[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).

[gunapologist99]

Exactly - agent forwarding is the laziest and fastest path to getting severely pwned, but the irony is that the alternatives are actually fairly simple and fast, if someone is willing to take the time to adjust their process a little bit.

[pritambaral]

> agent forwarding is the laziest and fastest path to getting severely pwned

Only for people who don't know what they are doing. Usually, such people also make poor replacement decisions that are even less secure.

> the alternatives are actually fairly simple and fast, if someone is willing to take the time to adjust their process a little bit.

I often need to work on code in ephemeral containers. Is there an "actually fairly simple and fast" method I can use to be able git pull and push to and from these ephemeral containers that:

1. doesn't require too much adjustment (a little bit is okay); and

2. is not less secure than agent forwarding with confirmation?

[dumpsterdiver]

> Only for people who don't know what they are doing.

By that do you just mean that no services are openly exposed on the system? To my understanding, if any vulnerable service is remotely exposed then it's not at all safe to use agent forwarding with the affected version of openssh.

[pritambaral]

By that I mean to use the now-20-year-old 'ssh-add -c' flag. It'd seem practically no one is aware that an ssh agent does NOT have to silently and automatically sign just any and every auth request, given just how frequently I see people decrying "No! If you forward your agent that means a remote host has full access to everything in your agent! Never forward your agent ever! No buts!"

With the '-c' flag, if a remote host tries to use my agent, I get a graphical dialog on my local machine asking me if I want to let my agent do it. If I'm not expecting the dialog, I can just say no, and now I know the remote is compromised.

Actually, I go a step further. Because it is possible to accidentally accept a signing request by pressing the enter key that I meant to press for sth else, I make my agent require a passphrase on every use, not just a yes/no dialog.

In checking my machines for this CVE, I discovered my agent has yet another layer of security built-in. I use gpg-agent, relying on its famed security posture. Turns out, when forwarded, gpg-agent supports nothing except signing. It does not support adding keys from a remote, let alone fancy operations like loading a PKCS11 provider chosen by a remote.

----

Securing a forwarded agent has nothing to do with whether there are openly exposed services on a remote system. The remote system could have malicious code that entered it not necessarily through an exposed service. A compromised npm package in the supply chain, for instance, is sufficient. It doesn't matter how it got there. What matters is: when it does, can it abuse a forwarded agent. Hence the '-c' flag to ssh-add.

[dumpsterdiver]

I was not aware of that functionality in ssh-add. Thank you!

Man page reference:

"-c Indicates that added identities should be subject to confirmation before being used for authentication. Confirmation is performed by ssh-askpass(1). Successful confirmation is signaled by a zero exit status from ssh-askpass(1), rather than text entered into the requester."

Edit: After thinking about it more, I think I may have misunderstood how the -c parameter would perform in regards to this CVE.

Would a confirmation prompt actually suffice to prevent this attack? Also, it begs the question, does this exploit rely upon someone already forwarding to a malicious server? I've only read the CVE, and skimmed the reporter's blog, so I don't know with certainty one way or the other.

If the exploit does require that the forwarding has already taken place, -c couldn't really help, right? The decision was made to allow it. I hope I'm not sounding contrarian, I'm genuinely curious about how this would play out.

[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.

[gunapologist99]

> 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.

That's not how agent forwarding works. An attacker on the remote server can piggyback on your SSH session and do anything else desired, so your remote git repo is still compromised, but the blast radius of these remote keys is much smaller. (in infosec, we'd usually call this least privilege but separation of duties also applies)

All of this is still possible even with gpg-agent, even if this particular RCE doesn't apply to you, so "Never Use Agent Forwarding" still applies.

[pritambaral]

> An attacker on the remote server can piggyback on your SSH session and do anything else desired

This myth is about 20 years out of date. See what the '-c' flag for ssh-add does. It was added in OpenSSH 3.6 back in 2003.

In fact, I can prove it to you. Take my pubkey from GitHub (same username) and put it on a host you control. Tell me to ssh into it with my agent forwarded and see if that gives you access to my GitHub account.

----

> All of this is still possible even with gpg-agent

Even without the 'confirm each use' flag, gpg-agent with a zero TTL visually asks for the decryption key on each use. There _are_ some agents out there that have no support for visual confirmations and yet happily accept the '-c' flag (looking at you, GNOME), but gpg-agent isn't one of them.

----

> That's not how agent forwarding works

You seem to be misreading. I'm not claiming that's how agent forwarding works. I'm saying that's how your suggestion of creating a keypair on a remote host works.

It _is_ less secure because it requires those keys to be resident on the remote. If the remote is compromised, decrypting the key in the compromised machine's memory is strictly insecure compared to doing it on my local machine with an agent. Once captured from the compromised remote, those keys can be exfiltrated and used repeatedly. But, if an agent is somehow tricked into signing an unauthorised request, that access is still limited to one use only.

[gunapologist99]

>> > An attacker on the remote server can piggyback on your SSH session and do anything else desired

> This myth is about 20 years out of date.

This hole didn't simply disappear when -c was added.

The vulnerability is simply that the socket file containing the connection back to your agent is accessible by anyone who managed to escalate to root on the remote host.

You're making several assumptions:

#1: someone is using -c

#2: that -c even does anything on their platform

#3: the user pays attention to them and is untrickable

#4: there are no bugs in the ssh-agent or gpg-agent on the client machine

Any one of these being false renders all protection from -c moot; worse yet, a bug in the ssh-agent (or gpg-agent, if that's your poison) like the one in the subject of this post can be leveraged into complete client takeover.

> Once captured from the compromised remote, those keys can be exfiltrated and used repeatedly.

that is true, which is why they should be tightly scoped.

> But, if an agent is somehow tricked into signing an unauthorised request, that access is still limited to one use only.

That one use only is all that is needed. An attacker might install another pubkey, start up another socket process, or even rootkit the remote box if you have sudo, doas or if there are any privilege escalation vulns.

These situations are identical in that the remote box is pwned, but only one of these tries to limit the exploits to just that one remote box and not every other host your keys have access to.

[pritambaral]

> This hole didn't simply disappear when -c was added. The vulnerability is simply that ...

Why are you conflating the two? You're making claims that sth has always been utterly, completely broken, and the only evidence you can cite for it is sth that was revealed to the world a few days ago?

This CVE is a secvuln. No one is arguing against that. Secvulns happen. No software is bug-free. Does that mean every software everywhere is suddenly utterly completely broken?

Actually, while we are on the topic, why not argue banning SSH entirely. After all, each SSH connection is a connection back to the host where the `ssh` client runs. Tomorrow, there could be a secvuln discovered in the `ssh` binary that can be exploited by simply printing the right characters to stdout. In fact, this very vector has been used before to pwn vulnerable terminal emulators, even over ssh.

----

> ... which is why they should be tightly scoped.

As can forwarded agents (see 'IdentityAgent' in `man ssh_config`). In fact, this is how I separate client projects from each other and my personal projects. (I didn't do it for security, rather for the convenience of not tripping any 'max keys allowed' limits, but hey, I'll take the security benefit too!)

----

> That one use only is all that is needed. ... tries to limit the exploits to just that one remote box and not every other host your keys have access to.

You're making several assumptions:

#1: someone is disciplined enough to use tightly scoped keys

#2: that they bother to rotate all those ephemeral keys without fail

#3: that the sheer inconvenience of constantly updating keys doesn't bother them enough to say 'screw this!'

Guess what the weakest link is when it comes to computer security? The human factor. You're asking humans to go through way too much hassle they're not going to care about. Which means they'll voluntarily break the security of the system without care (and, of course, without understanding).

You also forget that:

1. Agents can be scoped too.

2. Jumpboxes are often used to jump to a large number (most often, all) of servers. No organization is going around creating point-to-point jump links between servers or dedicating a separate jumpbox for every destination server.

3. Your model, when exploited, gives the attacker repeatable, lasting access. You say, "That one use only is all that is needed", and you're correct, but only for the most determined and prepared attackers. Once-only accidental access is better than repeatable lasting access for the simple fact that most attackers are aiming for only the latter.

4. Your model is also more susceptible to silent persistent malware. My method has the benefit that exploited remotes are discovered before the exploit is given lateral movement access.

----

> ... who managed to escalate to root on the remote host.

No root needed. DAC enough. I say this because I run untrusted code in VMs/containers I ssh into, with my agent forwarded. And I consider arbitrary npm/python/etc. packages automatically untrusted.