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Message-ID: <20180817183154.GA18661@eldamar.local>
Date: Fri, 17 Aug 2018 20:31:54 +0200
From: Salvatore Bonaccorso <carnil@...ian.org>
To: oss-security@...ts.openwall.com
Subject: Re: OpenSSH Username Enumeration
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
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