New: dropbear: norootlogin (-w) no longer works when PAM is enabled

https://bugs.busybox.net/show_bug.cgi?id=12181

Bug ID: 12181
Summary: dropbear: norootlogin (-w) no longer works when PAM is
enabled
Product: buildroot
Version: 2019.02.5
Hardware: All
OS: Linux
Status: NEW
Severity: critical
Priority: P5
Component: Other
Assignee: unassigned at buildroot.uclibc.org
Reporter: jan.dumon at septentrio.com
CC: buildroot at uclibc.org
Target Milestone: ---

This affects dropbear 2018.76 in 2019.02.5 and earlier.

The vanilla dropbear 2018.76 doesn't have this problem, it is introduced (ironically) by the patch for CVE-2018-15599 in
https://git.busybox.net/buildroot/commit?id=4a3b0ba38fde05e8f8c3512d516d86803efa44c0

It only happens when PAM is enabled & used.

When invoked with the -w command line argument, root logins should be disallowed, but when someone attempts to login as root, the following happens:

- login attempt for root
- password is asked
- norootlogin is set but pam is invoked anyway
- pam validates the password and an 'already authenticated flag' gets set
- login still fails (as is expected) and password is asked again
- regardless of input, root can log in because he was authenticated before. (completely unexpected !!!)

It's still required to have a proper password but the root login should not be allowed per the -w command line argument.

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https://bugs.busybox.net/show_bug.cgi?id=12181

--- Comment #1 from Jan Dumon <[email protected]> --- 2019.08 is not affected as it uses a more recent dropbear (2019.78) and no longer has the CVE patch.
This bug is only applicable to the LTS (2019.02.xx)

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https://bugs.busybox.net/show_bug.cgi?id=12181

Peter Korsgaard <[email protected]> changed:

What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |ASSIGNED

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--- Comment #2 from Peter Korsgaard <[email protected]> ---
(In reply to Jan Dumon from comment #1)
So that presumably means that it has gotten fixed upstream. Looking at the git history:

git shortlog DROPBEAR_2018.76..DROPBEAR_2019.78 -- svr-authpam.c Matt Johnston (1):
Wait to fail invalid usernames

vincentto13 (1):
Fix for issue successfull login of disabled user (#78)

Presumably this is fixed by the 2nd patch. Can you try applying this patch to the 2019.02.x version and test:

commit a0aa2749813331134452f80bb8a808bdc871ba41

Author: vincentto13 <[email protected]>
Date:   Wed Mar 20 15:03:40 2019 +0100

This commit introduces fix for scenario:
1. Root login disabled on dropbear
2. PAM authentication model enabled

While login as root user, after prompt for password user is being notified about login failrue, but after second attempt of prompt for password within same session, login becames succesfull.

---
 svr-authpam.c | 1 +
 1 file changed, 1 insertion(+)

index d201bc9..e236db4 100644
@@ -275,6 +275,7 @@ void svr_auth_pam(int valid_user) {
                /* PAM auth succeeded but the username isn't allowed in for

another reason
(checkusername() failed) */ send_msg_userauth_failure(0, 1);

+               goto cleanup;
        }

        /* successful authentication */

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--- Comment #3 from Jan Dumon <[email protected]> --- (In reply to Peter Korsgaard from comment #2)

Tried it and it works, no more root logins with -w.

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Peter Korsgaard <[email protected]> changed:

What |Removed |Added
----------------------------------------------------------------------------

         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #4 from Peter Korsgaard <[email protected]> --- Fixed by
https://git.buildroot.org/buildroot/commit/?id=ae81527917e74b9751df4e7bfb7a0b2688362eac,
thanks

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