Bypass ssh key file permission check

Question

I have an encrypted FAT volume (for compatibility) containing a private key file and other sensitive data.

I want to connect to my server through SSH using my private key, but of course, as FAT doesn't support file permission, it ignores my key saying its permissions are too open.

So currently I'm copying it somewhere else on my hard drive with 0600 permissions, using it and then securely erasing it, but it's a pain.

Is there a way to bypass permission check on this very ssh/scp command line ?

Edit: Precision: it was a TrueCrypt volume on OS X.

On the solution: The accepted answer below solved my problem (using a SSH key file located on a TrueCrypt volume with Mac OS X), but it is a workaround. Looks like there is no way to "bypasssh key file permission check".

Answer 1

Adding the key from stdin worked for me:

cat /path/to/id_rsa | ssh-add -k -

Answer 2

AFAIK, there is no way to bypass the keyfile permission check with ssh or ssh-add (and you can't trick it with named pipe or such). Besides, you do not actually want to trick ssh, but just to be able to use your key files.

Indeed, TrueCrypt volume is supposed to keep your data private, so mounting the volumes as world-readable (default behaviour of TrueCrypt) is not really optimum. If you're using a FAT-formatted volume, you really should ajust the mount options, as Dan Carley suggested.

Although mount options aren't yet correctly supported by TrueCrypt for OS X (even if you launch TC using the command line interface and the mount options from the man page - already tried), OS X does support mount option defaults based on the volume name.

You need to know your user id (usually 501 if you are the first/only user of the computer). You can get it with "id -u".

Let' say you volume name is "PRIVATE" (volume names are in capitals), and your uid is 501, all you have to do is adding this line to /etc/fstab :

LABEL=PRIVATE none msdos -u=501,-m=700

You need to be root to create/edit this file (it is not present in default OSX install) :

sudo vim /etc/fstab

Next time you mount the volume, it'll have permission 700 and owner id 501.

This also works with USB drives (which are usually formatted in FAT, too).

Answer 3

As a crazy workaround, you could make a disk image of an ext2 volume containing your private key and mount it as a loop device, then use your ssh key from there.

Make a 1MB empty file:

dd if=/dev/zero of=diskimg bs=1024 count=1024

Format it ext2 (Press Y when it says it isn't a device):

mke2fs diskimg

Mount it somewhere (as root):

mount -t ext2 -o loop diskimg /my/path/to/diskimg

Now you have a tiny ext2 filesystem that you can set permissions on. You could write a script to mount it and make sure those permissions have the right UID/GID based on whatever system you're on (since the UIDs may mismatch). It also requires sudo/root access to work.

Answer 4

What about adding StrictModes no to your /etc/ssh/sshd_config (and the reload/restart sshd) ?

edit: oops, this option is server-side only :/

Answer 5

Another way to do this is with process substitution:

ssh-add <(cat ./file.key)

..or similar for ssh.

This has the advantage of not making ssh-add invoke external programs (that might not be available) for passphrase retrieval like it does when it detects it's being piped to (i.e. cat file.key | ssh-add -).

Make sure to use modern shells (bash or zsh) because /bin/sh might not support process substitution.

Answer 6

If I recall correctly, ssh-agent does not check for key permissions. So this might work:

[ -S "$SSH_AUTH_SOCK" ] || eval $(ssh-agent)

ssh-add path/to/id_rsa

Answer 7

My recipe for dealing with Cygwin/Windows permissions and ssh keys in cygwin is as follows.

open first cygwin64 terminal, start ssh-agent there

eval $(ssh-agent)

change permissions of (any) key just before adding to the agent

chmod 400 my_key.pem
ssh-add my_key.pem

open future cygwin64 terminals out of this one to ensure they inherit the environment and have the ssh-agent with the key(s)

Answer 8

I'm also using a vfat formatted Truecrypt volume for the same compatibility reasons. But on Linux, using tcplay to mount it from a script.

In such a case, instead of editing fstab, the solution is to directly add the umask=077 option to the mount command.

This is the relevant part of the script I use:

uid=$(id -u)
gid=$(id -g)
... losetup and tcplay commands ...
sudo mount -o nodev,nosuid,uid=$uid,gid=$gid,umask=077 /dev/mapper/$mnt_name $mnt_point

Answer 9

The key reason is u need a linux environment.

As for me,I previously conneted the server with xfpt. So i stickup it to the server and change the Permissions of the key-file for 600 like this enter image description here

And after stickup it back ,ssh worked.

Answer 10

For users with apache server running on windows server 2019, if you need to use Net::SFTP::Foreign to access SFTP with ssh file (not with password), you need to set the ssh file sole permission to "SYSTEM" (right-click->properties->security->edit), remove inheritance (right-click->properties->security->advance) and also removed the owner's name (right-click->properties->details->Remove properties and Personal Information) or change file's owner to "SYSTEM".

Code part, need to add ENV variables below $ENV{"PROGRAMDATA"}="C:\ProgramData"; $ENV{"USERNAME"}="SYSTEM";

Hope this helps!

Answer 11

To bypass SSH key file permission checks, you must modify file permissions or use the "StrictModes no" directive in your SSH configuration. This allows SSH to ignore key file permissions, potentially compromising your security. Proceed with caution and consider alternatives such as fixing permissions or using an SSH agent for secure key management.

Answer 12

Can you modify your mount options (umask, uid and gid) to suit?