While I agree that TLSv1.0 and TLSv1.1 should be avoided as much as
possible, I believe this document fails to consider that there are old
systems that are still in use that cannot be upgraded. Strict
implementation of the MUST NOT rules in this document can even prevent
those systems from being upgraded at all, even when upgrades are
available. Strict implementation of the MUST NOT rules in this
document can also make old embedded systems with built-in servers
effectively unusable or require the operators of such systems to disable
TLS entirely.
In general, it should not be assumed that old systems can be upgraded,
or that old systems are feasibly replaced with newer systems. There
are several reasons for that.
* One is that operating system vendors sometimes stop supporting old
hardware, and client or server software vendors stop supporting old
operating systems.
* Some platforms are certified for medical use with specific versions
of operating systems for which OS or software upgrades would require
recertification, and the manufacturers of such systems do not always
recertify their platforms with the latest operating systems.
* Some embedded systems either do not have provision for firmware
upgrades and/or are operated on disconnected networks so that
upgrades are cumbersome (and may violate company security policies);
those products sometimes do not have the option of firmware updates
because there is no revenue stream to support them and they wouldn't
be used anyway. And yet, it's common for embedded systems to be
configured, queried, or monitored using HTTP[S].
* I have also worked on products for manufacturing environments for
which upgrades were forbidden; any firmware upgrade would have
required shutting down the assembly line for days and retesting the
whole thing.
* Finally, sometimes software or firmware "upgrades" take away
functionality present in earlier versions, so that the "upgrade" may
make that computer useless for its intended purpose.
In general, there are two kinds of problems caused by disabling of TLS
1.0/1.1 in implementations:
1. Old clients cannot talk to newer servers
Again, sometimes clients run on old machines that cannot be upgraded or
replaced. When servers refuse to support old TLS versions, an old
client may refuse to work at all. It is not always feasible to
download the same file from a different machine or different client program.
I have seen this happen when trying to upgrade some software on an old
MacBook Pro. The software I was trying to download could only be
downloaded from Apple using Safari on a Mac. Apple's server would not
use a version of TLS compatible with the old version of Safari I had,
and there were no upgrades to that version of Safari. I tried
downloading the software from another (non-Apple) computer; the server
would not let me do so. I didn't have a more current Mac to use,
didn't wish to buy one, and the pandemic made using someone else's Mac
infeasible.
The best idea I came up with was to set up a web proxy that supported
more recent versions of TLS, and configure Safari to communicate via
that web proxy. But I never found time to do that.
I'm not saying the RFC should be fixed for me, but rather, that I've
personally experienced a situation that many other people undoubtedly
have experienced and will experience after publication of this RFC.
(Some servers are already following these recommendations.)
I have also worked with systems operated by a major petroleum producer
(who will remain unamed) who had (unsurprisingly) very elaborate
security measures. Their internal networks were inaccessible from
outside systems except via multiple layers of remote desktop. So any
client software to be used had to be software already vetted and
installed on their internal machines. But presumably because of the
difficulty of vetting new software, the only browser available was MSIE
5 [don't remember which version of Windows]. (I know this because I
had to update product software to use GIF files instead of PNG, add some
JS polyfills, and avoid some HTML5 features, in order to be compatible
with their browsers). I cite this only as another example that one
cannot reasonably expect all client and server to be current, or even
nearly so.
In some of these cases (when the client cannot be upgraded) an
appropriate remedy may be to install a web proxy to allow the old client
to communicate with the server. Of course this can still come with
risks, including perhaps exposure of the network traffic between the
client and the web proxy.
In other cases, server operators might do well to consider whether, for
their specific users, services, and content, TLS >= 1.2 is really an
appropriate constraint to impose. For example the ietf.org web server
is currently supporting older versions of TLS in order to make IETF
documents as widely available as possible, even on legacy systems. IMO,
that practice makes sense given IETF's mission and the fact that all of
the documents on that server are intended to be publicly available.
(though if other services besides document download are provided on the
same server, particularly if they use password-over-TLS authentication,
either requiring a higher version of TLS for those services, or hosting
those services on a different server that does require >= TLS 1.2, might
make good sense)
2. Newer clients cannot talk to old servers
Perhaps naively, I assume that most servers reachable via the public
Internet are upgradeable, and can/should be upgraded to support at least
TLS 1.2. Certainly I would recommend against exposing any server to the
public Internet, for which security upgrades are not routinely
provided. So I'm /not/ talking about those servers here.
My concern here is with embedded systems that support HTTPS for
configuration, monitoring, or control of industrial hardware. The
security concerns are both very real (monitoring/controlling vital
manufacturing or infrastructure and/or with potential risk to human
lives) and often (unfortunately) not given proper regard. Frankly,
most operators of such systems probably use (non-TLS) HTTP or other
cleartext interfaces to configure, monitor, and/or control such devices,
and rely on the illusion of a completely disconnected network to let
them sleep at night. But some of those embedded devices do support
TLS, even if it's old TLS (likely with self-signed certs... TLS really
wasn't designed to work with embedded systems that don't have DNS
names.) Again, upgrades may be unavailable, or infeasible, for many of
these embedded devices.
Client programs in such environments may be a mixture of web browsers
and/or dedicated special-purpose clients (which may be interactive or
non-interactive) or proxies (e.g. to adapt a proprietary interface to
something like OPC-UA). If the client can just keep using old systems
with old web browsers, they can keep using those browsers to
control/configure/monitor those embedded devices, but if the client
system is replaced with newer hardware and a newer OS and browser, this
may fail. For special-purpose clients the situation is slightly
different - the vendors of those clients will presumably be aware of the
limitations of the embedded systems they talk to - but deprecating TLS
1.0 and 1.1 from libraries may make support more difficult.
For newer interactive clients I believe the appropriate action when
talking to a server that doesn't support TLS >= 1.2 is to (a) warn the
user, and (b) treat the connection as if it were insecure. (so no
"lock" icon, for example, and the usual warnings about submitting
information over an insecure channel.)
For newer non-interactive clients an appropriate action might be to
permit the client to refuse TLS < 1.2 by default but be explicitly
configurable to use earlier versions.
For web browsers, the use of browser plugins might permit the occasional
use of TLS < 1.2 (since most users won't need it) without the increased
attack surface for every user. A web proxy might also work as a
solution for web browsers and some other http clients.
3. Fixes to the draft
I believe the draft needs to explicitly discuss some of these cases, and
suggest workarounds for such cases, rather than simply say "MUST NOT".
IMO the best result from saying "MUST NOT" is that IETF security
documents will be more likely to be disregarded since they're making
poor recommendations for many cases. That's not good, but worse
results are also possible, including that the effect of these
recommendations will be to actually reduce security for many operations,
at least in the short term.
I realize that "MUST NOT" is attractive because it is simple and clear
language, and that "SHOULD NOT" (even though it's the right language per
2119 definitions) may be seen as allowing too much weasel room. I'm
not sure how best to address that. "MUST NOT... except..." could also be
confusing. Saying "MUST NOT" but also discussing exceptions in the
document may be seen as conflicting or confusing. I don't have a
strong opinion about how to resolve that dilemma, but I do think some
exceptional cases should be acknowledged somehow. (or maybe in a
separate RFC?)
There are probably other cases that I haven't considered. It's easy to
make the mistake of assuming that every application that uses TLS is
like the web (e.g. interactive), but of course that's not the case.
The Right Thing to do for an SMTP client relaying mail, for example, is
probably to use the highest TLS version supported by the server, rather
than disconnect the SMTP session and resend the message as cleartext.
Even SSL 1.0 seems better than cleartext in that case. Of course, the
client could bounce the message, but given that the common expectation
is that mail will be relayed in cleartext if necessary, bouncing seems
like the Wrong Thing to do absent an originator-specified requirement
for hop-by-hop encryption.
Similarly if a user is trying to read his/her mail from an old mobile
phone, should that user's ability to read mail cease to work because the
mail server now requires TLS 1.2 or better? Old phones don't necessarily
have upgrades available. And while a lot of cellular networks actively
encourage upgrading of old phones by pessimizing their service (newer
phones may make better use of radio bandwidth and/or support more
frequencies), I am not in a position to say what works for the entire
world. I have heard that old phones are still usable, and in demand, in
many countries.
4. Future
This is out of scope for this draft, but I also believe that these kinds
of compatibility issues may need to be analyzed every time the IETF
recommends to deprecate something.
Keith
_______________________________________________
TLS mailing list
TLS@ietf.org
https://www.ietf.org/mailman/listinfo/tls
