7 ms·

So this basically means to scan for this exploit remotely we'd need the private key of the attacker which we don't have. Only other option is to run detection s

So this basically means to scan for this exploit remotely we'd need the private key of the attacker which we don't have. Only other option is to run detection scripts locally. Yikes.

Thorrez 2y ago
The tweet says "unreplayable". Can someone explain how it's not replayable? Does the backdoored sshd issue some challenge that the attacker is required to sign?
- candiodari 2y ago
  What it does is this: RSA_public_decrypt verifies a signature on the client's (I think) host key by a fixed Ed448 key, and then if it verifies, passes the payload to system(). If you send a request to SSH to associate (agree on a key for private communications), signed by a specific private key, it will send the rest of the request to the "system" call in libc, which will execute it in bash. So this is quite literally a "shellcode". Except, you know, it's on your system.
  - Thorrez 2y ago
    That sounds repayable though. If I did a tcpdump of the attacker attacking my system, I could replay that attack against someone other system. For it to not be replayable, there needs to be some challenge issued by the backdoored sshd. Of course since the backdoor was never widely deployed and is now public, I think it's unlikely the attacker will attempt to use it. So whether it's replayable doesn't have a practical impact now. I'm only asking about replayability because I'm curious how it's unreplayable.
tialaramex 2y ago
One completely awful thing some scanners might choose to do is if you're offering RSA auth (which most SSH servers are and indeed the SecSH RFC says this is Mandatory To Implement) then you're "potentially vulnerable" which would encourage people to do password auth instead. Unless we find that this problem has somehow infested a lot of real world systems that seems to me even worse than the time similar "experts" decided that it was best to demand people rotate their passwords every year or so thereby ensuring the real security is reduced while on paper you claim you improved it.
- danparsonson 2y ago
  Have to admit I've never understood why password auth is considered so much worse than using a cert - surely a decent password (long, random, etc) is for all practical purposes unguessable, and so you're either using a private RSA key that no-one can guess, or a password that no-one can guess, and then what's the difference? With the added inconvenience of having to pass around a certificate if you want to login to the same account from from multiple sources.
  - dboreham 2y ago
    It depends what happens to the password. Typically it's sent as a bearer credential. But there are auth schemes (not widely used these days) where the password isn't sent over the wire.
    - solardev 2y ago
      Isn't it pretty standard practice to salt and hash the password client-side before sending it over the wire?
      - cayde 2y ago
        No, usually it's sent in plain text to the server, however encapsulated.
        solardev 2y ago
        Wow, really? Ten years ago, it was drilled into me to never send a password like that, especially since the server shouldn't have the plain version anyway (so no reason for the client to send it). https://owasp.org/www-community/OWASP_Application_Security_FAQ https://owasp.org/www-community/OWASP_Application_Security_F... says "Salted hash for transmitting passwords is a good technique. This ensures that the password can not be stolen even if the SSL key is broken." I didn't want to believe you, but man, I just checked a few websites in the network inspector... and it seems like GMail, Hackernews, Wordpress, Wix, and Live.com all just sent it in plaintext with only SSL encryption :( That's a bit disappointing. But TIL. Thanks for letting me know!
CodesInChaos 2y ago
It might be possible to use timing information to detect this, since the signature verification code appears to only run if the client public key matches a specific fingerprint. The backdoor's signature verification should cost around 100us, so keys matching the fingerprint should take that much longer to process than keys that do not match it. Detecting this timing difference should at least be realistic over LAN, perhaps even over the internet, especially if the scanner runs from a location close to the target. Systems that ban the client's IP after repeated authentication failures will probably be harder to scan. (https://bench.cr.yp.to/results-sign.html https://bench.cr.yp.to/results-sign.html lists Ed448 verification at around 400k cycles, which at 4GHz amounts to 100us)
- pstrateman 2y ago
  However only probabilistic detection is possible that way and really 100us variance over the internet would require many many detection attempts to discern.
- Thorrez 2y ago
  According to[1], the backdoor introduces a much larger slowdown, without backdoor: 0m0.299s, with backdoor: 0m0.807s. I'm not sure exactly why the slowdown is so large. [1] https://www.openwall.com/lists/oss-security/2024/03/29/4 https://www.openwall.com/lists/oss-security/2024/03/29/4
  - CodesInChaos 2y ago
    The effect of the slowdown on the total handshake time wouldn't work well for detection, since without a baseline you can't tell if it's slow due to the backdoor, or due to high network latency or a slow/busy CPU. The relative timing of different steps in the TCP and SSH handshakes on the other hand should work, since the backdoor should only affect one/some steps (RSA verification), while others remain unaffected (e.g. the TCP handshake).

So this basically means to scan for this exploit remotely we'd need the private key of the attacker which we don't have. Only other option is to run detection scripts locally. Yikes.

Thorrez 2y ago

The tweet says "unreplayable". Can someone explain how it's not replayable? Does the backdoored sshd issue some challenge that the attacker is required to sign?

candiodari 2y ago
What it does is this: RSA_public_decrypt verifies a signature on the client's (I think) host key by a fixed Ed448 key, and then if it verifies, passes the payload to system(). If you send a request to SSH to associate (agree on a key for private communications), signed by a specific private key, it will send the rest of the request to the "system" call in libc, which will execute it in bash. So this is quite literally a "shellcode". Except, you know, it's on your system.
- Thorrez 2y ago
  That sounds repayable though. If I did a tcpdump of the attacker attacking my system, I could replay that attack against someone other system. For it to not be replayable, there needs to be some challenge issued by the backdoored sshd. Of course since the backdoor was never widely deployed and is now public, I think it's unlikely the attacker will attempt to use it. So whether it's replayable doesn't have a practical impact now. I'm only asking about replayability because I'm curious how it's unreplayable.

tialaramex 2y ago

One completely awful thing some scanners might choose to do is if you're offering RSA auth (which most SSH servers are and indeed the SecSH RFC says this is Mandatory To Implement) then you're "potentially vulnerable" which would encourage people to do password auth instead. Unless we find that this problem has somehow infested a lot of real world systems that seems to me even worse than the time similar "experts" decided that it was best to demand people rotate their passwords every year or so thereby ensuring the real security is reduced while on paper you claim you improved it.

danparsonson 2y ago
Have to admit I've never understood why password auth is considered so much worse than using a cert - surely a decent password (long, random, etc) is for all practical purposes unguessable, and so you're either using a private RSA key that no-one can guess, or a password that no-one can guess, and then what's the difference? With the added inconvenience of having to pass around a certificate if you want to login to the same account from from multiple sources.
- dboreham 2y ago
  It depends what happens to the password. Typically it's sent as a bearer credential. But there are auth schemes (not widely used these days) where the password isn't sent over the wire.
  - solardev 2y ago
    Isn't it pretty standard practice to salt and hash the password client-side before sending it over the wire?
    - cayde 2y ago
      No, usually it's sent in plain text to the server, however encapsulated.
      - solardev 2y ago
        Wow, really? Ten years ago, it was drilled into me to never send a password like that, especially since the server shouldn't have the plain version anyway (so no reason for the client to send it). https://owasp.org/www-community/OWASP_Application_Security_FAQ https://owasp.org/www-community/OWASP_Application_Security_F... says "Salted hash for transmitting passwords is a good technique. This ensures that the password can not be stolen even if the SSL key is broken." I didn't want to believe you, but man, I just checked a few websites in the network inspector... and it seems like GMail, Hackernews, Wordpress, Wix, and Live.com all just sent it in plaintext with only SSL encryption :( That's a bit disappointing. But TIL. Thanks for letting me know!

CodesInChaos 2y ago

It might be possible to use timing information to detect this, since the signature verification code appears to only run if the client public key matches a specific fingerprint. The backdoor's signature verification should cost around 100us, so keys matching the fingerprint should take that much longer to process than keys that do not match it. Detecting this timing difference should at least be realistic over LAN, perhaps even over the internet, especially if the scanner runs from a location close to the target. Systems that ban the client's IP after repeated authentication failures will probably be harder to scan. (https://bench.cr.yp.to/results-sign.html https://bench.cr.yp.to/results-sign.html lists Ed448 verification at around 400k cycles, which at 4GHz amounts to 100us)

pstrateman 2y ago
However only probabilistic detection is possible that way and really 100us variance over the internet would require many many detection attempts to discern.
Thorrez 2y ago
According to[1], the backdoor introduces a much larger slowdown, without backdoor: 0m0.299s, with backdoor: 0m0.807s. I'm not sure exactly why the slowdown is so large. [1] https://www.openwall.com/lists/oss-security/2024/03/29/4 https://www.openwall.com/lists/oss-security/2024/03/29/4
- CodesInChaos 2y ago
  The effect of the slowdown on the total handshake time wouldn't work well for detection, since without a baseline you can't tell if it's slow due to the backdoor, or due to high network latency or a slow/busy CPU. The relative timing of different steps in the TCP and SSH handshakes on the other hand should work, since the backdoor should only affect one/some steps (RSA verification), while others remain unaffected (e.g. the TCP handshake).