jdev - 2020-07-04

in Gajim you can trust a self signed cert that a server offers

what Gajim does is, it adds it to a kind of cert store (not the system one)

and later if it gets UNKNOWN_CA, it just checks the store if the user trusts this cert

now if someone knows i trust a specific self signed cert, could the creator of that cert impersonate another server?

i think no, because changing stuff like domain within the cert, would change the fingerprint of the cert, and then the certs are not equal anymore, hence new trust decision

is this correct?

Is that trust store per account or 'global'?

global

basically a server offers a cert, and i get the fingerprint and look if there is a cert with that fingerprint in the trust store

question is, changing the domain a cert is valid for, changes the fingerprint

or not?

the fingerprint for the entire cert would cover the entire cert

including all the names

it sounds like it may be possible for such a server to impersonate another server you have an account on, if you connect there somehow?

do you verify the names in self-signed certs tho?

no, thats just one step in the verify process

of course i let the ssl lib first verify the cert normally

afterwards it offers me errors

like UNKNOWN_CA

if UNKNOWN_CA is the *only* error on the cert

i check the trust store

in prosody there are no name checks if the CA is untrusted, I don't know how your code works

of course if htere are other errors like, BAD_IDENTITY or other stuff

this is shown to the user

hence if the someone manipulates the trusted cert, so it links to another domain, to circumvent a BAD_IDENTITY error

it would change the fingerprint of the cert

hence Gajim would not find it anymore in the trust store

and raise UNKNOWN_CA

You know, since this is security related, it might be best to quietly test whether it's exploitable before talking about it in public :)

i dont think this is exploitable, and even if, nobody is forced to accept self signed certs

but i wouldnt know how otherwise i can support self signed certs

without presenting the user on every connect with a cert error dialog

What happens if the server later switches to a CA-issued cert?

IMO it would make sense to have such trust decisions be saved per account, just to be sure.

its verified ok and user is not notified about it

yes Zash thats what i was thining about

i have no concrete reason, but if feels a bit better

Some users would probably want a warning if the certificate changed from a self-signed to a CA-issued one.

Depending on why a self-signed cert was in used at all.

If it's just a temporary cert until a CA issued one can be gotten, then it probably doesn't matter.

If it's someone who doesn't trust CAs at all running the server, then it'd be suspicious if it suddenly had a CA-issued one.

yeah probably right

but i dont care about that one guy that wants that thing