program	O
searches	O
the	O
value	O
it	O
reads	O
from	O
$_GET["file"]	O
for	O
directory	B-Protocol
traversal	I-Protocol
sequences	O
and	O
exits	O
if	O
it	O
finds	O
one	O
.	O
</s>
<s>
When	O
double-URI-encoded	O
payload	O
%252E%252E%252F%252E%252E%252F%252E%252E%252F%252E%252E%252Fetc%252Fpasswd	O
is	O
used	O
,	O
the	O
value	O
of	O
$_GET["file"]	O
will	O
be	O
%2E%2E%2F%2E%2E%2F%2E%2E%2F%2E%2E%2Fetc%2Fpasswd	O
which	O
does	O
n't	O
contain	O
any	O
directory	B-Protocol
traversal	I-Protocol
sequence	O
and	O
thus	O
passes	O
through	O
the	O
filter	O
and	O
will	O
be	O
given	O
to	O
the	O
urldecode	O
function	O
which	O
returns	O
../../../../etc/passwd	O
,	O
resulting	O
in	O
a	O
successful	O
attack	O
.	O
</s>
<s>
This	O
opens	O
the	O
possibility	O
for	O
XSS	O
attacks	O
that	O
incorporate	O
their	O
payload	O
into	O
the	O
HTTP	B-Protocol
GET	O
parameter	O
name	O
.	O
</s>
