<s>
OpenSSH	B-Language
(	O
also	O
known	O
as	O
OpenBSD	B-Language
Secure	I-Language
Shell	I-Language
)	O
is	O
a	O
suite	O
of	O
secure	O
networking	B-Architecture
utilities	O
based	O
on	O
the	O
Secure	B-Protocol
Shell	I-Protocol
(	O
SSH	B-Protocol
)	O
protocol	O
,	O
which	O
provides	O
a	O
secure	O
channel	O
over	O
an	O
unsecured	O
network	O
in	O
a	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
architecture	I-Operating_System
.	O
</s>
<s>
OpenSSH	B-Language
started	O
as	O
a	O
fork	B-Application
of	O
the	O
free	B-Application
SSH	B-Protocol
program	O
developed	O
by	O
Tatu	O
Ylönen	O
;	O
later	O
versions	O
of	O
Ylönen	O
's	O
SSH	B-Protocol
were	O
proprietary	B-Application
software	I-Application
offered	O
by	O
SSH	B-Protocol
Communications	O
Security	O
.	O
</s>
<s>
OpenSSH	B-Language
was	O
first	O
released	O
in	O
1999	O
and	O
is	O
currently	O
developed	O
as	O
part	O
of	O
the	O
OpenBSD	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
OpenSSH	B-Language
is	O
not	O
a	O
single	O
computer	O
program	O
,	O
but	O
rather	O
a	O
suite	O
of	O
programs	O
that	O
serve	O
as	O
alternatives	O
to	O
unencrypted	O
protocols	O
like	O
Telnet	B-Protocol
and	O
FTP	B-Protocol
.	O
</s>
<s>
OpenSSH	B-Language
is	O
integrated	O
into	O
several	O
operating	B-General_Concept
systems	I-General_Concept
,	O
namely	O
Microsoft	B-Application
Windows	I-Application
,	O
macOS	B-Application
and	O
most	O
Linux	B-Application
operating	I-Application
systems	I-Application
,	O
while	O
the	O
portable	O
version	O
is	O
available	O
as	O
a	O
package	O
in	O
other	O
systems	O
.	O
</s>
<s>
OpenBSD	B-Language
Secure	I-Language
Shell	I-Language
was	O
created	O
by	O
OpenBSD	B-Operating_System
developers	O
as	O
an	O
alternative	O
to	O
the	O
original	O
SSH	B-Protocol
software	O
by	O
Tatu	O
Ylönen	O
,	O
which	O
is	O
now	O
proprietary	B-Application
software	I-Application
.	O
</s>
<s>
Although	O
source	O
code	O
is	O
available	O
for	O
the	O
original	O
SSH	B-Protocol
,	O
various	O
restrictions	O
are	O
imposed	O
on	O
its	O
use	O
and	O
distribution	O
.	O
</s>
<s>
OpenSSH	B-Language
was	O
created	O
as	O
a	O
fork	B-Application
of	O
Björn	O
Grönvall	O
's	O
OSSH	O
that	O
itself	O
was	O
a	O
fork	B-Application
of	O
Tatu	O
Ylönen	O
's	O
original	O
free	B-Application
SSH	B-Protocol
1.2.12	O
release	O
,	O
which	O
was	O
the	O
last	O
one	O
having	O
a	O
license	O
suitable	O
for	O
forking	O
.	O
</s>
<s>
The	O
OpenSSH	B-Language
developers	O
claim	O
that	O
their	O
application	O
is	O
more	O
secure	O
than	O
the	O
original	O
,	O
due	O
to	O
their	O
policy	O
of	O
producing	O
clean	O
and	O
audited	O
code	O
and	O
because	O
it	O
is	O
released	O
under	O
the	O
BSD	B-Operating_System
license	I-Operating_System
,	O
the	O
open-source	O
license	O
to	O
which	O
the	O
word	O
open	O
in	O
the	O
name	O
refers	O
.	O
</s>
<s>
OpenSSH	B-Language
first	O
appeared	O
in	O
OpenBSD	B-Operating_System
2.6	O
.	O
</s>
<s>
Developments	O
since	O
then	O
have	O
included	O
the	O
addition	O
of	O
ciphers	O
(	O
e.g.	O
,	O
ChaCha20-Poly1305	B-Algorithm
in	O
6.5	O
of	O
January	O
2014	O
)	O
,	O
cutting	O
the	O
dependency	O
on	O
OpenSSL	B-Language
(	O
6.7	O
,	O
October	O
2014	O
)	O
and	O
an	O
extension	O
to	O
facilitate	O
public-key	B-Application
discovery	O
and	O
rotation	O
for	O
trusted	O
hosts	O
(	O
for	O
transition	O
from	O
DSA	O
to	O
Ed25519	O
public	O
host	O
keys	O
,	O
version	O
6.8	O
of	O
March	O
2015	O
)	O
.	O
</s>
<s>
On	O
19	O
October	O
2015	O
,	O
Microsoft	O
announced	O
that	O
OpenSSH	B-Language
will	O
be	O
natively	O
supported	O
on	O
Microsoft	B-Application
Windows	I-Application
and	O
accessible	O
through	O
PowerShell	B-Application
,	O
releasing	O
an	O
early	O
implementation	O
and	O
making	O
the	O
code	O
publicly	O
available	O
.	O
</s>
<s>
OpenSSH-based	O
client	O
and	O
server	O
programs	O
have	O
been	O
included	O
in	O
Windows	B-Operating_System
10	I-Operating_System
since	O
version	O
1803	O
.	O
</s>
<s>
The	O
SSH	B-Protocol
client	I-Protocol
and	O
key	O
agent	O
are	O
enabled	O
and	O
available	O
by	O
default	O
,	O
and	O
the	O
SSH	B-Protocol
server	I-Protocol
is	O
an	O
optional	O
Feature-on-Demand	O
.	O
</s>
<s>
In	O
October	O
2019	O
protection	O
for	O
private	B-Application
keys	I-Application
at	O
rest	O
in	O
RAM	O
against	O
speculation	O
and	O
memory	O
side-channel	O
attacks	O
were	O
added	O
in	O
OpenSSH	B-Language
8.1	O
.	O
</s>
<s>
OpenSSH	B-Language
is	O
developed	O
as	O
part	O
of	O
the	O
OpenBSD	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Rather	O
than	O
including	O
changes	O
for	O
other	O
operating	B-General_Concept
systems	I-General_Concept
directly	O
into	O
OpenSSH	B-Language
,	O
a	O
separate	O
portability	O
infrastructure	O
is	O
maintained	O
by	O
the	O
OpenSSH	B-Language
Portability	O
Team	O
,	O
and	O
"	O
portable	O
releases	O
"	O
are	O
made	O
periodically	O
.	O
</s>
<s>
This	O
infrastructure	O
is	O
substantial	O
,	O
partly	O
because	O
OpenSSH	B-Language
is	O
required	O
to	O
perform	O
authentication	O
,	O
a	O
capability	O
that	O
has	O
many	O
varying	O
implementations	O
.	O
</s>
<s>
This	O
model	O
is	O
also	O
used	O
for	O
other	O
OpenBSD	B-Operating_System
projects	I-Operating_System
such	O
as	O
OpenNTPD	B-Application
.	O
</s>
<s>
The	O
OpenSSH	B-Language
suite	O
includes	O
the	O
following	O
command-line	B-Device
utilities	I-Device
and	O
daemons	B-Operating_System
:	O
</s>
<s>
,	O
a	O
replacement	O
for	O
,	O
and	O
to	O
allow	O
shell	B-Operating_System
access	I-Operating_System
to	O
a	O
remote	O
machine	O
.	O
</s>
<s>
,	O
a	O
tool	O
to	O
inspect	O
and	O
generate	O
the	O
RSA	B-Architecture
,	O
DSA	O
and	O
elliptic-curve	O
keys	O
that	O
are	O
used	O
for	O
user	O
and	O
host	O
authentication	O
.	O
</s>
<s>
,	O
which	O
scans	O
a	O
list	O
of	O
hosts	O
and	O
collects	O
their	O
public	B-Application
keys	I-Application
.	O
</s>
<s>
,	O
the	O
SSH	B-Protocol
server	I-Protocol
daemon	B-Operating_System
.	O
</s>
<s>
The	O
OpenSSH	B-Language
server	O
can	O
authenticate	O
users	O
using	O
the	O
standard	O
methods	O
supported	O
by	O
the	O
SSH	B-Protocol
protocol	O
:	O
with	O
a	O
password	O
;	O
public-key	B-Application
authentication	O
,	O
using	O
per-user	O
keys	O
;	O
host-based	O
authentication	O
,	O
which	O
is	O
a	O
secure	O
version	O
of	O
'	O
s	O
host	O
trust	O
relationships	O
using	O
public	B-Application
keys	I-Application
;	O
keyboard-interactive	O
,	O
a	O
generic	O
challenge	O
–	O
response	O
mechanism	O
,	O
which	O
is	O
often	O
used	O
for	O
simple	O
password	O
authentication	O
,	O
but	O
which	O
can	O
also	O
make	O
use	O
of	O
stronger	O
authenticators	O
such	O
as	O
tokens	O
;	O
and	O
Kerberos/GSSAPI	O
.	O
</s>
<s>
The	O
server	O
makes	O
use	O
of	O
authentication	O
methods	O
native	O
to	O
the	O
host	O
operating	B-General_Concept
system	I-General_Concept
;	O
this	O
can	O
include	O
using	O
the	O
BSD	B-Operating_System
Authentication	I-Operating_System
system	O
or	O
pluggable	B-Application
authentication	I-Application
modules	I-Application
(	O
PAM	O
)	O
to	O
enable	O
additional	O
authentication	O
through	O
methods	O
such	O
as	O
one-time	O
passwords	O
.	O
</s>
<s>
However	O
,	O
this	O
occasionally	O
has	O
side	O
effects	O
:	O
when	O
using	O
PAM	O
with	O
OpenSSH	B-Language
,	O
it	O
must	O
be	O
run	O
as	O
root	B-Application
,	O
as	O
root	B-Application
privileges	I-Application
are	O
typically	O
required	O
to	O
operate	O
PAM	O
.	O
</s>
<s>
OpenSSH	B-Language
versions	O
after	O
3.7	O
(	O
16	O
September	O
2003	O
)	O
allow	O
PAM	O
to	O
be	O
disabled	O
at	O
run-time	O
,	O
so	O
regular	O
users	O
can	O
run	O
sshd	O
instances	O
.	O
</s>
<s>
On	O
OpenBSD	B-Operating_System
,	O
OpenSSH	B-Language
uses	O
a	O
dedicated	O
user	O
by	O
default	O
to	O
drop	O
privileges	O
and	O
perform	O
privilege	O
separation	O
in	O
accordance	O
with	O
the	O
principle	O
of	O
least	O
privilege	O
,	O
applied	O
throughout	O
the	O
operating	B-General_Concept
system	I-General_Concept
including	O
the	O
Xenocara	B-Application
X	B-Operating_System
server	I-Operating_System
.	O
</s>
<s>
OpenSSH	B-Language
includes	O
the	O
ability	O
to	O
set	O
up	O
a	O
secured	O
channel	O
through	O
which	O
data	O
sent	O
to	O
local	O
,	O
client-side	B-Operating_System
Unix	B-Protocol
domain	I-Protocol
sockets	I-Protocol
or	O
local	O
,	O
client-side	B-Operating_System
TCP	B-Protocol
ports	O
may	O
be	O
"	O
forwarded	B-Protocol
"	O
(	O
sent	O
across	O
the	O
secured	O
channel	O
)	O
for	O
routing	O
on	O
the	O
server	O
side	O
;	O
when	O
this	O
forwarding	O
is	O
set	O
up	O
,	O
the	O
server	O
is	O
instructed	O
to	O
send	O
that	O
forwarded	B-Protocol
data	O
to	O
some	O
socket	O
or	O
TCP	B-Protocol
host/port	O
(	O
the	O
host	O
could	O
be	O
the	O
server	O
itself	O
,	O
"	O
localhost	O
"	O
;	O
or	O
,	O
the	O
host	O
may	O
be	O
some	O
other	O
computer	O
,	O
so	O
that	O
it	O
appears	O
to	O
the	O
other	O
computer	O
that	O
the	O
server	O
is	O
the	O
originator	O
of	O
the	O
data	O
)	O
.	O
</s>
<s>
The	O
forwarding	O
of	O
data	O
is	O
bidirectional	O
,	O
meaning	O
that	O
any	O
return	O
communication	O
is	O
itself	O
forwarded	B-Protocol
back	O
to	O
the	O
client-side	B-Operating_System
in	O
the	O
same	O
manner	O
;	O
this	O
is	O
known	O
as	O
an	O
"	O
SSH	B-Protocol
tunnel	O
"	O
,	O
and	O
it	O
can	O
be	O
used	O
to	O
multiplex	O
additional	O
TCP	B-Protocol
connections	O
over	O
a	O
single	O
SSH	B-Protocol
connection	O
since	O
2004	O
,	O
to	O
conceal	O
connections	O
,	O
to	O
encrypt	O
protocols	O
that	O
are	O
otherwise	O
unsecured	O
,	O
and	O
to	O
circumvent	O
firewalls	O
by	O
sending/receiving	O
all	O
manner	O
of	O
data	O
through	O
one	O
port	O
that	O
is	O
allowed	O
by	O
the	O
firewall	O
.	O
</s>
<s>
For	O
example	O
,	O
an	O
X	B-Operating_System
Window	I-Operating_System
System	I-Operating_System
tunnel	O
may	O
be	O
created	O
automatically	O
when	O
using	O
OpenSSH	B-Language
to	O
connect	O
to	O
a	O
remote	O
host	O
,	O
and	O
other	O
protocols	O
,	O
such	O
as	O
HTTP	B-Protocol
and	O
VNC	B-Protocol
,	O
may	O
be	O
forwarded	B-Protocol
easily	O
.	O
</s>
<s>
Tunneling	O
a	O
TCP-encapsulating	O
payload	O
(	O
such	O
as	O
PPP	B-Protocol
)	O
over	O
a	O
TCP-based	O
connection	O
(	O
such	O
as	O
SSH	B-Protocol
's	O
port	B-Protocol
forwarding	I-Protocol
)	O
is	O
known	O
as	O
"	O
TCP-over-TCP	O
"	O
,	O
and	O
doing	O
so	O
can	O
induce	O
a	O
dramatic	O
loss	O
in	O
transmission	O
performance	O
(	O
a	O
problem	O
known	O
as	O
"	O
TCPmeltdown	O
"	O
)	O
,	O
which	O
is	O
why	O
virtual	B-Application
private	I-Application
network	I-Application
software	O
may	O
instead	O
use	O
for	O
the	O
tunnel	O
connection	O
a	O
protocol	O
simpler	O
than	O
TCP	B-Protocol
.	O
</s>
<s>
However	O
,	O
this	O
is	O
often	O
not	O
a	O
problem	O
when	O
using	O
OpenSSH	B-Language
's	O
port	B-Protocol
forwarding	I-Protocol
,	O
because	O
many	O
use	O
cases	O
do	O
not	O
entail	O
TCP-over-TCP	O
tunneling	O
;	O
the	O
meltdown	O
is	O
avoided	O
because	O
the	O
OpenSSH	B-Language
client	O
processes	O
the	O
local	O
,	O
client-side	B-Operating_System
TCP	B-Protocol
connection	O
in	O
order	O
to	O
get	O
to	O
the	O
actual	O
payload	O
that	O
is	O
being	O
sent	O
,	O
and	O
then	O
sends	O
that	O
payload	O
directly	O
through	O
the	O
tunnel	O
's	O
own	O
TCP	B-Protocol
connection	O
to	O
the	O
server	O
side	O
,	O
where	O
the	O
OpenSSH	B-Language
server	O
similarly	O
"	O
unwraps	O
"	O
the	O
payload	O
in	O
order	O
to	O
"	O
wrap	O
"	O
it	O
up	O
again	O
for	O
routing	O
to	O
its	O
final	O
destination	O
.	O
</s>
<s>
In	O
addition	O
,	O
some	O
third-party	O
software	O
includes	O
support	O
for	O
tunnelling	O
over	O
SSH	B-Protocol
.	O
</s>
<s>
These	O
include	O
DistCC	B-Device
,	O
CVS	B-Application
,	O
rsync	B-Application
,	O
and	O
Fetchmail	B-Protocol
.	O
</s>
<s>
On	O
some	O
operating	B-General_Concept
systems	I-General_Concept
,	O
remote	O
file	B-Application
systems	I-Application
can	O
be	O
mounted	O
over	O
SSH	B-Protocol
using	O
tools	O
such	O
as	O
sshfs	B-Application
(	O
using	O
FUSE	B-Application
)	O
.	O
</s>
<s>
An	O
ad	O
hoc	O
SOCKS	B-Application
proxy	I-Application
server	O
may	O
be	O
created	O
using	O
OpenSSH	B-Language
.	O
</s>
<s>
This	O
allows	O
more	O
flexible	O
proxying	O
than	O
is	O
possible	O
with	O
ordinary	O
port	B-Protocol
forwarding	I-Protocol
.	O
</s>
<s>
Beginning	O
with	O
version	O
4.3	O
,	O
OpenSSH	B-Language
implements	O
an	O
OSI	O
layer	O
2/3	O
tun-based	O
VPN	B-Application
.	O
</s>
<s>
This	O
is	O
the	O
most	O
flexible	O
of	O
OpenSSH	B-Language
's	O
tunnelling	O
capabilities	O
,	O
allowing	O
applications	O
to	O
transparently	O
access	O
remote	O
network	O
resources	O
without	O
modifications	O
to	O
make	O
use	O
of	O
SOCKS	B-Application
.	O
</s>
<s>
OpenSSH	B-Language
supports	O
the	O
following	O
public	B-Application
key	I-Application
types	O
:	O
</s>
<s>
Before	O
version	O
5.2	O
of	O
OpenSSH	B-Language
,	O
it	O
was	O
possible	O
for	O
an	O
attacker	O
to	O
recover	O
up	O
to	O
14	O
bits	O
of	O
plaintext	O
with	O
a	O
success	O
probability	O
of	O
2−14	O
.	O
</s>
<s>
A	O
local	O
privilege	O
escalation	O
vulnerability	O
existed	O
in	O
OpenSSH	B-Language
6.8	O
to	O
6.9	O
(	O
)	O
due	O
to	O
world-writable	O
(	O
622	O
)	O
TTY	O
devices	O
,	O
which	O
was	O
believed	O
to	O
be	O
a	O
denial	O
of	O
service	O
vulnerability	O
.	O
</s>
<s>
With	O
the	O
use	O
of	O
the	O
TIOCSTI	O
ioctl	B-Operating_System
,	O
it	O
was	O
possible	O
for	O
authenticated	O
users	O
to	O
inject	O
characters	O
into	O
other	O
users	O
terminals	O
and	O
execute	O
arbitrary	O
commands	O
on	O
Linux	B-Application
.	O
</s>
<s>
Malicious	O
or	O
compromised	O
OpenSSH	B-Language
servers	O
could	O
read	O
sensitive	O
information	O
on	O
the	O
client	O
such	O
as	O
private	O
login	O
keys	O
for	O
other	O
systems	O
,	O
using	O
a	O
vulnerability	O
that	O
relies	O
on	O
the	O
undocumented	O
connection-resuming	O
feature	O
of	O
the	O
OpenSSH	B-Language
client	O
,	O
which	O
is	O
called	O
roaming	O
,	O
enabled	O
by	O
default	O
on	O
the	O
client	O
,	O
but	O
not	O
supported	O
on	O
the	O
OpenSSH	B-Language
server	O
.	O
</s>
<s>
This	O
applies	O
to	O
versions	O
5.4	O
(	O
released	O
on	O
8	O
March	O
2010	O
)	O
to	O
7.1	O
of	O
the	O
OpenSSH	B-Language
client	O
,	O
and	O
was	O
fixed	O
in	O
OpenSSH7.1p2	O
,	O
released	O
on	O
14	O
January	O
2016	O
.	O
</s>
<s>
In	O
February	O
2001	O
,	O
Tatu	O
Ylönen	O
,	O
Chairman	O
and	O
CTO	O
of	O
SSH	B-Protocol
Communications	O
Security	O
informed	O
the	O
OpenSSH	B-Language
development	O
mailing	O
list	O
that	O
the	O
company	O
intended	O
to	O
assert	O
its	O
ownership	O
of	O
the	O
"	O
SSH	B-Protocol
"	O
and	O
"	O
Secure	B-Protocol
Shell	I-Protocol
"	O
trademarks	O
,	O
and	O
sought	O
to	O
change	O
references	O
to	O
the	O
protocol	O
to	O
"	O
SecSH	O
"	O
or	O
"	O
secsh	O
"	O
,	O
in	O
order	O
to	O
maintain	O
control	O
of	O
the	O
"	O
SSH	B-Protocol
"	O
name	O
.	O
</s>
<s>
He	O
proposed	O
that	O
OpenSSH	B-Language
change	O
its	O
name	O
in	O
order	O
to	O
avoid	O
a	O
lawsuit	O
,	O
a	O
suggestion	O
that	O
developers	O
resisted	O
.	O
</s>
<s>
OpenSSH	B-Language
developer	O
Damien	O
Miller	O
replied	O
urging	O
Ylönen	O
to	O
reconsider	O
,	O
arguing	O
that	O
"	O
SSH	B-Protocol
"	O
had	O
long	O
since	O
been	O
a	O
generic	O
trademark	O
.	O
</s>
<s>
At	O
the	O
time	O
,	O
"	O
SSH	B-Protocol
"	O
,	O
"	O
Secure	B-Protocol
Shell	I-Protocol
"	O
and	O
"	O
ssh	B-Protocol
"	O
had	O
appeared	O
in	O
documents	O
proposing	O
the	O
protocol	O
as	O
an	O
open	O
standard	O
.	O
</s>
<s>
After	O
study	O
of	O
the	O
USPTO	O
trademark	O
database	O
,	O
many	O
online	O
pundits	O
opined	O
that	O
the	O
term	O
"	O
ssh	B-Protocol
"	O
was	O
not	O
trademarked	O
,	O
merely	O
the	O
logo	O
using	O
the	O
lower	O
case	O
letters	O
"	O
ssh	B-Protocol
"	O
.	O
</s>
<s>
In	O
addition	O
,	O
the	O
six	O
years	O
between	O
the	O
company	O
's	O
creation	O
and	O
the	O
time	O
when	O
it	O
began	O
to	O
defend	O
its	O
trademark	O
,	O
and	O
that	O
only	O
OpenSSH	B-Language
was	O
receiving	O
threats	O
of	O
legal	O
repercussions	O
,	O
weighed	O
against	O
the	O
trademark	O
's	O
validity	O
.	O
</s>
<s>
Both	O
developers	O
of	O
OpenSSH	B-Language
and	O
Ylönen	O
himself	O
were	O
members	O
of	O
the	O
IETF	O
working	O
group	O
developing	O
the	O
new	O
standard	O
;	O
after	O
several	O
meetings	O
this	O
group	O
denied	O
Ylönen	O
's	O
request	O
to	O
rename	O
the	O
protocol	O
,	O
citing	O
concerns	O
that	O
it	O
would	O
set	O
a	O
bad	O
precedent	O
for	O
other	O
trademark	O
claims	O
against	O
the	O
IETF	O
.	O
</s>
<s>
The	O
participants	O
argued	O
that	O
both	O
"	O
Secure	B-Protocol
Shell	I-Protocol
"	O
and	O
"	O
SSH	B-Protocol
"	O
were	O
generic	O
terms	O
and	O
could	O
not	O
be	O
trademarks	O
.	O
</s>
