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Date: Fri, 17 Aug 2018 23:02:08 +0200
From: Dariusz Tytko <dariusz.tytko@...uritum.pl>
To: oss-security@...ts.openwall.com
Subject: Re: OpenSSH Username Enumeration

Hi,

We have reported this bug on 16th July 2018. Publication of the write-up
is waiting for the official patch.

W dniu 17.08.2018 o 20:31, Salvatore Bonaccorso pisze:
> Hi,
>
> On Wed, Aug 15, 2018 at 09:05:58AM -0700, Qualys Security Advisory wrote:
>> Hi all,
>>
>> We sent the following email to openssh@...nssh.com and
>> distros@...openwall.org about an hour ago, and it was decided that we
>> should send it to oss-security@...ts.openwall.com right away (as far as
>> we know, no CVE has been assigned to this issue yet):
>>
>> ========================================================================
>>
>> While reviewing the latest OpenSSH commits, we stumbled across:
>>
>> https://github.com/openbsd/src/commit/779974d35b4859c07bc3cb8a12c74b43b0a7d1e0
>>
>> Date:   Tue Jul 31 03:10:27 2018 +0000
>>     delay bailout for invalid authenticating user until after the packet
>>     containing the request has been fully parsed. Reported by Dariusz Tytko
>>     and Michal Sajdak; ok deraadt
>>
>> We realized that without this patch, a remote attacker can easily test
>> whether a certain user exists or not (username enumeration) on a target
>> OpenSSH server:
>>
>>   87 static int
>>   88 userauth_pubkey(struct ssh *ssh)
>>   89 {
>>  ...
>>  101         if (!authctxt->valid) {
>>  102                 debug2("%s: disabled because of invalid user", __func__);
>>  103                 return 0;
>>  104         }
>>  105         if ((r = sshpkt_get_u8(ssh, &have_sig)) != 0 ||
>>  106             (r = sshpkt_get_cstring(ssh, &pkalg, NULL)) != 0 ||
>>  107             (r = sshpkt_get_string(ssh, &pkblob, &blen)) != 0)
>>  108                 fatal("%s: parse request failed: %s", __func__, ssh_err(r));
>>
>> The attacker can try to authenticate a user with a malformed packet (for
>> example, a truncated packet), and:
>>
>> - if the user is invalid (it does not exist), then userauth_pubkey()
>>   returns immediately, and the server sends an SSH2_MSG_USERAUTH_FAILURE
>>   to the attacker;
>>
>> - if the user is valid (it exists), then sshpkt_get_u8() fails, and the
>>   server calls fatal() and closes its connection to the attacker.
>>
>> We believe that this issue warrants a CVE; it affects all operating
>> systems, all OpenSSH versions (we went back as far as OpenSSH 2.3.0,
>> released in November 2000), and is easier to exploit than previous
>> OpenSSH username enumerations (which were all timing attacks):
>>
>> https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2003-0190
>> https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2006-5229
>> https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-6210
> This new issue got assigned CVE-2018-15473 by MITRE.
>
> Regards,
> Salvatore

-- 
Dariusz Tytko

securitum.pl - bezpieczeństwo systemów IT.
sekurak.pl   - piszemy o bezpieczeństwie.


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