I'm forwarding verbatim a bug report from Thorsten Glaser (also at the attached URL).  The intemperate language is not mine, but I decided against editing it.

ssh of course doesn't document "--" as part of its interface, so IMO all the uses of "should" here are with reference to an idealised interface rather than the documentation.  Nevertheless, it does seem that it would be extremely useful to have a reliable way to disambiguate between options to ssh itself and options that should be passed to the remote command.

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Subject: command line parsing with -- between option and non-option arguments completely broken

Hi,

in the process of me fixing CVE-2017-12836 a user noticed a
problem with OpenSSH’s command line parsing.

I’ve verified these on OpenSSH 5.3 (MirBSD) and 7.5p1 (Debian).

So, to begin with, this command _should_ spawn xeyes:

$ ssh -oProxyCommand=xeyes vuxu.org

This command _could_ spawn xeyes on glibc systems, but
probably shouldn’t on POSIX or BSD systems:

$ ssh vuxu.org -oProxyCommand=xeyes

This command properly does not spawn xeyes but tries to
resolve “-oProxyCommand=xeyes” as hostname, correctly failing:

$ ssh -- -oProxyCommand=xeyes

This command *must not* spawn xeyes, but does:

$ ssh -- vuxu.org -oProxyCommand=xeyes

This instead must execute “-oProxyCommand=xeyes” as command
on the remote side.

Interestingly enough, this command works the same and also
mustn’t but also doesn’t:

$ ssh vuxu.org -- -oProxyCommand=xeyes

Now it gets completely weird, this doesn’t spawn xeyes either:

$ ssh -- vuxu.org -- -oProxyCommand=xeyes

This “should” execute “--” as command with “-oProxyCommand=xeyes”
as its first option on the remote site, but judging from the error
| mksh: ProxyCommand=xeyes: unknown option
it instead passes “-oProxyCommand=xeyes” as option to a shell on
the remote side.

I don’t do the security theatre, but this could perhaps be considered
missing command escaping on the remote side (passing what would be a
command as an option to the remote shell) in addition to completely
fucked up option parsing on the local side.

This was first reported by nickserv-auth’d user jn__ on #musl on
Freenode IRC, and leah2 forwarded it to me as current de-facto
maintainer of GNU CVS because I considered adding “--” between
option and nōn-option arguments sufficient for fixing the afore‐
mentioned CVE, judging this effective enough with normal command
line parsing rules (as in getopt(3) on OpenBSD) and given the
.Sx SYNOPSIS
in the ssh manpage.