Topic "SSH2 RSA Padding bug - true or false?"

Author Message
lnx.kid
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Joined: 2003-10-01
Posts: 2
Dear all SSH and WinSCP gurus,

I've recently come across the following snippet in one of my WinSCP3 logfiles:

<snip>
Server version: SSH-2.0-OpenSSH_3.1p1
We believe remote version has SSH2 RSA padding bug
</snip>

Now, the 'We believe....' part is what has me concerned....should I be? Where can I find more info on the 'SSH2 RSA padding bug' (hopefully a fix) and how is WinSCP determining that my server is vulnerable? TIA,

lnx.kid

*sorry if this is duplicate, I tried my best to search this forum and Google!
_________________
kandah-mahnos-kandah
martin
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Site Admin
Joined: 2002-12-10
Posts: 24530
Location: Prague, Czechia
lnx.kid wrote:
<snip>
Server version: SSH-2.0-OpenSSH_3.1p1
We believe remote version has SSH2 RSA padding bug
</snip>

Now, the 'We believe....' part is what has me concerned....should I be? Where can I find more info on the 'SSH2 RSA padding bug' (hopefully a fix) and how is WinSCP determining that my server is vulnerable?

The bugs detection comes from Putty, so I have very little knowledge about it.

Quote from "The bugs panel" of Putty documentation.

Quote:
Versions below 3.3 of OpenSSH require SSH2 RSA signatures to be padded with zero bytes to the same length as the RSA key modulus. The SSH2 draft specification says that an unpadded signature MUST be accepted, so this is a bug. A typical symptom of this problem is that PuTTY mysteriously fails RSA authentication once in every few hundred attempts, and falls back to passwords.

If this bug is detected, PuTTY will pad its signatures in the way OpenSSH expects. If this bug is enabled when talking to a correct server, it is likely that no damage will be done, since correct servers usually still accept padded signatures because they're used to talking to OpenSSH.

This is an SSH2-specific bug.

There is also chance that you've "forced the bug detection" on Bugs tab of login dialog.
_________________
Martin Prikryl
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