On Thu, May 10, 2018 at 1:26 PM, David Woodhouse <dwmw2 at infradead.org> wrote: > On Thu, 2018-05-10 at 01:38 +0000, Ryan Taylor wrote: >> >> If this is correct, using the servercert option is a significant security problem. >> >> Perhaps the servercert option is not intended to be used for any sort of security guarantee >> whatsoever. However if that is the case, there should probably be a warning in the man page, and >> also printed on standard output, kind of like overriding a web browser to connect to a site with an >> untrusted certificate. Users who have not done the math or read the man page carefully may be using >> this option with the mistaken belief that it provides some security assurance via a sort of >> makeshift certificate pinning. >> >> Or, this could be fixed by requiring a complete match of the hash instead of just 4 characters. >> What are the "certain testing use-cases" and how important are they compared to the security >> considerations? > > Hi Ryan, thanks for looking at this. > > As Dan says, if a user provides a truncated hash then we do require the > match to be at the beginning of the hash. It's not just *any* > substring. Even so, yes: 4 characters isn't many, so there's a fair > chance of a false match. Ultimately, Don't Do That Then. > > I'm happy to improve the documentation to make it clear that there is > absolutely no excuse for a user to use anything less than the full hash > in "production" use, or indeed in any circumstance where cut-and-paste > works. > > I vaguely recall that I had similar reservations when this feature was > first added (Nikos, was it you?). But I *have* found it useful > occasionally in testing, in strange setups where I haven't been able to > cut and paste for some reason. At the time, we were forcibly removing > the "--no-cert-check" option because I threw my toys out of the pram > after finding people advocating its use in production ? so even a 4- > char match was better than that! :) Yes it was me :) The small substring match helps when running openconnect in a VM without copy+paste or similar setups. There is indeed no excuse to try to save some bytes on production. Btw. we also have the more standard pin-sha256 pinning which reduces typing even less (doesn't seem to be in a released version) regards, Nikos
