Let PAM know about accepted pubkey?

[Johan Ström]

Hi,

first of: my familiarity with OpenSSH/Pam code-base is very limited..
Please excuse me if some of this does not make any sense or seems stupid!

I'm investigating if it is possible for a PAM module to find out which
public key was accepted (when 'AuthenticationMethods
publickey,keyboard-interactive' is used). From my digging in the source,
it seems it is currently not.

Would it be possible to provide this information? Maybe using
do_pam_putenv()? Would there be any security implications of doing this?

The reason I'm asking is that I'm looking into using 2FA (i.e. pubkey +
pam_google_authenticator) for *some* of my SSH keys. For example, a SSH
privkey on my trusted box can login without verification code, but
another SSH privkey on my not-as-trusted box requires a verification
code to login.

My initial thought was to implemented it through a pubkey whitelist in
the specific PAM module, but other solutions might also be possible.
Maybe a PAM module configured as "sufficient", which looks in a pubkey
whitelist and accepts immediately, skipping any further PAM modules
would work.


Thanks for any feedback!

Regards
Johan (who is not on the list and appreciates a direct CC!)

[Damien Miller]

Hi,
first of: my familiarity with OpenSSH/Pam code-base is very limited.. Please
excuse me if some of this does not make any sense or seems stupid!
I'm investigating if it is possible for a PAM module to find out which public
key was accepted (when 'AuthenticationMethods publickey,keyboard-interactive'
is used). From my digging in the source, it seems it is currently not.
Would it be possible to provide this information? Maybe using do_pam_putenv()?
Would there be any security implications of doing this?

There is an open bug about this. I do hope to make the public keys used during
authentication available in the environment (and to PAM), but it won't happen
until 7.3 at the earliest.

-d

[Johan Ström]

Post by Damien Miller

Hi,
first of: my familiarity with OpenSSH/Pam code-base is very limited.. Please
excuse me if some of this does not make any sense or seems stupid!
I'm investigating if it is possible for a PAM module to find out which public
key was accepted (when 'AuthenticationMethods publickey,keyboard-interactive'
is used). From my digging in the source, it seems it is currently not.
Would it be possible to provide this information? Maybe using do_pam_putenv()?
Would there be any security implications of doing this?

There is an open bug about this. I do hope to make the public keys used during
authentication available in the environment (and to PAM), but it won't happen
until 7.3 at the earliest.

Ah, great! I also got an off-list message from Damien, the author of
said bug:

https://bugzilla.mindrot.org/show_bug.cgi?id=2408

I've added my +1 on the ticket to express my interest, hoping it
attracts some interest!