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PuTTY private key passphrase stealing attack
From: Jan Schejbal <jan.mailinglisten () googlemail com>
Date: Tue, 01 Jun 2010 02:47:07 +0200
PuTTY, a SSH client for Windows, requests the passphrase to the ssh key
in the console window used for the connection. This could allow a
malicious server to gain access to a user's passphrase by spoofing that
We assume that the user is using key-bases ssh auth with ssh and
connects using PuTTY. PuTTY now asks for the passphrase to the key. The
user enters the passphrase. If the passphrase is wrong, PuTTY will now
request the passphrase again after stating that it was wrong. If the
passphrase is correct, the connection to the server is established.
A malicious/manipulated server could then display "Wrong passphrase" and
ask for the passphrase again. If the user enters it again, it is sent to
the malicious server.
As far as I can see, there are only two ways how the user might detect it:
1. The real "Wrong passphrase" message is displayed without delay. After
entering the correct passphrase, a small delay occurs.
2. The prompt contains the name of the key as stored on the client.
Often the same name is used in the authorized_keys file on the server,
giving it to the attacker. Maybe it is also possible for the server to
remotely read the screen contents or duplicate it using some xterm
control sequences, so users should not rely on it.
(See also the attached screenshot, where you can see that there is no
I assume that there are more similar issues like this one using
different authentication modes etc.
This can be exploited using a modified .bashrc file. This means that
once an attacker has gained access to a user account on the server, he
can try this to gain the passphrase to the key.
As a malicious server is required, the attack probability is not very
high. Without the keyfile, the passphrase is worthless to the attacker
unless it is used in multiple places. However, key-based auth is
supposed to be secure even with untrusted/malicious servers.
The possibility of such spoofing attacks is known:
Load the key into the Pageant agent before esablishing the connection
Other software affected:
Probably many console-based SSH tools have similar issues.
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- PuTTY private key passphrase stealing attack Jan Schejbal (Jun 01)