<s>
The	O
Point-to-Point	B-Protocol
Tunneling	I-Protocol
Protocol	I-Protocol
(	O
PPTP	B-Protocol
)	O
is	O
an	O
obsolete	O
method	O
for	O
implementing	O
virtual	B-Application
private	I-Application
networks	I-Application
.	O
</s>
<s>
PPTP	B-Protocol
has	O
many	O
well	O
known	O
security	O
issues	O
.	O
</s>
<s>
PPTP	B-Protocol
uses	O
a	O
TCP	B-Protocol
control	O
channel	O
and	O
a	O
Generic	B-Protocol
Routing	I-Protocol
Encapsulation	I-Protocol
tunnel	O
to	O
encapsulate	O
PPP	B-Protocol
packets	O
.	O
</s>
<s>
Many	O
modern	O
VPNs	B-Application
use	O
various	O
forms	O
of	O
UDP	B-Protocol
for	O
this	O
same	O
functionality	O
.	O
</s>
<s>
The	O
PPTP	B-Protocol
specification	O
does	O
not	O
describe	O
encryption	O
or	O
authentication	O
features	O
and	O
relies	O
on	O
the	O
Point-to-Point	B-Protocol
Protocol	I-Protocol
being	O
tunneled	O
to	O
implement	O
any	O
and	O
all	O
security	O
functionalities	O
.	O
</s>
<s>
The	O
PPTP	B-Protocol
implementation	O
that	O
ships	O
with	O
the	O
Microsoft	B-Application
Windows	I-Application
product	O
families	O
implements	O
various	O
levels	O
of	O
authentication	O
and	O
encryption	O
natively	O
as	O
standard	O
features	O
of	O
the	O
Windows	O
PPTP	B-Protocol
stack	O
.	O
</s>
<s>
The	O
intended	O
use	O
of	O
this	O
protocol	O
is	O
to	O
provide	O
security	O
levels	O
and	O
remote	O
access	O
levels	O
comparable	O
with	O
typical	O
VPN	B-Application
products	O
.	O
</s>
<s>
A	O
specification	O
for	O
PPTP	B-Protocol
was	O
published	O
in	O
July	O
1999	O
as	O
RFC	O
2637	O
and	O
was	O
developed	O
by	O
a	O
vendor	O
consortium	O
formed	O
by	O
Microsoft	O
,	O
Ascend	O
Communications	O
(	O
today	O
part	O
of	O
Nokia	O
)	O
,	O
3Com	O
,	O
and	O
others	O
.	O
</s>
<s>
PPTP	B-Protocol
has	O
not	O
been	O
proposed	O
nor	O
ratified	O
as	O
a	O
standard	O
by	O
the	O
Internet	O
Engineering	O
Task	O
Force	O
.	O
</s>
<s>
A	O
PPTP	B-Protocol
tunnel	O
is	O
instantiated	O
by	O
communication	O
to	O
the	O
peer	O
on	O
TCP	B-Protocol
port	O
1723	O
.	O
</s>
<s>
This	O
TCP	B-Protocol
connection	O
is	O
then	O
used	O
to	O
initiate	O
and	O
manage	O
a	O
GRE	B-Protocol
tunnel	I-Protocol
to	O
the	O
same	O
peer	O
.	O
</s>
<s>
The	O
PPTP	B-Protocol
GRE	B-Protocol
packet	O
format	O
is	O
non	O
standard	O
,	O
including	O
a	O
new	O
acknowledgement	O
number	O
field	O
replacing	O
the	O
typical	O
routing	O
field	O
in	O
the	O
GRE	B-Protocol
header	O
.	O
</s>
<s>
However	O
,	O
as	O
in	O
a	O
normal	O
GRE	B-Protocol
connection	O
,	O
those	O
modified	O
GRE	B-Protocol
packets	O
are	O
directly	O
encapsulated	O
into	O
IP	B-Protocol
packets	O
,	O
and	O
seen	O
as	O
IP	B-Protocol
protocol	I-Protocol
number	O
47	O
.	O
</s>
<s>
The	O
GRE	B-Protocol
tunnel	I-Protocol
is	O
used	O
to	O
carry	O
encapsulated	O
PPP	B-Protocol
packets	O
,	O
allowing	O
the	O
tunnelling	O
of	O
any	O
protocols	O
that	O
can	O
be	O
carried	O
within	O
PPP	B-Protocol
,	O
including	O
IP	B-Protocol
,	O
NetBEUI	B-Protocol
and	O
IPX	B-Protocol
.	O
</s>
<s>
In	O
the	O
Microsoft	O
implementation	O
,	O
the	O
tunneled	O
PPP	B-Protocol
traffic	O
can	O
be	O
authenticated	O
with	O
PAP	B-Protocol
,	O
CHAP	B-Protocol
,	O
MS-CHAP	B-Algorithm
v1/v2	I-Algorithm
.	O
</s>
<s>
PPTP	B-Protocol
has	O
been	O
the	O
subject	O
of	O
many	O
security	O
analyses	O
and	O
serious	O
security	O
vulnerabilities	O
have	O
been	O
found	O
in	O
the	O
protocol	O
.	O
</s>
<s>
The	O
known	O
vulnerabilities	O
relate	O
to	O
the	O
underlying	O
PPP	B-Protocol
authentication	O
protocols	O
used	O
,	O
the	O
design	O
of	O
the	O
MPPE	B-Protocol
protocol	O
as	O
well	O
as	O
the	O
integration	O
between	O
MPPE	B-Protocol
and	O
PPP	B-Protocol
authentication	O
for	O
session	O
key	O
establishment	O
.	O
</s>
<s>
MS-CHAP-v1	O
is	O
fundamentally	O
insecure	O
.	O
</s>
<s>
Tools	O
exist	O
to	O
trivially	O
extract	O
the	O
NT	O
Password	O
hashes	O
from	O
a	O
captured	O
MSCHAP-v1	O
exchange	O
.	O
</s>
<s>
When	O
using	O
MS-CHAP-v1	O
,	O
MPPE	B-Protocol
uses	O
the	O
same	O
RC4	B-Algorithm
session	O
key	O
for	O
encryption	O
in	O
both	O
directions	O
of	O
the	O
communication	O
flow	O
.	O
</s>
<s>
MS-CHAP-v2	O
is	O
vulnerable	O
to	O
dictionary	O
attacks	O
on	O
the	O
captured	O
challenge	O
response	O
packets	O
.	O
</s>
<s>
In	O
2012	O
,	O
it	O
was	O
demonstrated	O
that	O
the	O
complexity	O
of	O
a	O
brute-force	O
attack	O
on	O
a	O
MS-CHAP-v2	O
key	O
is	O
equivalent	O
to	O
a	O
brute-force	O
attack	O
on	O
a	O
single	B-Algorithm
DES	I-Algorithm
key	O
.	O
</s>
<s>
An	O
online	O
service	O
was	O
also	O
demonstrated	O
which	O
is	O
capable	O
of	O
decrypting	O
a	O
MS-CHAP-v2	O
MD4	O
passphrase	O
in	O
23	O
hours	O
.	O
</s>
<s>
MPPE	B-Protocol
uses	O
the	O
RC4	B-Algorithm
stream	O
cipher	O
for	O
encryption	O
.	O
</s>
<s>
EAP-TLS	B-Protocol
is	O
seen	O
as	O
the	O
superior	O
authentication	O
choice	O
for	O
PPTP	B-Protocol
;	O
however	O
,	O
it	O
requires	O
implementation	O
of	O
a	O
public-key	B-Architecture
infrastructure	I-Architecture
for	O
both	O
client	O
and	O
server	O
certificates	O
.	O
</s>
<s>
Most	O
networks	O
that	O
use	O
PPTP	B-Protocol
have	O
to	O
apply	O
additional	O
security	O
measures	O
or	O
be	O
deemed	O
completely	O
inappropriate	O
for	O
the	O
modern	O
internet	O
environment	O
.	O
</s>
