full-disclosure - Re: PuTTY private key passphrase stealing attack

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Message-Id: <A845D897-0529-41AF-BC27-4B7177665214@sarenet.es>
Date: Tue, 1 Jun 2010 12:37:16 +0200
From: Borja Marcos <borjam@...enet.es>
To: Jan Schejbal <jan.mailinglisten@...glemail.com>
Cc: full-disclosure@...ts.grok.org.uk, bugtraq@...urityfocus.com
Subject: Re: PuTTY private key passphrase stealing attack

On Jun 1, 2010, at 2:47 AM, Jan Schejbal wrote:

> 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 prompt.
> 
> 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.

This kind of attack is a real classic, the in-band problem inherent to any text terminal. Reading of the venerable and now forgotten classic by Wood and Kochan, "Unix System Security", published in 1985 should still be mandatory. Moreover, many of these in-band risks are applicable to window systems, which exhibit even worse properties. See the fuss with "tab-nabbing" now.

Borja.

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