To secure SSH, the more I care about a system the farther down this list I go.
1. Disable root
2. Use keys, disable passwords
3. Install fail2ban
4. Restrict incoming subnets
5. MathAuthTries = 1
6. Run on high random port
7. Use port knocking
From an accountability standpoint, if each person logs in to the root account directly with a distinct private/public key, you can still have full individual accountability.
I'd think the complexity side of the sudo strategy is self-evident, so perhaps I'm not understanding the part that needs explaining.
I don't find 'sudo' to be complex at all. If in your situation I would have had the ability to log in as root, in my situation I will be in the 'wheel' group. It seems very straightforward to me.
I think you misunderstand my point. You have increased the complexity of securing the system. You now have each user's login shells, all of the joys of what those login shells touch, the sudo program and its configuration, all added to the attack surface.
For accountability the openssh server already logs the key fingerprint.
The main security implications of sudo are a false sense of security and the risks related to having a password on your account and typing it in all the time (how often have you spilled it into a bash history?).
Sudo can't provide a dependable audit trail because it is trivially circumvented ('sudo bash'). It doesn't protect you from local-to-root exploits either.
It doesn't even protect you from yourself but rather makes your critical commands more complicated and error prone (shell globbing/escaping, pipes and redirects, etc.).
What is the false sense of security created by sudo? You don't have to have a password on your account to use it. You can use NOPASSWD or you can use pam_ssh_agent_auth to verify ssh keys if you are very paranoid. (Users still should not log into the servers with passwords.)
sudo doesn't provide a dependable audit trail, but neither does anything else. If you are root, why not just fix the logs? To fix this, you must use snoopy to log commands to syslog, then syslog to a different computer. This makes both 'sudo' and root's ssh keys as accountable as one could ask for, I think. (As I could ask for, anyway.)
It protects you from yourself in the sense that you are meant to think about what you type if it begins with "sudo." If you are running as your own user, it's much harder for a stray 'rm' to bring down the system. You might clobber your own files, but then you just restore from your backup.
What is the false sense of security created by sudo? You don't have to have a password on your account to use it.
You seem to think sudo adds security even when you don't use a sudo password. That's precisely the false sense it creates.
snoopy
On vanilla Linux there is no dependable audit trail after someone becomes root, period.
that you are meant to think about what you type if it begins with "sudo."
Because you don't think when you type into a root shell?
The mythical "stray rm" is a strawman. I've never heard of one happening in the real world. If you're that careless then sudo won't save you either.
What I do see often is people spending minutes on trial & error because sudo turns even trivial commands into a minefield when a pipe/redirect, the shell environment, globbing or a loop gets involved.
sudo doesn't provide a dependable audit trail, but neither does anything else. If you are root, why not just fix the logs? To fix this, you must use snoopy to log commands to syslog, then syslog to a different computer.
Wait, you just said "neither does anything else", and then gave an example of how to create an audit trail. ;-)
In truth, you should use auditd or similar systems to really have a proper audit trail.
You're absolutely right that sudo doesn't make an audit trail more worse. It just doesn't make it any better either. It does, however, create a bunch more ways that someone can hack your system.
It protects you from yourself in the sense that you are meant to think about what you type if it begins with "sudo."
How about you write a shell script on your own system, call it "sudo", and have it do "ssh root@admin.system $*"? You see what I mean? Maybe you are used to using sudo, but there is no reason the "oh noes, now I need to be careful mode" has to be a privilege escalation command on the host your are administrating. In fact, it shouldn't be. It really should be before you have logged in.
If your threat model has someone stealing your private key and breaking/obtaining the password to it, none of the other measures are going to mean anything as they have root on your box. With root they can bypass the access restrictions, knocking, whatever else. 2FA with an external device would probably make more sense than any of them and be significantly less irritating.
That assumes that stealing your private key is the same thing as sending arbitrary packets from your machine, otherwise things like restricting subnets/ip addresses and port knocking could still be useful layers. How different exfil of a private key from being able to send arbitrary packets on the user's machine is...probably not that different under most circumstances I'd guess.
Or they have your discarded dead drive from your backup disk that you forgot to enable full disk encryption on. Or they rooted a machine that you were using "ssh -A" on to forward your agent, and have been able to temporarily hijack your SSH agent session. Or there's an information disclosure vulnerability in your web browser that allows them to read files from your hard drive, though without control of what files they can read, and by chance the attacker got lucky and one of the files was .ssh/id_rsa.
While the above description goes way further than I probably would, and yeah, 2FA would be more reasonable, there are good reasons to practice defense in depth, and try to design systems that have a chance of staying secure even if one of your assumptions fails like "private key is compromised".
If your threat model is someone stealing and cracking your private key, you really ought to be more worried that they are going to use their time machine to go back in time and kill your mother.
Both actions happen at the same time, someone phishes you, captures your input and they have the password for it as well. Now that they've got a shell on your box IP address restrictions are bypassed as well, as is port knocking. This is completely plausible, time machines are not. I read a (private) incident report very recently where half a company had their devices spear phished in order to gain access to a single internal server.
I'd recommend encrypting your private keys if needed with a pass-phrase. I'd also recommend still requiring a sudo password on the other end and sending auth events to a auditing server.
2 factor with a token is ideal, I suppose I should have added that to the list, but that is outside of feasibility for most users.
Still, for someone who doesn't have RSA tokens etc available, assuming your remote machine doesn't get owned and someone doesn't extract your pass-phrase with a wrench, that list makes for a very secure system.
I'd also recommend still requiring a sudo password on the other end and sending auth events to a auditing server.
You should be auditing everything already, and adding in sudo now adds another attack vector. You also now have two different accounts that can be manipulated to compromise a system.
Comments
To secure SSH, the more I care about a system the farther down this list I go.
1. Disable root
This doesn't add any security, it only makes your admin life miserable.
I keep trying to explain to people that if anything this adds complexity to the security problem...
Can you explain this to me?
We do it at work for accountability purposes, but I'm not sure I'm clear on the security implications.
From an accountability standpoint, if each person logs in to the root account directly with a distinct private/public key, you can still have full individual accountability.
I'd think the complexity side of the sudo strategy is self-evident, so perhaps I'm not understanding the part that needs explaining.
I don't find 'sudo' to be complex at all. If in your situation I would have had the ability to log in as root, in my situation I will be in the 'wheel' group. It seems very straightforward to me.
I think you misunderstand my point. You have increased the complexity of securing the system. You now have each user's login shells, all of the joys of what those login shells touch, the sudo program and its configuration, all added to the attack surface.
For accountability the openssh server already logs the key fingerprint.
The main security implications of sudo are a false sense of security and the risks related to having a password on your account and typing it in all the time (how often have you spilled it into a bash history?).
Sudo can't provide a dependable audit trail because it is trivially circumvented ('sudo bash'). It doesn't protect you from local-to-root exploits either.
It doesn't even protect you from yourself but rather makes your critical commands more complicated and error prone (shell globbing/escaping, pipes and redirects, etc.).
Fair enough (wrt accountability).
What is the false sense of security created by sudo? You don't have to have a password on your account to use it. You can use NOPASSWD or you can use pam_ssh_agent_auth to verify ssh keys if you are very paranoid. (Users still should not log into the servers with passwords.)
sudo doesn't provide a dependable audit trail, but neither does anything else. If you are root, why not just fix the logs? To fix this, you must use snoopy to log commands to syslog, then syslog to a different computer. This makes both 'sudo' and root's ssh keys as accountable as one could ask for, I think. (As I could ask for, anyway.)
It protects you from yourself in the sense that you are meant to think about what you type if it begins with "sudo." If you are running as your own user, it's much harder for a stray 'rm' to bring down the system. You might clobber your own files, but then you just restore from your backup.
What is the false sense of security created by sudo? You don't have to have a password on your account to use it.
You seem to think sudo adds security even when you don't use a sudo password. That's precisely the false sense it creates.
snoopy
On vanilla Linux there is no dependable audit trail after someone becomes root, period.
that you are meant to think about what you type if it begins with "sudo."
Because you don't think when you type into a root shell?
The mythical "stray rm" is a strawman. I've never heard of one happening in the real world. If you're that careless then sudo won't save you either.
What I do see often is people spending minutes on trial & error because sudo turns even trivial commands into a minefield when a pipe/redirect, the shell environment, globbing or a loop gets involved.
Wait, you just said "neither does anything else", and then gave an example of how to create an audit trail. ;-)
In truth, you should use auditd or similar systems to really have a proper audit trail.
You're absolutely right that sudo doesn't make an audit trail more worse. It just doesn't make it any better either. It does, however, create a bunch more ways that someone can hack your system.
How about you write a shell script on your own system, call it "sudo", and have it do "ssh root@admin.system $*"? You see what I mean? Maybe you are used to using sudo, but there is no reason the "oh noes, now I need to be careful mode" has to be a privilege escalation command on the host your are administrating. In fact, it shouldn't be. It really should be before you have logged in.
If your threat model has someone stealing your private key and breaking/obtaining the password to it, none of the other measures are going to mean anything as they have root on your box. With root they can bypass the access restrictions, knocking, whatever else. 2FA with an external device would probably make more sense than any of them and be significantly less irritating.
That assumes that stealing your private key is the same thing as sending arbitrary packets from your machine, otherwise things like restricting subnets/ip addresses and port knocking could still be useful layers. How different exfil of a private key from being able to send arbitrary packets on the user's machine is...probably not that different under most circumstances I'd guess.
Or they have your discarded dead drive from your backup disk that you forgot to enable full disk encryption on. Or they rooted a machine that you were using "ssh -A" on to forward your agent, and have been able to temporarily hijack your SSH agent session. Or there's an information disclosure vulnerability in your web browser that allows them to read files from your hard drive, though without control of what files they can read, and by chance the attacker got lucky and one of the files was .ssh/id_rsa.
While the above description goes way further than I probably would, and yeah, 2FA would be more reasonable, there are good reasons to practice defense in depth, and try to design systems that have a chance of staying secure even if one of your assumptions fails like "private key is compromised".
If your threat model is someone stealing and cracking your private key, you really ought to be more worried that they are going to use their time machine to go back in time and kill your mother.
Both actions happen at the same time, someone phishes you, captures your input and they have the password for it as well. Now that they've got a shell on your box IP address restrictions are bypassed as well, as is port knocking. This is completely plausible, time machines are not. I read a (private) incident report very recently where half a company had their devices spear phished in order to gain access to a single internal server.
Oh, someone capturing your private key's passphrase is something to be genuinely worried about.
Someone brute forcing said passphrase? Yeah no.
I'd recommend encrypting your private keys if needed with a pass-phrase. I'd also recommend still requiring a sudo password on the other end and sending auth events to a auditing server.
2 factor with a token is ideal, I suppose I should have added that to the list, but that is outside of feasibility for most users.
Still, for someone who doesn't have RSA tokens etc available, assuming your remote machine doesn't get owned and someone doesn't extract your pass-phrase with a wrench, that list makes for a very secure system.
You should be auditing everything already, and adding in sudo now adds another attack vector. You also now have two different accounts that can be manipulated to compromise a system.