[sullivanmatt]

This sounds way worse than it is.

To be clear, the "remote" part of the code execution is that an attacker controlling your destination server can cause your client to run an attacker-controlled payload, if the client is forwarding their credentials (`ssh -A`). Most people don't tend to make connections to arbitrary SSH hosts, and certainly they don't do it while forwarding their credentials along.

It's a neat attack, and I applaud the Qualys team on their find, but this is not any sort of emergency situation for 99.99% of systems.

[gunapologist99]

I beg to differ: this does not sound way worse than it is. If anything, it's understating the issue.

Not only can it be exploited across a wide variety of clients across multiple platforms, but all that's required is that you're using key forwarding.

This is devastating, because it's not just that you control the destination server and steal the keys, but you can take over the user's entire workstation.

Once you've got the user's entire workstation, you potentially have access to everything else they have, from their email, to other SSH hosts, to key loggers, to Git repos. This is about as bad as it gets, and all because someone is using Agent Forwarding.

Best of all, the victim has no idea that they've been completely compromised. They can live inside your machine for years, upgrade their sploits, and generally exfiltrate all of your secrets.

Never use agent forwarding. Just don't. "Agent forwarding should be enabled with caution" in the man page is another massive understatement. Even if you think you need it, check the other responses in this thread for examples of how to work around it.

[dumpsterdiver]

> Never use agent forwarding

Agreed. As this exploit proves, it's not even safe to log into your own servers using ssh forwarding if any service is exposed remotely, because if an attacker compromises that exposed service and gains root then they could extend the attack to your workstation, and that's a huge deal - especially considering that you have the private key to log into that server on your computer (so it's not an unsafe bet there might be other keys).

[e12e]

> Never use agent forwarding.

Just to add to this, with the new -J/ProxyJump directive, it's become (even) easier to login through a ssh host without needing to enable agent forwarding (Given that you're connecting through a not-ancient host running a reasonable version of openssh - beware of firewall/appliances stuck on ancient sshd and/or proprietary/"mini" versions).

[pritambaral]

Agent Forwarding is not a trivial thing to take lightly, but a knee-jerk reaction "ban it entirely" is too much.

I forward my agent by default because I've set it up securely. My setup is safe from this exploit too (I use gpg-agent as my SSH Agent). In return I get the seamless convenience I cannot get through any other method. Jump hosts are fine (and I use them too) but there is no way I'd be able to do remote git operations in ephemeral dev containers without the peace of mind (and safety) that agent forwarding gives me.

Creating keys on remote dev envs for git operations is _less_ secure than agent forwarding, even when those keys are encrypted (passphrase protected) at rest, because they have to be loaded into memory on the (potentially compromised) remote host.

[justsomeadvice0]

Lots of people end up with AgentForward on by default as a sort of "make it work" fix, and lots of people use `git+ssh` on untrusted servers. Here's an example:

https://abyssdomain.expert/@filippo/109659699817863532

TBF this is a vulnerable config either way; but RCE on the client shouldn't be possible.

[doubled112]

I've been using a separate SSH config for git for a long time now. Nice to see it wasn't just paranoia.

Among the settings are explicitly disabling agent forwarding, and using a git specific identity (SSH key).

[tinus_hn]

I’m not so sure git is secure against a malicious server, even if you’re not simply pulling in a Makefile written by the attacker.

[TechBro8615]

The attacker controlled destination server could be a compromised host, so this enables lateral movement from a deployed VM or remote dev machine into a developer laptop.

[ivlad]

“git pull” over SSH and have your system RCEd? I’d say it levels above “neat”.

[franga2000]

You'd need to go out of your way to make git pull do agent forwarding and I can't really think of a reason why anyone would.

[e28eta]

I don’t know how prevalent it is as a network architecture, but it seems like a bastion host / jump box would be a juicy target for this exploit, since it’d let the attacker jump upstream.

[jmalicki]

Sure, but first they have to root the bastion box.

If you root the bastion box, you have user credentials for anything inside the network. Controlling the user's laptop seems unlikely to be your most profitable next step.

[akerl_]

Increasingly, the role of a bastion host is served either by something like Teleport, which handles authn/z and proxying without needing forwarded agents, or newer options in OpenSSH like ProxyJump where you hop via a bastion host but without ever forwarding your agent.

[malux85]

Yeah if I’m reading the technical analysis right, your conditions that you mention have to be correct and also the attacker must have “poisoned” library files on the targets machine so they can dlopen them, is that right?

Pretty unlikely

[Arnavion]

The libraries are on the client's machine, not the server's. And they're not "poisoned"; the default distro-provided libs already provide the remote execution capabiity (eclipse-titan, libkf5sonnetui5, libns3-3v5 and systemd-boot packages from Ubuntu 22.04).

[sullivanmatt]

There must be a specific set of libs present on the victim (client), correct. Qualys claims that stock Ubuntu Desktop systems often have these libs, and that they haven't looked into whether other distros tend to.

But yes, your point stands. Huge number of preconditions here to fulfill.

[paulmd]

Are you saying that if you SFTP in to a client machine to upload a file to their server, it’s expected behavior that you’re willing to give them root on your machine?

[gunapologist99]

Aren't all SSH hosts potentially attacker-controlled? ;)

[nocsi]

Yea, but there’s a security boundary wherein you don’t want the SSH host to be executing code in your environment. Of course, the attackers can backdoor sshd to log credentials, setup init scripts on the host to execute code every client login and other shenanigans.