Trying to find the right place to bring this up to, as I'm not sure whether shadow-utils is shipping this out like this or if it's the distributions that have distributed it like this.

I'm seeing on Red Hat 9 and Debian 12 that login.defs is shipping with SUB_UID_* configuration resulting in /etc/subuid and /etc/subgid entries for local accounts being added to the system.

This can have unintended consequences, as it assumes the systems are running in an environment where there would be no ID's on the network that fall within the ranges handed out by this configuration.

The issue is that the subordinate ID's can easily become the effective UID/GID of the account they're assigned to. For example, my local account had an entirely unexpected subordinate ID range assigned to it which happens to overlap with a user ID on my network. With minimal effort I'm able to become that ID and write to the users NFS home directory.

jon@localhost ~]$ id;uname -a;cat /etc/subuid
uid=1000(jon) gid=1000(jon) groups=1000(jon),10(wheel)
Linux localhost.localdomain 5.14.0-503.14.1.el9_5.aarch64 #1 SMP PREEMPT_DYNAMIC Fri Nov 15 07:14:03 EST 2024 aarch64 aarch64 aarch64 GNU/Linux
jon:100000:65536
[jon@localhost ~]$ getent passwd msteph
msteph:x:108472:108472::/mnt/home/msteph:/bin/bash
[jon@localhost ~]$ ls -al subdo;getcap subdo
-rwxr-xr-x 1 jon jon 72936 Dec 22 23:12 subdo
[jon@localhost ~]$ ./subdo -u 108472 -g 108472 -r 1 bash
bash: /home/jon/.bashrc: Permission denied
bash-5.1$ id
uid=108472(msteph) gid=108472(msteph) groups=108472(msteph),65534(nobody)
bash-5.1$ cd ~msteph
bash-5.1$ ls
bash-5.1$ ls -al
total 16
drwx------ 3 msteph nobody 99 Dec 22 23:19 .
drwxr-xr-x. 8 nobody nobody 85 Dec 22 23:18 ..
-rw------- 1 msteph msteph 20 Dec 22 23:19 .bash_history
-rw-r--r-- 1 msteph nobody 18 Apr 30 2024 .bash_logout
-rw-r--r-- 1 msteph nobody 141 Apr 30 2024 .bash_profile
-rw-r--r-- 1 msteph nobody 492 Apr 30 2024 .bashrc
drwxr-xr-x 4 msteph nobody 39 Dec 20 09:10 .mozilla
bash-5.1$ touch testing123
bash-5.1$ ls -aal
total 16
drwx------ 3 msteph nobody 117 Dec 22 23:23 .
drwxr-xr-x. 8 nobody nobody 85 Dec 22 23:18 ..
-rw------- 1 msteph msteph 20 Dec 22 23:19 .bash_history
-rw-r--r-- 1 msteph nobody 18 Apr 30 2024 .bash_logout
-rw-r--r-- 1 msteph nobody 141 Apr 30 2024 .bash_profile
-rw-r--r-- 1 msteph nobody 492 Apr 30 2024 .bashrc
drwxr-xr-x 4 msteph nobody 39 Dec 20 09:10 .mozilla
-rw-r--r-- 1 msteph msteph 0 Dec 22 23:23 testing123
bash-5.1$