Secure TMP Folder on Webserver against execution

Introduction

Often web servers (or hosting subscriptions) are writing in tmp folder. Therefore, it's good to keep the tmp folder clean and safe. A good way to secure your tmp folder on a linux machine is to disallow script execution in the tmp folder. I will show you how to check and secure your tmp folder.

Please be aware that sometimes server control panels need execution within the tmp folder.

Step 1 - Login to your web server

First, login to your server via SSH. Be sure you have root permissions.

Step 2 - Check if a tmp folder already exists!

With the following command you can see if there's already a tmp folder:

root@server:~# mount | egrep --color -w '^(tmpfs|/tmp)|/tmp'

Please copy the whole command except the # sign.

The output can be something like this if a dedicated partition is used for tmp: "/filesystems/tmp_fs on /tmp type ext3 (rw,nosuid,noexec,relatime,data=ordered)"
If not, the output probably looks like: "tmpfs on /tmp type tmpfs (rw,nosuid,nodev)"

If there is no such output, please continue.

Step 3 - Create a 4GB partition

Run the three commands one after the other.

root@server:~# mkdir /filesystems # create a new folder
root@server:~# dd if=/dev/zero of=/filesystems/tmp_fs seek=2048 count=2048 bs=1M # create a 2GB file called tmp_fs
root@server:~# mkfs.ext3 /filesystems/tmp_fs # create an ext3 filesystem on the newly created file

Step 4 - Make sure your partition will start with a reboot

We will add the following code into /etc/fstab.

Open fstab:

root@server:~# nano /etc/fstab

Insert the following at the end of the file:

/filesystems/tmp_fs /tmp ext3 noexec,nosuid,loop 0 0

Important: in the fstab file, always keep an empty row after the whole content, otherwise fstab won't work after a reboot.

Save the file and continue.

Step 5 - Backup the current tmp folder

root@server:~# cd /
root@server:~# cp -R /tmp /tmp_backup

Step 6 - Mount the new tmp folder

This will be done with the noexec, nosuid, and rw options.

root@server:~# mount -o loop,noexec,nosuid,rw /filesystems/tmp_fs /tmp

Set the correct permissions:

root@server:~# chmod 1777 /tmp

Step 7 - Copy old files to the new location

root@server:~# cp -R /tmp_backup/* /tmp/
root@server:~# rm -rf /tmp_backup

Step 8 - Check tmp folder and try execution (Optional)

Check if new partition exists:

root@server:~# df -h
root@server:~# mount

Check if you can execute any script:

root@server:~# cp /bin/ls /tmp
root@server:~# chmod 755 /tmp/ls

Run ls:

root@server:~# /tmp/ls

If everything worked, you should now get the following output "Permission denied".

You can clean up with following command:

root@server:~# rm /tmp/ls

Conclusion

Your web server is now secured against bad scripts and attacks on the tmp folder. If you have any questions or feedback, please feel free to contact me.

License: MIT