<s>
DNS	B-Protocol
Certification	B-Protocol
Authority	I-Protocol
Authorization	I-Protocol
(	O
CAA	O
)	O
is	O
an	O
Internet	B-Application
security	I-Application
policy	O
mechanism	O
that	O
allows	O
domain	O
name	O
holders	O
to	O
indicate	O
to	O
certificate	B-Architecture
authorities	I-Architecture
whether	O
they	O
are	O
authorized	O
to	O
issue	O
digital	O
certificates	O
for	O
a	O
particular	O
domain	O
name	O
.	O
</s>
<s>
It	O
does	O
this	O
by	O
means	O
of	O
a	O
new	O
"	O
CAA	O
"	O
Domain	B-Protocol
Name	I-Protocol
System	I-Protocol
(	O
DNS	B-Protocol
)	O
resource	O
record	O
.	O
</s>
<s>
It	O
was	O
drafted	O
by	O
computer	O
scientists	O
Phillip	O
Hallam-Baker	O
and	O
Rob	O
Stradling	O
in	O
response	O
to	O
increasing	O
concerns	O
about	O
the	O
security	O
of	O
publicly	O
trusted	O
certificate	B-Architecture
authorities	I-Architecture
.	O
</s>
<s>
A	O
series	O
of	O
incorrectly	O
issued	O
certificates	O
from	O
2001	O
onwards	O
damaged	O
trust	O
in	O
publicly	O
trusted	O
certificate	B-Architecture
authorities	I-Architecture
,	O
and	O
accelerated	O
work	O
on	O
various	O
security	O
mechanisms	O
,	O
including	O
Certificate	B-Protocol
Transparency	I-Protocol
to	O
track	O
mis-issuance	O
,	O
HTTP	B-Protocol
Public	I-Protocol
Key	I-Protocol
Pinning	I-Protocol
and	O
DANE	B-Protocol
to	O
block	O
mis-issued	O
certificates	O
on	O
the	O
client-side	B-Operating_System
,	O
and	O
CAA	O
to	O
block	O
mis-issuance	O
on	O
the	O
certificate	B-Architecture
authority	I-Architecture
side	O
.	O
</s>
<s>
CA/Browser	O
Forum	O
discussion	O
began	O
shortly	O
afterward	O
,	O
and	O
in	O
March	O
2017	O
they	O
voted	O
in	O
favor	O
of	O
making	O
CAA	O
implementation	O
mandatory	O
for	O
all	O
certificate	B-Architecture
authorities	I-Architecture
by	O
September	O
2017	O
.	O
</s>
<s>
At	O
least	O
one	O
certificate	B-Architecture
authority	I-Architecture
,	O
Comodo	O
,	O
failed	O
to	O
implement	O
CAA	O
before	O
the	O
deadline	O
.	O
</s>
<s>
A	O
2017	O
study	O
by	O
the	O
Technical	O
University	O
of	O
Munich	O
found	O
many	O
instances	O
where	O
certificate	B-Architecture
authorities	I-Architecture
failed	O
to	O
correctly	O
implement	O
some	O
part	O
of	O
the	O
standard	O
.	O
</s>
<s>
,	O
Qualys	O
reports	O
that	O
still	O
,	O
only	O
6.8	O
%	O
of	O
the	O
150,000	O
most	O
popular	O
TLS-supporting	O
websites	O
use	O
CAA	O
records	O
.	O
</s>
<s>
Certificate	B-Architecture
authorities	I-Architecture
implementing	O
CAA	O
perform	O
a	O
DNS	B-Protocol
lookup	O
for	O
CAA	O
resource	O
records	O
,	O
and	O
if	O
any	O
are	O
found	O
,	O
ensure	O
that	O
they	O
are	O
listed	O
as	O
an	O
authorized	O
party	O
before	O
issuing	O
a	O
digital	O
certificate	O
.	O
</s>
<s>
flag	O
A	O
flags	O
byte	O
which	O
implements	O
an	O
extensible	B-Architecture
signaling	O
system	O
for	O
future	O
use	O
.	O
</s>
<s>
,	O
only	O
the	O
issuer	O
critical	O
flag	O
has	O
been	O
defined	O
,	O
which	O
instructs	O
certificate	B-Architecture
authorities	I-Architecture
that	O
they	O
must	O
understand	O
the	O
corresponding	O
property	O
tag	O
before	O
issuing	O
a	O
certificate	O
.	O
</s>
<s>
iodef	O
This	O
property	O
specifies	O
a	O
method	O
for	O
certificate	B-Architecture
authorities	I-Architecture
to	O
report	O
invalid	O
certificate	O
requests	O
to	O
the	O
domain	O
name	O
holder	O
using	O
the	O
Incident	O
Object	O
Description	O
Exchange	O
Format	O
.	O
</s>
<s>
,	O
not	O
all	O
certificate	B-Architecture
authorities	I-Architecture
support	O
this	O
tag	O
,	O
so	O
there	O
is	O
no	O
guarantee	O
that	O
all	O
certificate	O
issuances	O
will	O
be	O
reported	O
.	O
</s>
<s>
This	O
property	O
allows	O
domain	O
holders	O
to	O
publish	O
contact	O
information	O
in	O
DNS	B-Protocol
.	O
</s>
<s>
Third	O
parties	O
monitoring	O
certificate	B-Architecture
authority	I-Architecture
behavior	O
might	O
check	O
newly	O
issued	O
certificates	O
against	O
the	O
domain	O
's	O
CAA	O
records	O
.	O
</s>
<s>
specifies	O
"	O
accounturi	O
"	O
and	O
"	O
validationmethods	O
"	O
parameters	O
which	O
allow	O
users	O
to	O
specify	O
desired	O
methods	O
of	O
domain	O
control	O
validation	O
as	O
defined	O
in	O
ACME	B-Protocol
protocol	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
website	O
administrator	O
can	O
bind	O
a	O
domain	O
they	O
control	O
to	O
particular	O
account	O
registered	O
with	O
their	O
desired	O
Certification	B-Architecture
Authority	I-Architecture
.	O
</s>
<s>
A	O
draft	O
of	O
the	O
first	O
extension	O
to	O
the	O
CAA	O
standard	O
was	O
published	O
on	O
October	O
26	O
,	O
2016	O
,	O
proposing	O
a	O
new	O
account-uri	O
token	O
to	O
the	O
end	O
of	O
the	O
issue	O
property	O
,	O
which	O
ties	O
a	O
domain	O
to	O
a	O
specific	O
Automated	B-Protocol
Certificate	I-Protocol
Management	I-Protocol
Environment	I-Protocol
account	O
.	O
</s>
<s>
To	O
indicate	O
that	O
only	O
the	O
certificate	B-Architecture
authority	I-Architecture
identified	O
by	O
ca.example.net	O
is	O
authorized	O
to	O
issue	O
certificates	O
for	O
example.com	O
and	O
all	O
subdomains	O
,	O
one	O
may	O
use	O
this	O
CAA	O
record	O
:	O
</s>
<s>
To	O
indicate	O
that	O
certificate	B-Architecture
authorities	I-Architecture
should	O
report	O
invalid	O
certificate	O
requests	O
to	O
an	O
email	O
address	O
and	O
a	O
Real-time	O
Inter-network	O
Defense	O
endpoint	O
:	O
</s>
<s>
To	O
use	O
a	O
future	O
extension	O
of	O
the	O
protocol	O
,	O
for	O
example	O
,	O
one	O
which	O
defines	O
a	O
new	O
future	O
property	O
,	O
which	O
needs	O
to	O
be	O
understood	O
by	O
the	O
certificate	B-Architecture
authority	I-Architecture
before	O
they	O
can	O
safely	O
proceed	O
,	O
one	O
may	O
set	O
the	O
issuer	O
critical	O
flag	O
:	O
</s>
<s>
In	O
early	O
2020	O
,	O
Let	B-Application
's	I-Application
Encrypt	I-Application
disclosed	O
that	O
their	O
software	O
improperly	O
queried	O
and	O
validated	O
CAA	O
records	O
potentially	O
affecting	O
over	O
3	O
million	O
certificates	O
.	O
</s>
<s>
Let	B-Application
's	I-Application
Encrypt	I-Application
worked	O
with	O
customers	O
and	O
site	O
operators	O
to	O
replace	O
over	O
1.7	O
million	O
certificates	O
,	O
but	O
decided	O
not	O
to	O
revoke	O
the	O
rest	O
to	O
avoid	O
client	O
downtime	O
and	O
since	O
the	O
affected	O
certificates	O
would	O
all	O
expire	O
in	O
less	O
than	O
90	O
days	O
.	O
</s>
