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The Date field in the Release file is useful to avoid allowing an
attacker to 'downgrade' a user to earlier Release files (and hence to
older states of the archieve with open security bugs). It is also needed
to allow a user to define min/max values for the validation of a Release
file (with or without the Release file providing a Valid-Until field).
APT wasn't formally requiring this field before through and (agrueable
not binding and still incomplete) online documentation declares it
optional (until now), so we downgrade the error to a warning for now to
give repository creators a bit more time to adapt – the bigger ones
should have a Date field for years already, so the effected group should
be small in any case.
It should be noted that earlier apt versions had this as an error
already, but only showed it if a Valid-Until field was present (or the
user tried to used the configuration items for min/max valid-until).
Closes: 809329
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This doesn't allow all tests to run cleanly, but it at least allows to
write tests which could run successfully in such environments.
Git-Dch: Ignore
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Valid-Until protects us from long-living downgrade attacks, but not all
repositories have it and an attacker could still use older but still
valid files to downgrade us. While this makes it sounds like a security
improvement now, its a bit theoretical at best as an attacker with
capabilities to pull this off could just as well always keep us days
(but in the valid period) behind and always knows which state we have,
as we tell him with the If-Modified-Since header. This is also why this
is 'silently' ignored and treated as an IMSHit rather than screamed at
the user as this can at best be an annoyance for attackers.
An error here would 'regularily' be encountered by users by out-of-sync
mirrors serving a single run (e.g. load balancer) or in two consecutive
runs on the other hand, so it would just help teaching people ignore it.
That said, most of the code churn is caused by enforcing this additional
requirement. Crisscross from InRelease to Release.gpg is e.g. very
unlikely in practice, but if we would ignore it an attacker could
sidestep it this way.
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