OpenSSH through 7.7 is prone to a user enumeration vulnerability due to not delaying bailout for an invalid authenticating user until after the packet containing the request has been fully parsed, related to auth2-gss.c, auth2-hostbased.c, and auth2-pubkey.c.
CVE: CVE-2018-15473
Write up from https://www.openwall.com/lists/oss-security/2018/08/15/5
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:
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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;
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if the user is valid (it exists), then sshpkt_get_u8() fails, and the server calls fatal() and closes its connection to the attacker.