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OpenSSH Username Enumeration

Hi all, We sent the following email to openssh () openssh com and distros () vs openwall org about an hour ago, and it was decided that we should send it to oss-security () lists 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 We also believe that this should be posted to oss-security right away: the issue (commit) is already public, and if we spotted it, then others (not so well intentioned) did too. We are at your disposal for questions, comments, and further discussions. Thank you very much! With best regards, -- the Qualys Security Advisory team

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