A user code is eight characters from a twenty-five character alphabet,
which is a large space but a fixed one, and the endpoint that checked
them had no opinion about how often you asked. That is the guessing
attack RFC 8628 section 5.2 asks implementations to limit, and nothing
here did.
Wrong codes are now counted per caller address over a rolling window,
and the endpoint stops answering once the budget is gone. Getting a code
right is not charged for, so somebody who mistypes once and then succeeds
is not walking towards a lockout. A caller whose address cannot be
established shares one bucket with every other such caller, which makes
stripping the headers that say where you are buy a smaller budget rather
than an unlimited one.
The counter lives in the general-purpose store and is approximate. The
number that decides this is whether somebody is working through the code
space, and a handful either way does not change that answer.
A six-digit code has a million values, and nothing was counting how many
of them a caller tried. The code travelled in an encrypted cookie the
caller held, verification compared against that cookie, and a wrong
answer simply re-rendered the form. Nobody has to be the person the code
was mailed to: type somebody else's address into the first screen and the
code goes to their mailbox while the cookie stays with you. At that point
the only thing between a stranger and an account is a million requests,
and the constant-time comparison protecting the code was guarding a door
you could just keep knocking on.
Guesses are now counted on the server, under a name that changes with
every code. That placement is the point: a counter kept beside the code,
in the cookie, is a counter the guesser can wind back by replaying an
older copy. Starting over is still allowed and still costs a fresh code
sent to the mailbox being aimed at, which is where somebody notices. A
correct code spends its record too, so its remaining guesses do not carry
into the next one.
The cookie also lived for twenty-four hours, which made the pin a
password with a million possible values and a day to try them. Ten
minutes now, and the code stops being accepted when the clock says so
rather than when the cookie happens to go away.
Resend had no limit either, so the button was a way to mail a stranger as
fast as requests go out. Codes to one address are spaced, and one attempt
at signing in can only ask for so many.
Both refusals say the same thing on purpose. Which of the two it was is a
fact about somebody else's mailbox.
Anybody could ask for a device code and be handed a link with the user
code already in it. Following that link started a sign-in, and finishing
the sign-in approved the grant. So sending somebody the link was enough:
they saw an ordinary sign-in prompt, completed it, and whoever kept the
device code polled and collected their access and refresh tokens. The
victim never saw a question, because there was not one.
There is now. Signing in says who the browser belongs to; it does not say
the person meant to hand an account to a program somewhere else. Those
are two questions and only the second authorizes anything, so the flow
ends at a page that names the program, shows the code back so it can be
compared with what the device is displaying, and offers Approve and Deny.
Approving is a POST carrying a value from the cookie, so another site
cannot submit it on somebody's behalf. Denial moved onto the same page:
it used to be a GET anyone could fire, which meant a link scanner could
cancel a real sign-in and a stranger with a user code could grief one.
Three more things that were wrong underneath.
The grant was read, modified and written back as a whole record. A poll
that read a pending grant and then wrote its bookkeeping erased an
approval that landed in between, and the client polled a dead grant until
it expired. Grants moved to a table, where approving is one conditional
update and redeeming is one delete that returns what it deleted, so
neither party can undo the other and two polls cannot both be served.
Tokens were minted when the person clicked and left sitting in storage
until collected. They are minted at redemption now, so the lifetime the
client is told about starts when it receives them, and a grant nobody
collects leaves no usable refresh token behind.
The client identifier was never checked, at either end. It is validated
when the grant is created and has to match when the code is redeemed —
without that, a leaked code is redeemable by anyone, and the identifier
the token carries is whatever the last caller claimed. The device code
is also stored as a hash now, since it is the credential the tokens are
handed to.
The store is an interface because the issuer cannot reach the database,
and because the guarantees are the point: every method is one operation,
and no caller reads a grant, decides, and writes it back.
A program with no browser — the desktop app — had a client for RFC 8628 and
nothing to point it at. This serves the other half: a device authorization
request that hands back a code, a page a person enters that code on, and a
token endpoint that answers the poll.
Both of the paths the client already implements are now reachable. Polling
faster than the advertised interval gets slow_down, and each warning widens
the interval so ignoring one costs more than the last; refusing gets
access_denied, so a request nobody started stops instead of being polled until
it ages out. The interval is capped, because it only ever grows and a code has
to stay pollable for the whole of its life.
The codes live in the same storage as the other short-lived grants rather than
in a table, since that is what they are. User codes are drawn from an alphabet
with no vowels and no look-alike pairs, and are accepted back in whatever case
and spacing a person retyped them in.
