UNIX and Linux setUID advice and guidance

Published 16/06/2017 | By AFC

It is a topic that often comes up on client engagements, usually when running structured build reviews of Linux “gold builds”, but occasionally when trying to explain in detail how we used a Linux system to pivot internally.

SetUID and setGID files are inevitably a risk, potentially allowing attackers to elevate privileges to root from a basic user. When shared out on SMB or NFS shares they can spread the risk even further.

One good approach would be:

“The only good setUID is a chmod -s setuid” i.e. deactivate this particular functionality.

However, sysadmins may (understandably) get a bit wobbly when we suggest this and come up with a number of fairly valid reasons why this is tricky. They have a point to some extent, but this is 2017 and we should be biting some of these bullets.

Yes, reducing the risk in this area is tough, but it is in fact a good start in defining least privilege security controls. You can learn how little or how much power individual users require and manage this accordingly. You can also test your changes before rolling them out and sanity check the advice given.

At the very least we need a list of files that should not be setUID that we can give to sysadmins and, with confidence, recommend removing or changing the permissions. A pragmatic strategy for mitigating the risks incurred would also be good.

So what are they?

SetUID/setGID bits are file permissions set on binary files when we need them to run with the permissions of the owner (setUID) or the group (setGID) that owns the file, usually a root or equivalent user. Historically this functionality was entrenched in UNIX and Linux and was necessary, up to a point, for a system to function as intended.

Many vendors are now working toward eliminating the requirement for these permissions and UNIX based systems can be configured, with some care, not to use them.

We potentially have 3 categories of setUID and setGID files:

The “naughty” list

May have become setUID by accident, installation of third-party software, malicious activity or sloppy coding and should almost certainly not be allowed
tar, find, vi, etc
Likely high risk to get root easily
Epic fail -rwsrw-rw- More common than you would think
Vendor supplied software often a culprit

The “remove” list

Redundant, legacy or not really required to be setUID
rcp, arping, etc
Likely low risk but should be easy to remove

The “allowed” list

OS may fail to function as expected if setUID is removed without care
For example passwd, sudo
Should be mitigated to minimise risks
Many can be reclassified as 2 above and have the setUID bit removed

We report on categories 1 (naughty) and 2 (remove) scoring appropriate to the risk and ease of exploitation. We report on 3 (allowed) with recommendations to mitigate.

For example, a setUID on the find command (or any shell command with abilities to run other programs or shells) would be a high risk vulnerability allowing regular users to become root fairly easily.

How to deal with these issues?

The approach to recommend to customers:

Audit – find all setUID and setGID files on the system locally and on all filesystems not mounted nosetuid
Validate – elfsign and cross checks against vendor checksums
Eliminate where possible – uninstall, chmod -s or rm
Mitigate where necessary – FIM, auditing, remote logging, mount NAS nosetuid

This applies primarily to common Linux distributions. Solaris and AIX for example may have slightly different filenames and locations, but similar principles apply.

The ubiquitous sudo or other equivalent tools can be used to configure fine grained privilege management without the requirement for setUID permissions. Your version of sudo must be up to date, the sudoers configuration files must be secure and have a strictly limited set of commands allowed per user or group.

Customers need to consider how the remediation and mitigation steps fit within their overall strategies for user and privilege management alongside local and remote file permissions.

For example:

Do they have a good FIM solution and can they utilise remote logging as a mitigation? Do they wish to work at the standard build level and have a hardened standard build? If so, then maybe they’re in the right place to look at removing these permissions?

In particular File Integrity Monitoring, which should focus on the most vulnerable files on a system, should track any changes to the files with the setUID/setGID bit set. Any changes should be audited and remotely logged to ensure that an attacker cannot cover their tracks.

Other mitigation techniques include SELinux and other RBAC mechanisms on Linux, Trusted Execution and Trusted Computing Base on AIX and RBAC on Solaris.

More recent Linux kernels (starting 2.6.26) can make use of capabilities which provide fine-grained control over superuser permissions, allowing use of the root user to be avoided.

Sysadmins need some encouragement and support in both remediation and mitigation. If something unexpected does not work upon removing a setUID file permission, it is important to try to use that as a learning experience and a consequence of taking least privilege as far as possible.

At the very least tighten up your sudo configuration and remove as many setUIDs as you can.

Finding setUID and setGID files

# find / -type f -a \( -perm -u+s -o -perm -g+s \) -exec ls -l {} \;

Example output:

Here is a setUID file:

-rws--x--x. 1 root root 23960 Sep 23  2016 /usr/bin/chfn

And here is a setGID file:

-r-xr-sr-x. 1 root tty 15392 May  4  2014 /usr/bin/wall

Just for fun

Snapshot a VM of your favourite Linux distro and run the following command (this will break your system in odd ways, so never do it to a production system) and run the following commands:

# find / -type f -perm -u+s -exec ls -l {} \; |awk '{print $NF}'|grep -v proc &amp;gt; /var/tmp/setUID.lst
# find / -type f -perm -g+s -exec ls -l {} \; |awk '{print $NF}'|grep -v proc &amp;gt; /var/tmp/setGID.lst
# for a in `cat /var/tmp/setUID.lst`; do chmod u-s $a; done
# for a in `cat /var/tmp/setGID.lst`; do chmod g-s $a; done

See what breaks.

You can revert to the snapshot if your nerve does not hold.

You should be able to reboot the machine and log in as a normal user. After that you will discover the limitations you have imposed upon yourself and the operating system.

The lists (mid 2017) which are not exhaustive

N.B. PATHs to the files may vary…

The “naughty” list

/usr/bin/awk
/bin/tar
/usr/bin/python
/usr/bin/script
/usr/bin/man
/usr/bin/ssh
/usr/bin/scp
/usr/bin/git
/usr/bin/find
/usr/bin/gdb
/usr/bin/pico
/usr/bin/nano
/usr/bin/zip
/usr/bin/vi
/usr/sbin/lsof
/bin/cat
/usr/bin/vim
/usr/bin/gvim

The point with this list is that they should not really be setUID or setGID and they all have command options that could spawn a shell or otherwise engage in mischief.

The “remove” list

/usr/bin/rcp
/usr/bin/rlogin
/usr/bin/rsh
/usr/libexec/openssh/ssh-keysign
/usr/lib/openssh/ssh-keysign
/sbin/netreport
/usr/sbin/usernetctl
/usr/sbin/userisdnctl
/usr/sbin/pppd
/usr/bin/lockfile
/usr/bin/mail-lock
/usr/bin/mail-unlock
/usr/bin/mail-touchlock
/usr/bin/dotlockfile
/usr/bin/arping
/usr/sbin/uuidd
/usr/bin/mtr
/usr/lib/evolution/camel-lock-helper-1.2
/usr/lib/pt_chown
/usr/lib/eject/dmcrypt-get-device
/usr/lib/mc/cons.saver

Consider removing as a good mitigation strategy.

Here’s some others that are usually installed setUID, however, on a hardened system they can usually have their setUID/setGID bits removed:

/bin/mount
/bin/umount
/usr/bin/at
/usr/bin/newgrp
/usr/bin/ssh-agent
/sbin/mount.nfs
/sbin/umount.nfs
/sbin/mount.nfs4
/sbin/umount.nfs4
/usr/bin/crontab
/usr/bin/wall
/usr/bin/write
/usr/bin/screen
/usr/bin/mlocate
/usr/bin/chage
/usr/bin/chfn
/usr/bin/chsh
/bin/fusermount
/usr/bin/Xorg
/usr/bin/X
/usr/lib/dbus-1.0/dbus-daemon-launch-helper
/usr/lib/vte/gnome-pty-helper
/usr/lib/libvte9/gnome-pty-helper
/usr/lib/libvte-2.90-9/gnome-pty-helper

The “allowed” list

/bin/ping
/bin/su
/sbin/pam_timestamp_check
/sbin/unix_chkpwd
/usr/bin/gpasswd
/usr/bin/locate
/usr/bin/passwd
/usr/libexec/utempter/utempter
/usr/sbin/lockdev
/usr/sbin/sendmail.sendmail
/usr/bin/expiry
/bin/ping6
/usr/bin/traceroute6.iputils
/usr/bin/pkexec
/usr/bin/sudo
/usr/bin/sudoedit
/usr/sbin/postdrop
/usr/sbin/postqueue
/usr/sbin/suexec
/usr/lib/squid/ncsa_auth
/usr/lib/squid/pam_auth
/usr/kerberos/bin/ksu
/usr/sbin/ccreds_validate

Final words

It’s worth noting that we know these lists won’t work for everyone; systems are complex and behaviour is intertwined. That said, no matter what your opinion on the necessity or otherwise of a setUID/setGID bit on a given file, we’d like to think the approach of knowing which files have the setUID/setGID bit and why is something we can all get on board with.

UNIX and Linux setUID advice and guidance

So what are they?

The “naughty” list

The “remove” list

The “allowed” list

How to deal with these issues?

Finding setUID and setGID files

Just for fun

The lists (mid 2017) which are not exhaustive

The “naughty” list

The “remove” list

The “allowed” list

Final words

Some good references

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