88 points by zdw 3 days ago | 15 comments
nubinetwork 36 minutes ago
While it wouldn't prevent the issue they described, I prefer to pull, rather than push. My thinking is, if you pull, you're still connected. If you push, as soon as the push finishes, you're locked out.
chasil 5 hours ago
I have a few observations about this article.

Generally, try not to use SCP. It has been a crufty old program from the Berkeley R-Utilities, but newer OpenSSH releases have rewritten it to use the sftp-server server instead. There will be wildly different behavior between these implementations.

The backend SCP changes are documented here:

https://lwn.net/Articles/835962/

If you need something that SFTP cannot do, then use tar on both sides.

PuTTY has implemented their pscp to prefer the sftp-server for many years, in a long prediction of the eventual abandonment. Their pscp implementation is a better drop-in replacement than the OpenSSH solutions.

The allure of SCP is retry on failure, which is somewhat more difficult with SFTP:

  until scp source.txt user@target:dir/
  do echo target down; sleep 300
  done
Converting that to pscp is much easier than SFTP.

I also have an older rhel5 system where I am running tinysshd to use better SSH crypto. Due to upgrades, NFS is now squashing everything to nobody, so I had to disable precisely these checks to let users login with their authorized_keys. I can post the code if anybody is curious.

1 hour ago
elevation 3 hours ago
I occasionally use `scp` around my network and have for years. It works great and its simple interface is easy to remember. I don't want to sftp if I have to use tar on both sides. I might type rsync and but then I remember something about the trailing slash will cause the command to behave differently the second time. I just don't need yet another syntax I'll misremember. As long as scp is in my distro's repositories, I'll be using it.
fsckboy 2 hours ago
easy to remember, if you don't use trailing slashes ever, it will just work every time
eikenberry 2 hours ago
> Their pscp implementation is a better drop-in replacement than the OpenSSH solutions.

What makes it a better drop in replacement?

chasil 2 hours ago
Several reasons.

-PuTTY pscp allows raw passwords on the command line, or from a file. OpenSSH is unreasonable in refusing to do this.

-Scripting can adapt to a .netrc easily; OpenSSH will never do this.

-Modern OpenSSH is a nightmare when using legacy crypto, while pscp is fluid. There is nothing wrong with hmac-md5, and no reason to refuse it. I will take PuTTY or dropbear in a heartbeat over these burned bridges and workarounds.

https://www.openssh.org/legacy.html

-pscp does not link to dozens of libraries as ssh/scp does, so it is easier to build with less dependency. The ldd output of ssh and scp on rhel9 is 23 libraries, while PuTTY is 3 [package obtained from EPEL].

-pscp strongly leans to SFTP on the backend and can be directed to use it exclusively, so there is no ambiguity.

-Using pscp with a retry on fail is much easier than sftp -b.

-The wacky cipher control on rhel8 does not impact the PuTTY tools.

That is an extensive list.

jolmg 19 minutes ago
> -PuTTY pscp allows raw passwords on the command line, or from a file. OpenSSH is unreasonable in refusing to do this.

You can use `sshpass` to force it through the command line. However, arguments can be viewed by any process through `/proc`, `ps`, etc. It's pretty reasonable to not support exposure of the password like that, especially since you can force it through another tool if you really, really need to.

extraduder_ire 2 hours ago
>If you need something that SFTP cannot do, then use tar on both sides.

Wouldn't tar do the exact same thing to that file's permissions?

chasil 1 hour ago
Likely, but maintaining hard links is more of what I was thinking.
Onavo 41 minutes ago
What I want is to be able to drag and drop files in my remote server to and from my desktop as if it's an NFS/NAS. What's the best option for this that will fully saturate the link?
jolmg 1 hour ago
SCP protocol is fine and convenient as long as people understand that the remote file arguments are server-side shell code, and the consequences that implies.

You get the benefit of being able to e.g. get your last download off your desktop to your laptop like this:

  scp -TO desktop:'downloads/*(oc[1])' .
or this if you're on bash:

  scp -TO desktop:'$(ls -t downloads/* | head -1)' .
or pull a file from a very nested project dir for which you have setup dynamic directories (or shell variables if you're on bash):

  scp -TO desktop:'~foo/config/database.yml' config/

  scp -TO desktop:'$FOO_DIR/config/database.yml' config/
Just don't pull files from an SCP server that may be malicious. Use on trusted servers. If you do the following on your home dir:

  scp -TOr malicious:foo/ .
That may overwrite .ssh/authorized_keys, .zshrc, etc. because `foo/` is server-side shell code. The client can't say that `.zshrc` resulting from the evaluation of `foo/` doesn't make sense, because it might in the remote shell language.

> If you need something that SFTP cannot do, then use tar on both sides.

No reason to make things inconvenient between personal, trusted computers, just because there may be malicious servers out there where one has no reason to SCP.

Something else to note is that your suggestion of using `tar` like `ssh malicious 'tar c foo/' | tar x` faces basically the exact same problem. The server can be malicious and return .ssh/authorized_keys, .zshrc, etc. in the archive for `tar x` to overwrite locally basically exactly the same way. This goes with the point of this SE answer:

> I'd say a lot of Unix commands become unsafe if you consider a MITM on SSH possible. A malicious sudo could steal your password, a malicious communication client could read your mails/instant messages, etc. Saying that replacing scp with sftp when talking to a compromised server will somehow rectify the situation is very optimistic to say the least. [...] In short, if you don't pay attention to which servers you SSH into, there's a high risk for you to be screwed no matter which tools you use, and using sftp instead of scp will be only marginally safer. --- https://unix.stackexchange.com/questions/571293/is-scp-unsaf...

I think this whole problem with SCP just stems from not having properly documented this aspect in the manpage, so people expected it to just take filepaths.

mistrial9 5 hours ago
you sound so wise and produce excellent reference, but in the next breath you show NFS in use?

signed -confused

eikenberry 2 hours ago
What would you use for remote mounting filesystems? I don't know of any that are simply superior (w/o caveats/tradeoffs).
gchamonlive 3 hours ago
Why is it so self-evident that NFS is bad?
chasil 1 hour ago
I upvoted you, and yes, cleartext NFS is a concern.

I had it wrapped in stunnel TLS, but I ripped that out recently as I am retiring and the new staff is simply not capable of maintaining that configuration.

My users were yelling, and the patch to tinysshd to omit all permissions checks silenced the complaints. No, it's not pretty.

jamiesonbecker 26 minutes ago
That's a common and easy footgun with scp/rsync and ~/.ssh perms/authorized_keys drift, even with experienced sysadmins

If you DO have direct access, this should fix it up quickly as another user with sudo perms:

    username=bob
    sudo mkdir -p /home/$username/.ssh/
    echo "ssh-ed25519 AAAA....insertyourpubkeyhere" | sudo tee /home/$username/.ssh/authorized_keys
    sudo chown $username:$username /home/$username
    sudo chmod u+rwX,go-rwX /home/$username -R
optionally:

    echo "$username ALL=(ALL) NOPASSWD: ALL" | sudo tee /etc/sudoers.d/$username
Not to shill too hard, but this specific failure mode is basically why Userify exists (est. 2011): local accounts on every box + a tiny outbound-only agent that polls and overwrites desired state (keys, perms, sudo role). So if a deploy step clobbers authorized_keys or breaks perms/sudo for managed users, the next poll (~90s) just re-converges it. Also kills sessions when a user is removed. No inbound ports to nodes; no PAM/NSS; designed to be auditable.

Totally sympathetic to the author here - I did something similar last year with an ec2 instance and was getting ready to shut it down, snap and attach so I could repair it, but then userify magically fixed it. :)

Shim (old but readable): https://github.com/userify/shim/blob/master/shim.py#L308

(obligatory: https://userify.com)

procaryote 5 hours ago
This is a useful tip!

but also... who has a dir with 777 permissions? Is that something people do nowadays?

easterncalculus 4 hours ago
My guess would be mounting an NTFS partition - with ntfs-3g it will load everything as 777 just by default, since it can’t translate the permissions.
1 hour ago
SoftTalker 2 hours ago
I've seen users who have every file set to 777. They do it to "avoid permissions issues"
chasil 5 hours ago
Well, everybody has 1777 as /tmp (with the sticky bit).

  $ ll -d /tmp
  drwxrwxrwt. 20 root root 4096 Mar  3 12:19 /tmp
  $ mkdir mytmp
  $ chmod 1777 mytmp
  $ ll -d mytmp
  drwxrwxrwt. 1 luser lgroup 0 Mar  3 12:19 mytmp
huflungdung 1 hour ago
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zahlman 3 days ago
I assume using `./*` rather than `.` in the `scp` command would have worked around the issue?
hrmtst93837 2 hours ago
Using './*' would have avoided this in most shells because ordinary globbing excludes dotfiles, so .ssh and authorized_keys are not matched. In my experience scp is brittle for bulk syncs, so I run rsync -a --exclude='.ssh' --dry-run ./ user@host:~/target to verify before I commit the changes. I keep an out of band recovery path, like a temporary deploy key, a nonprivileged rescue user, or console access, as the only reliable way to avoid being locked out at 3AM.
Biganon 2 hours ago
The problem was not scp'ing the .ssh/ directory. The problem was scp'ing a directory whose permissions were 777, and "mapping" it (cannot find a better term) to a remote directory, which happened to be the home directory. The remote home directory therefore had its permissions changed to 777, which was deemed "too open" by openssh which refuses to use any file in it.
malicka 5 hours ago
Yes, since it would’ve copied the globbed files, rather than the current directory itself.
tracker1 4 hours ago
I accidentally nuked my hosted server's network stack with a config error... my bigger mistake was generating a massive random password for the root account... the remote terminal management console didn't support pasting and the default config only gave you like 30s to login.... not fun at all.

Script all the things. double-check your scripts... always be backing up.

jonathanlydall 3 hours ago
> the remote terminal management console didn't support pasting and the default config only gave you like 30s to login

I would have used AutoHotkey or something similar in such a scenario.

gchamonlive 3 hours ago
Also a gentle reminder that backups without periodic drills are just binary blobs. I had an instance where for some reason my Borg backups where corrupted. Only caught them with periodic drills.
LoganDark 3 hours ago
You did not transfer the files within a directory. You transferred the directory itself, via `.`. That is why scp changed the permissions of your home directory itself; if you instead had transferred via `*` I am sure you would not have had this problem.
impure 5 hours ago
Ah, file permissions. My old friend. Good thing this happened on a 'local' server and not a remote VPS.
MomsAVoxell 4 hours ago
Done stupid stuff like this enough times that I just use tar, and also make a sandbox directory to receive it, to double-check whats going to happen, before un—tar’ing it again into the destination intended and/or do a manual move.

Too many burned fingers to not do this little dance almost every other time.

Actually, I lied, I just use rsync like an insane person.

crest 6 hours ago
It's nice to see people sharing their mistakes too.
sowbug 6 hours ago
Related: In my Bash logout script I have a chmod that fixes authorized_keys. It won't help with scp because that's non-interactive, but it has helped the other 999 times I've forgotten to clean up the mess I made during an ssh session.
TZubiri 5 hours ago
Getting locked out of a server must be a cannonical experienc in the sysadmin journey, like checking the logs to see you are being attacked as soon as your online, or trying to build your own linux from scratch without bloat.
roelschroeven 6 hours ago
tl;dr: I you scp -r to your homedir, expect scp to copy not just files and directories but their permissions as well (which I think isn't all that surprising).
ranger_danger 6 hours ago
It's not supposed to do that unless it's newly creating the destination, or you supplied the -p flag to preserve permissions... that's what the entire issue is about; it's a bug that was fixed in 10.3.
Calzifer 5 hours ago
I wouldn't even expect it on newly created stuff without the -p flag. Normal cp doesn't do it.
rhier1 4 hours ago
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binaryturtle 7 hours ago
When I load the site in my (slightly older) Firefox I just get some random junk and gibberish (markov chain generated nonsense?)

<bleep> that nonsense!

theblazehen 5 hours ago
I suspect you're hitting the page where they're running https://iocaine.madhouse-project.org/

Perhaps you got bot flagged or something

extraduder_ire 2 hours ago
That URL gives me a 418 I'm a teapot error with no body. I'm guessing they don't like my VPN.