Bug 3503

Summary:

OpenSSH tries executing banner as command

Product:

Portable OpenSSH

Reporter:

Mateusz <mateusz.gierblinski>

Component:

ssh

Assignee:

Assigned to nobody <unassigned-bugs>

Status:

CLOSED INVALID

Severity:

normal

CC:

dtucker

Priority:

Version:

8.8p1

Hardware:

amd64

OS:

Linux

Attachments:

Description	Flags
Proof of Concept	none

Description Mateusz 2022-11-17 03:39:46 AEDT

Created attachment 3626 [details]
Proof of Concept

Hi there, 

On default Fedora 37 installation I found an interesting issue. In my home directory I have the following config: 

Host redhat
	HostName 192.16.122.253
	User mto
	#Identityfile /home/mto/.ssh/id_ed25519
	ProxyCommand ssh -T -i /home/mto/.ssh/id_ed25519 mto@192.168.122.253

When I'm trying to connect, I receive the following message: 

-bash: line 1: $'SSH-2.0-OpenSSH_8.8\r': command not found

As you can see, OpenSSH tries to execute banner version as command. Based on the StackOverflow (link: https://unix.stackexchange.com/questions/269024/change-ssh-banner-which-is-grabbed-by-netcat) we can modify banner and it has to be exactly 11 characters long, otherwise the binary gets corrupted. 

Please refer to provided screenshot for proof. 

Thanks,
Mateusz

Comment 1 Darren Tucker 2022-11-17 08:42:38 AEDT

(In reply to Mateusz from comment #0)
[...]
> As you can see, OpenSSH tries to execute banner version as command.

That's because that's what you configured it to do, although it's not immediately obvious.

What's happening is that the proxy command is bringing up an interactive session, then the client tries to initiate an SSH connection with the proxycommand which is connected to a shell at the other end.  The client's initial banner is sent over the proxycommand to the that shell which tries to run it, fails, and returns the error that you see.

If we strip your example down to the proxycommand you can see this:

$ ssh -o 'ProxyCommand ssh -vvv 127.0.0.1' 127.0.01
[...]
debug2: shell request accepted on channel 0
debug2: channel 0: rcvd ext data 59
-bash: line 1: $'SSH-2.0-OpenSSH_9.0\r': command not found
debug2: channel 0: written 59 to efd 6

It's not clear what exactly you're trying to do, but you probably want to add a "-W [host]:[port]" to the proxycommand (or use ProxyJump).

> Based on the StackOverflow (link:
> https://unix.stackexchange.com/questions/269024/change-ssh-banner-
> which-is-grabbed-by-netcat) we can modify banner

That's different because it's referring to the banner string sent by the server side.  In your case the string is sent by the client.

Comment 2 Darren Tucker 2022-12-16 14:17:44 AEDT

We believe this is not actually a bug in OpenSSH.  If you have more details that indicate otherwise, please reopen this bug.

Comment 3 Damien Miller 2023-03-17 13:38:54 AEDT

OpenSSH 9.3 has been released. Close resolved bugs