One safety tip: disable SSH Agent Forwarding before you connect, otherwise the r... (2024)

The full command you want is:

 ssh -a -i /dev/null terminal.shop

to disable agent forwarding, as well as to not share your ssh public key with them, but that's just a little less slick than saying just:

 ssh terminal.shop

to connect.

I'm curious why you added `-i /dev/null`. IIUC, this doesn't remove ssh-agent keys.

If you want to make sure no keys are offered, you'd want:

 ssh -a -o IdentitiesOnly=yes terminal. Shop

I'm not sure if the `-i` actually prevents anything, I believe things other than /dev/null will still be tried in sequence.

Check for yourself with

 ssh -v -i /dev/null terminal.shop

vs

 ssh -v terminal.shop

What you're looking for is that there is no line that says something like

 debug1: Offering public key: /Users/fragmede/.ssh/id_rsa RSA SHA256:xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Upon further testing, the full command you want is:

 ssh -a -i /dev/null -o IdentityAgent=/dev/null terminal.shop

to forcibly disable a local identity agent from offering up its identities as well, and not just agent forwarding.

Upon further testing,

 ssh -o IdentitiesOnly=yes terminal.shop

still offers up my public key on my system (macOS, OpenSSH_9.6p1, LibreSSL 3.3.6), contrary to what StackOverflow and the Internet seems to think. Tested by hitting whoami.filippo.io, linked in child comment.

Aha, yes, `-o IdentityAgent=/dev/null` is better for my intent. I was confused that `-i` wasn't removing .ssh/id_rsa from the candidates, but that was ssh-agent.

 ssh -a -i /dev/null -o IdentityAgent=/dev/null terminal.shop

That looks pretty solid. Thanks!

For a cool example (deanonymization), see https://words.filippo.io/dispatches/whoami-updated/ (discussed at time: https://news.ycombinator.com/item?id=34301768). Someone has crawled public keys from GitHub (tbh I was surprised that GitHub publishes them) and set up a database.

Gitlab does the same.

I've seen provisioning scripts and even cloud-init if I'm not wrong supporting downloading keys in that manner.

From one side it's cool from other side allows to bypass of system administrator for keys update more easily.

> You can make a search for all users, which will tell you there are 97,616,627 users at the time of this writing, but you can only fetch at most 1000 results from a search, and they don’t come in any clear order, so you can’t just make the next search start where the previous one left off (or I didn’t figure out how).

> What you can do though is request accounts created in a certain time range. If you get the time range right, so that it has less than 1000 entries, you can paginate through it, and then request the next time range.

This reminds me of when I tried to add a google drive storage backend to camlistore/perkeep (because I had nearly-unlimited free quota at the time). One of the things a perkeep blobserver needs to be able to do enumerate all the blobs it has, in order. You can send millions of blobs to google drive without issue, but you can't directly paginate a search for them in sorted order.

You could just issue a search for all blobs under your perkeep drive folder, keep paginating the result until you run out of pages, and then sort in memory, but there's really no way of knowing how many blobs you're going to end up with and you might blow out your blobserver's memory.

Perkeep blobs are identified by blobrefs, SHA sums of the contents of the blob, so they look like sha-[0-9a-f]{64}. Google drive lets you search for files with a name prefix, so you can search for like /perkeep/sha-* and see if the result has a pagination token (indicating that there are more than 1000 results), and if so then you search for each of /perkeep/sha-0*, /perkeep/sha-1*, ... , /perkeep/sha-f*, each time checking to see whether there are too many matches. When there's not too many matches, you've found the prefix length that will let you fetch a bounded number of blobrefs, emit them to the perkeep client, and then release the memory before fetching more.

 /pk/sha-\* 1000+ results (non-empty pagination token) /pk/sha-0\* 1000+ results (non-empty pagination token) /pk/sha-00\* 1000+ results (non-empty pagination token) /pk/sha-000\* 193 results, sort these in memory and emit to client /pk/sha-001\* 179 results, sort these in memory and emit to client ... /pk/sha-fff\* 223 results, sort these in memory and emit to client

I didn't end up landing the patch before I lost interest, partly because it was pretty much the first golang I had tried writing. It was fun working out the above details, though.

> I tried to add a google drive storage backend to camlistore/perkeep (because I had nearly-unlimited free quota at the time)

This explains the quotas now :)

Offering your public key only allows them to identify the key and prove you have it. There is no security concern in sending this to an untrusted server.

Agent forwarding is a whole other beast.

Hm I thought I'd edited this. I was mistaken,

 ssh -o IdentitiesOnly=yes terminal.shop

works as expected, however I had an IdentityAgent set, and my key was being submitted via that route.

 ssh -o IdentitiesOnly=yes -o IdentityAgent=/dev/null terminal.shop

behaves as expected; same as

 ssh -a -i /dev/null -o IdentityAgent=/dev/null terminal.shop

Verified via whoami.filippo.io.

1. Why is this something that would be enabled by default.

2. Can't you disable agent forwarding in a config file, so as not to have to clutter the command line?

I think it’s disabled by default on all distros I’ve used. You could add an entry to /etc/ssh_config or ~/.ssh/ if you want.

(It’ll still offer public keys by default in the exchange, but that’s “just” a privacy issue, not a privilege escalation problem.)

I just ran it in a `tmpfs` without any credentials:

 $ bwrap --dev-bind / / --tmpfs ~ ssh terminal.shop

I think you may want to clear the environment (e.g., of `SSH_AUTH_SOCK`) as well as isolate in a PID namespace as well. I also reflexively `--as-pid-1 --die-with-parent`.

 bwrap --dev-bind / / --clearenv --tmpfs ~ --unshare-pid --as-pid-1 --die-with-parent ssh terminal.shop

(The `bwrap` manpage says “you are unlikely to use it directly from the commandline,” yet I use it like this all the time. If you do, too, then we should be friends!)

Honestly the only thing that you need is -a (and only if you made the bad choice to do agent forwarding by default). Sending your pubkey (and a signature, because the server pretends to accept your pubkey for some reason?) isn't a security risk and you're (in theory) going to be providing much more identifying information in the form of your CC...

(And as the siblings mentioned this won't work to prevent your key from being sent if you're using an agent)

SSH Agent Forwarding does not happen by default. You need to include the -A option in your ssh command, unless maybe you've enabled it globally in your ~/.ssh/config file.

They can't get your private keys, but they could "perform operations on the keys that enable them to authenticate using the identities loaded into the agent" (quoting the man page). This would also only be possible while you are connected.

This is only a threat if you enable agent forwarding for all hosts.

If you enable agent forwarding for all hosts then yes, data will be forwarded.

Your link says:

> Don’t enable agent forwarding when connecting to untrustworthy hosts. Fortunately, the ~/.ssh/config syntax makes this fairly simple

*disable ssh agent FORWARDING.

Which honestly should always be disabled. There are no trusted hosts.

That's baby+bathwater.

Just use ssh-add -c to have the ssh-agent confirm every use of a key.

TIL. Thanks! Gonna do wonders when working at places where I can't use a hardware key with physical confirmation of use.

My assessment still stands. Use proxyjump (-J) instead of proxy command whenever possible.

What can also help is specifying the right options right in ~/.ssh/config for certain hosts and domains: E.g. do "ForwardAgent no" globally, use a "Match *.my-trustworthy-company-domain.com" block and add "ForwardAgent yes" there.

Also very good for other options that are useful but problematic when used with untrustworthy target hosts, like ForwardX11, GSSAPIAuthentication, weaker *Algorithms (e.g. for those old Cisco boxes with no updates and similar crap).

Another neat trick is just using a ""Match *.my-trustworthy-company-domain.com" block" with an "IdentityFile ~/.ssh/secret-company-internal-key" directive. That key will then be used for those company-internal things, but not for any others, if you don't add it to the agent.

That makes more sense than my solution.

As far as I’m concerned the baby and the bath water is just a normal expression.

I thought it was something about the use of “confirm,” haha.

Mine too, and I think the post is fine also, but I have some idea of what that person was talking about. For a while, in some corporate environments, it was a recurring phenomenon to hear someone dismiss an urge to be cautious by saying "You're throwing out the baby with the bathwater."

So I can see where someone might count it toward buzzword bingo. But this post also offered an alternate solution when saying "baby+bathwater", so the bingo caller should refuse to score this one.

Your English is fine. That person was violating HN rules about snark (“Be kind. Don't be snarky. Converse curiously; don't cross-examine. Edit out swipes.”)

Learned that rule the hard way. It’s crucial to the success of HN and I am grateful dang corrected me.

I don't see a rule where joking is prohibited. People sure love their buzzwords though. Must bring them a feeling of synergy in these unprecedented times :)

Glad that at least a few people above got the joke

Default for the last 24 years according to https://github.com/openssh/openssh-portable/blame/385ecb31e1...

AFAIK, this doesn't solve the SSH agent problem - the problem is the agent has access to all of those keys regardless of the host you connect to.

So forwarding your SSH agent means an administrator of the system you're connected to could use any of those host keys loaded in the agent to connect to their associated machine.

> There are no trusted hosts.

...your own (headless) server that's in the same room as you, when you're using your laptop as a thin-client for it?

But if that's your standard then the laptop you're connecting from is not trusted either, and then you're not even allowed to use your own keys.

You're allowed to draw sensible boundaries.

Just to be clear, ssh agent forwarding is disabled by default and enabling it is always a hazard when connecting to machines that others also have access to.

Not at all specific to this.

Is it not standard practice to make different keys for different important services?

I have a private key for my prod server, a private key for GitHub, and a private junk key for authenticating to misc stuff. I can discard any without affecting anything else that's important.

If I authenticated with my junk key, would my other keys still be at risk?

> If I authenticated with my junk key, would my other keys still be at risk?

Yes, if you authenticate with your junk key (or no key), and SSH agent forwarding is enabled, you are still at risk. It lets the remote machine login to any server with any keys that are on your local SSH agent. Parent's link shows how this can be abused.

Fortunately, it's disabled by default, at least on newer versions.

TIL, the good news I guess is I only ssh into my hosting platforms and GitHub who have a reason to protect my data since I pay them.

Still I'll be sure to break up my keys more going forward and disable SSH forwarding.

disabling agent forwarding is the important bit.

But if you do want to break up your keys more, make sure you specify IdentityFile and Identities Only in the per host definitions in your ssh config.

By default assuming you use an ssh agent (no forwarding) with multiple keys and a default ssh config, the behavior is to just try to auth with every key in order.

So if you're worried about the ssh server identifying you, you're still exposing yourself. I don't think this is much of a concern but worth noting.

Slightly more important: you're wasting time during the initial connection to fail authentication a few times. This can matter more with higher latency

Even more important: sshd has a configurable number of times a client is allowed to fail authentication in a session attempt. If you have too many other keys in your agent you will just fail to auth before it tries the key that is actually valid for that host.

And for privacy, don’t let it know your identity or username:

 ssh -o PubkeyAuthentication=no -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no -a [email protected]

Otherwise, the remote server can probably identify who you are on platforms like GitHub.

This feature is not enabled by default; "ForwardAgent = yes" has to be in the config file.

The article you cited makes it clear that you can turn this on for specific hosts in your private SSH config (and probably should do it that way).

So why wouldn't you?

Turning on forwarding globally and then having to remember to disable it for some untrusted hosts with -a looks silly and error-prone to me.

You can configure the agent to confirm each key usage to have your cake and eat it too. :)

It's also good to see if any malicious process tries to make use of the agent locally!