<s>
A	O
downgrade	B-Protocol
attack	I-Protocol
,	O
also	O
called	O
a	O
bidding-down	O
attack	O
or	O
version	B-Protocol
rollback	I-Protocol
attack	I-Protocol
,	O
is	O
a	O
form	O
of	O
cryptographic	O
attack	O
on	O
a	O
computer	O
system	O
or	O
communications	O
protocol	O
that	O
makes	O
it	O
abandon	O
a	O
high-quality	O
mode	O
of	O
operation	O
(	O
e.g.	O
</s>
<s>
an	O
encrypted	B-Protocol
connection	I-Protocol
)	O
in	O
favor	O
of	O
an	O
older	O
,	O
lower-quality	O
mode	O
of	O
operation	O
(	O
e.g.	O
</s>
<s>
cleartext	O
)	O
that	O
is	O
typically	O
provided	O
for	O
backward	B-General_Concept
compatibility	I-General_Concept
with	O
older	O
systems	O
.	O
</s>
<s>
This	O
is	O
one	O
of	O
the	O
most	O
common	O
types	O
of	O
downgrade	B-Protocol
attacks	I-Protocol
.	O
</s>
<s>
Opportunistic	B-Protocol
encryption	I-Protocol
protocols	O
such	O
as	O
STARTTLS	B-Protocol
are	O
generally	O
vulnerable	O
to	O
downgrade	B-Protocol
attacks	I-Protocol
,	O
as	O
they	O
,	O
by	O
design	O
,	O
fall	O
back	O
to	O
unencrypted	O
communication	O
.	O
</s>
<s>
Websites	O
which	O
rely	O
on	O
redirects	O
from	O
unencrypted	O
HTTP	O
to	O
encrypted	O
HTTPS	O
can	O
also	O
be	O
vulnerable	O
to	O
downgrade	B-Protocol
attacks	I-Protocol
(	O
e.g.	O
,	O
sslstrip	O
)	O
,	O
as	O
the	O
initial	O
redirect	O
is	O
not	O
protected	O
by	O
encryption	O
.	O
</s>
<s>
Downgrade	B-Protocol
attacks	I-Protocol
are	O
often	O
implemented	O
as	O
part	O
of	O
a	O
Man-in-the-middle	O
(	O
MITM	O
)	O
attack	O
,	O
and	O
may	O
be	O
used	O
as	O
a	O
way	O
of	O
enabling	O
a	O
cryptographic	O
attack	O
that	O
might	O
not	O
be	O
possible	O
otherwise	O
.	O
</s>
<s>
Downgrade	B-Protocol
attacks	I-Protocol
have	O
been	O
a	O
consistent	O
problem	O
with	O
the	O
SSL/TLS	O
family	O
of	O
protocols	O
;	O
examples	O
of	O
such	O
attacks	O
include	O
the	O
POODLE	B-Protocol
attack	I-Protocol
.	O
</s>
<s>
Downgrade	B-Protocol
attacks	I-Protocol
in	O
the	O
TLS	O
protocol	O
take	O
many	O
forms	O
.	O
</s>
<s>
Researchers	O
have	O
classified	O
downgrade	B-Protocol
attacks	I-Protocol
with	O
respect	O
to	O
four	O
different	O
vectors	O
,	O
which	O
represents	O
a	O
framework	O
to	O
reason	O
about	O
downgrade	B-Protocol
attacks	I-Protocol
as	O
follows	O
:	O
</s>
<s>
web	O
browsers	O
)	O
to	O
protect	O
sensitive	O
domain	O
names	O
against	O
certain	O
types	O
of	O
downgrade	B-Protocol
attacks	I-Protocol
that	O
exploit	O
the	O
clients	O
 '	O
support	O
for	O
legacy	O
versions	O
or	O
non-recommended	O
ciphersuites	O
(	O
e.g.	O
</s>
<s>
those	O
that	O
do	O
not	O
support	O
forward	O
secrecy	O
or	O
authenticated	O
encryption	O
)	O
such	O
as	O
the	O
POODLE	B-Protocol
,	O
ClientHello	O
fragmentation	O
,	O
and	O
a	O
variant	O
of	O
the	O
DROWN	O
(	O
aka	O
"	O
the	O
special	O
drown	O
"	O
)	O
downgrade	B-Protocol
attacks	I-Protocol
.	O
</s>
<s>
Removing	O
backward	B-General_Concept
compatibility	I-General_Concept
is	O
often	O
the	O
only	O
way	O
to	O
prevent	O
downgrade	B-Protocol
attacks	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
if	O
a	O
Web	O
server	O
and	O
user	O
agent	O
both	O
implement	O
HTTP	B-Protocol
Strict	I-Protocol
Transport	I-Protocol
Security	I-Protocol
and	O
the	O
user	O
agent	O
knows	O
this	O
of	O
the	O
server	O
(	O
either	O
by	O
having	O
previously	O
accessed	O
it	O
over	O
HTTPS	O
,	O
or	O
because	O
it	O
is	O
on	O
an	O
"	O
HSTS	O
preload	O
list	O
"	O
)	O
,	O
then	O
the	O
user	O
agent	O
will	O
refuse	O
to	O
access	O
the	O
site	O
over	O
vanilla	O
HTTP	O
,	O
even	O
if	O
a	O
malicious	O
router	O
represents	O
it	O
and	O
the	O
server	O
to	O
each	O
other	O
as	O
not	O
being	O
HTTPS-capable	O
.	O
</s>
