<s>
A	O
directory	B-Protocol
traversal	I-Protocol
(	O
or	O
path	B-Protocol
traversal	I-Protocol
)	O
attack	O
exploits	O
insufficient	O
security	O
validation	O
or	O
sanitization	O
of	O
user-supplied	O
file	O
names	O
,	O
such	O
that	O
characters	O
representing	O
"	O
traverse	O
to	O
parent	O
directory	O
"	O
are	O
passed	O
through	O
to	O
the	O
operating	O
system	O
's	O
file	B-Application
system	I-Application
API	I-Application
.	O
</s>
<s>
An	O
affected	O
application	O
can	O
be	O
exploited	O
to	O
gain	O
unauthorized	O
access	O
to	O
the	O
file	B-Application
system	I-Application
.	O
</s>
<s>
A	O
typical	O
example	O
of	O
a	O
vulnerable	O
application	O
in	O
PHP	B-Application
code	O
is	O
:	O
</s>
<s>
Unix	O
/etc/passwd	O
is	O
a	O
common	O
file	O
used	O
to	O
demonstrate	O
directory	B-Protocol
traversal	I-Protocol
,	O
as	O
it	O
is	O
often	O
used	O
by	O
crackers	O
to	O
try	O
cracking	O
the	O
passwords	O
.	O
</s>
<s>
Directory	B-Protocol
traversal	I-Protocol
in	O
its	O
simplest	O
form	O
uses	O
the	O
../	O
pattern	O
.	O
</s>
<s>
Microsoft	B-Application
Windows	I-Application
and	O
DOS	B-Device
directory	B-Protocol
traversal	I-Protocol
uses	O
the	O
..	O
\	O
or	O
../	O
patterns	O
.	O
</s>
<s>
Directory	B-Protocol
traversal	I-Protocol
has	O
been	O
the	O
cause	O
of	O
numerous	O
Microsoft	O
vulnerabilities	O
.	O
</s>
<s>
Some	O
web	O
applications	O
attempt	O
to	O
prevent	O
directory	B-Protocol
traversal	I-Protocol
by	O
scanning	O
the	O
path	O
of	O
a	O
request	O
URI	B-Protocol
for	O
patterns	O
such	O
as	O
../	O
.	O
</s>
<s>
This	O
check	O
is	O
sometimes	O
mistakenly	O
performed	O
before	O
percent-decoding	O
,	O
causing	O
URIs	B-Protocol
containing	O
patterns	O
like	O
%2e%	O
2e/	O
to	O
be	O
accepted	O
despite	O
being	O
decoded	O
into	O
../	O
before	O
actual	O
use	O
.	O
</s>
<s>
Percent	O
decoding	O
may	O
accidentally	O
be	O
performed	O
multiple	O
times	O
;	O
once	O
before	O
validation	O
,	O
but	O
again	O
afterwards	O
,	O
making	O
the	O
application	O
vulnerable	O
to	O
Double	B-Protocol
percent-encoding	I-Protocol
attacks	I-Protocol
in	O
which	O
illegal	O
characters	O
are	O
replaced	O
by	O
their	O
double-percent-encoded	O
form	O
in	O
order	O
to	O
bypass	O
security	O
countermeasures	O
.	O
</s>
<s>
This	O
kind	O
of	O
vulnerability	O
notably	O
affected	O
versions	O
5.0	O
and	O
earlier	O
of	O
Microsoft	O
's	O
IIS	B-Application
web	O
server	O
software	O
.	O
</s>
<s>
This	O
is	O
specifically	O
forbidden	O
by	O
the	O
UTF-8	O
standard	O
,	O
but	O
has	O
still	O
led	O
to	O
directory	B-Protocol
traversal	I-Protocol
vulnerabilities	O
in	O
software	O
such	O
as	O
the	O
IIS	B-Application
web	O
server	O
.	O
</s>
<s>
Some	O
archive	O
formats	O
like	O
zip	B-General_Concept
allow	O
for	O
directory	B-Protocol
traversal	I-Protocol
attacks	I-Protocol
:	O
files	O
in	O
the	O
archive	O
can	O
be	O
written	O
such	O
that	O
they	O
overwrite	O
files	O
on	O
the	O
filesystem	B-Application
by	O
backtracking	O
.	O
</s>
<s>
Code	O
that	O
extracts	O
archive	O
files	O
can	O
be	O
written	O
to	O
check	O
that	O
the	O
paths	O
of	O
the	O
files	O
in	O
the	O
archive	O
do	O
not	O
engage	O
in	O
path	B-Protocol
traversal	I-Protocol
.	O
</s>
<s>
A	O
possible	O
algorithm	O
for	O
preventing	O
directory	B-Protocol
traversal	I-Protocol
would	O
be	O
to	O
:	O
</s>
<s>
Process	O
URI	B-Protocol
requests	O
that	O
do	O
not	O
result	O
in	O
a	O
file	O
request	O
,	O
e.g.	O
,	O
executing	O
a	O
hook	O
into	O
user	O
code	O
,	O
before	O
continuing	O
below	O
.	O
</s>
<s>
When	O
a	O
URI	B-Protocol
request	O
for	O
a	O
file/directory	O
is	O
to	O
be	O
made	O
,	O
build	O
a	O
full	O
path	O
to	O
the	O
file/directory	O
if	O
it	O
exists	O
,	O
and	O
normalize	O
all	O
characters	O
(	O
e.g.	O
,	O
%20	O
converted	O
to	O
spaces	O
)	O
.	O
</s>
<s>
(	O
This	O
is	O
PHP-specific	O
.	O
)	O
</s>
