Re: MD5 is insecure

[Dan Mahoney via NANOG <nanog () lists nanog org>]

> On Sep 4, 2025, at 00:51, Saku Ytti via NANOG <nanog () lists nanog org> wrote:
>
> I'm not educated on the subject matter, so it doesn't matter when I
> think that this absolutely is non-issue and does not impact SSH
> security.
>
> For people like me, could someone showcase how, given the MD5 hash,
> they successfully login to the device, not having access to the
> private key of the client. Don't explain to me why it works, show me
> how you login to Cisco device using this.
> Explaining won't work, because from my perspective in this thread it
> has been very well explained why it doesn't matter, why there is no
> security issue.

I had composed a long response here, but I think the tone of what I wrote in my blog speaks for itself, or I would have 
used different words, but at this point I don’t know if people have read the post, so I’ll rehash a bit.

The tone of my article was not “Holy crap, patch everything now”, it was “wow, that’s funny, they’re doing a thing that 
literally nobody else is doing that *theoretically* (regardless of what hash algo is used) increases the attack surface 
quite a lot, but it’s especially bad  if yet another flaw is discovered in an algorithm that’s already had many flaws 
discovered (and those flaws were known as of the most recent releases of the OS for these devices).  Why is nobody else 
doing what they’re doing?  I wonder."

Good thing they give me the option of using another algorithm.  Good thing that if I actually go to upload my full key, 
they keep it around, but give me the option of a hash for convenience.  Good thing I’m not trusting my entire network 
to these devices.  Good thing they make it easy to get patched software for without a service entitlement, and they 
also make it easy to get a service contract for a device I bought on the secondary market.  (These are all false 
statements).

It’s not just about md5, it’s about many other corners that were cut by the same vendor, including the one that bit 
Randy, with their p0wn-by-default “smart install” feature.

-Dan
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