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.
