User enumeration in Dropbear 2018.76 and earlier

Matthijs R. Koot koot at cyberwar.nl
Mon Aug 20 17:50:11 AWST 2018


Hi all,

The user enumeration issue in OpenSSH [0] also exists in Dropbear 2018.76
and earlier; at least going back to w/v2013.58 (didn't test with earlier
versions yet). It is specifically related to this code in svr-auth.c [1]:

----- 8< ----- 8< ----- 8< ----- 8< -----
#if DROPBEAR_SVR_PUBKEY_AUTH
        /* user wants to try pubkey auth */
        if (methodlen == AUTH_METHOD_PUBKEY_LEN &&
                        strncmp(methodname, AUTH_METHOD_PUBKEY,
                                AUTH_METHOD_PUBKEY_LEN) == 0) {
                if (valid_user) {
                        svr_auth_pubkey();
                } else {
                        /* pubkey has no failure delay */
                        send_msg_userauth_failure(0, 0);
                }
                goto out;
        }
#endif
----- 8< ----- 8< ----- 8< ----- 8< -----

The PoC released for OpenSSH [2] also works against Dropbear - which seems
remarkable because both have an entirely different code base and the issue
is not due to SSH specs.

Because the issue can be abused to test both for SSH and non-SSH users, it
can be abused to enumerate installed services/software by testing for
default/known service users.

To test an SSH service on 127.0.0.1:22022 for user 'admin', the PoC can be
executed as follows:

$ python ssh-check-username.py --port 22022 127.0.0.1 admin

Credits to /u/dbzjegrw8o6n0 for posting an initial observation at
/r/blackhat [3], which triggered a colleague and me to look into it.

Friendly regards,
Matthijs Koot (Secura BV, the Netherlands)

[0] http://seclists.org/oss-sec/2018/q3/124
[1] https://github.com/mkj/dropbear/blob/master/svr-auth.c#L175-L188
[2] https://bugfuzz.com/stuff/ssh-check-username.py
[3]
https://www.reddit.com/r/blackhat/comments/97ywnm/openssh_username_enumeration/e4e05n2/



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