SSH2 RSA Padding bug - true or false?

Advertisement

lnx.kid
Joined:
Posts:
2

SSH2 RSA Padding bug - true or false?

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

Reply with quote

Advertisement

martin
Site Admin
martin avatar
Joined:
Posts:
28,658
Location:
Prague, Czechia

Re: SSH2 RSA Padding bug - true or false?

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.

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.

Reply with quote

Advertisement

You can post new topics in this forum