Securing PHP-FPM with AppArmor

PHP-FPM is an ideal candidate to secure with AppArmor. Not only can the security of a web server be endangered by security bugs in PHP itself, it can also be affected by security holes in PHP applications. By confining PHP-FPM with AppArmor, we can limit abuse when a security hole is exploited, by preventing PHP-FPM for example from reading arbitrary files on your system or executing random binaries, which may contain a Linux backdoor or crypto-miner malware.

Preparation

First it is important that you run different PHP web applications as different users by running them in a different pool. The different pools can then be confined with their own separate AppArmor subprofile or hat, so that we can protect the different PHP applications from each other.

Then on Debian 12 Bookworm, I recommend that you upgrade to the AppArmor packages from my own repository, because they contain some important bug fixes, notably aa-logprof and aa-genprof supporting exec events in hats. Set up the bookworm-frehi repository in apt and upgrade AppArmor to the version available in this repository:

# apt install -t bookworm-frehi apparmor

Now we are ready to create our AppArmor profile for PHP-FPM. Debian already ships a basic profile /etc/apparmor.d/php-fpm as part of the package apparmor-profiles. However aa-logprof and aa-genprof will want to write everything in one single file, while the /etc/apparmor.d/php-fpm heavily relies on including other files and also the comments in that file will be lost. For that reason I choose to create my own profile from scratch with aa-genprof. So first I disable the php-fpm profile and remove it:

# aa-disable php-fpm
# rm /etc/apparmor.d/disable/php-fpm
# rmdir /etc/apparmor.d/php-fpm.d/

Generating a profile for /usr/sbin/php-fpm8.2

I start aa-genprof:

# aa-genprof /usr/sbin/php-fpm8.2

In another shell, I stop PHP-FPM:

# systemctl stop php8.2-fpm

and now I modify all pools in /etc/php/8.2/fpm/pool.d/*.conf and I add the apparmor_hat value to the name of the pool. For example:

# apparmor_hat = myblog

This instructs PHP-FPM to switch to this subprofile or hat for this pool. If every pool uses a different hat, we can isolate the different pools running different web applications from each other.

Now verify with aa-status whether /usr/sbin/php.php-fpm8.2 is in complain mode and if not, put it in complain mode manually with the aa-complain command:

# aa-complain /etc/apparmor.d/usr.sbin.php-fpm8.2

Now we start PHP-FPM:

# systemctl start php8.2-fpm

It’s a good idea to immediately press S to scan for the first events. Process them like I showed in my previous AppArmor tutorial. As usual, in case of doubt choose the more fine-grained option instead of broad abstractions.

I will discuss some specific things you will encounter.

You will see that PHP-FPM requests read access to /etc/passwd and /etc/group. It needs this in order to look up the UID and GID of the user and group value you have set in your FPM pools. As we are running our different pools in their own subprofiles, which don’t have access to these files, this is not a security problem: your PHP applications won’t have access to these files if you have properly set apparmor_hat in every pool.

At some point our PHP-FPM process switches to the profile /usr/sbin/php-fpm8.2//null-/usr/sbin/php-fpm8.2//myblog. This is because we have set apparmor_hat = myblog in the myblog pool. The null part in the profile name indicates that this profile does not exist yet and so AppArmor uses this as a temporary name. We will have to fix this manually later. Now accept this rule.

Profile:        /usr/sbin/php-fpm8.2
Exec Condition: ALL
Target Profile: /usr/sbin/php-fpm8.2//null-/usr/sbin/php-fpm8.2//myblog

 [1 - change_profile -> /usr/sbin/php-fpm8.2//null-/usr/sbin/php-fpm8.2//myblog,]
(A)llow / [(D)eny] / (I)gnore / Audi(t) / Abo(r)t / (F)inish

Continue processing all events, and when you are done save the profile and quit aa-genprof.

Check with aa-status in which mode the /usr/sbin/php-fpm8.2 profile is and switch it back to complain mode if necessary. Otherwise if it’s in enforce mode, your PHP applications will probably not work correctly any more.

We will now edit the file /etc/apparmor.d/usr.sbin.php-fpm8.2 in a text editor. Find the change_profile rules in that file, and replace them by this line:

change_profile -> /usr/sbin/php-fpm8.2//*,

This allows PHP-FPM to switch to all PHP-FPM hats.

In the file, you will also find some signal rules which refer to the temporary null profile. We need to fix these too like this:

  signal send set=kill peer=/usr/sbin/php-fpm8.2//*,<br />  signal send set=quit peer=/usr/sbin/php-fpm8.2//*,<br />  signal send set=term peer=/usr/sbin/php-fpm8.2//*,

Then within the profile /usr/sbin/php-fpm8.2 flags=(complain) {

create an empty myblog hat in complain mode:

  ^myblog flags=(complain) {
  }

Now reload the AppArmor profile:

# apparmor_parser -r /etc/apparmor.d/usr.sbin.php-fpm8.2

Restart aa-genprof:

# aa-genprof /usr/sbin/php-fpm8.2

and restart PHP-FPM in another shell:

# systemctl restart php8.2-fpm

Now exercise your web applications and process all generated events. Particularly test posting new items on your website and uploading pictures and other files. You will notice that the events triggered by the www pool, are now in the hat myblog, which is shown by aa-genprof as /usr/sbin/php-fpm8.2^myblog.

In WordPress I am using the WebP Express plugin, which automatically creates a webp file for every uploaded image by calling the command cwebp.

aa-genprof will show this event:

Profile:  /usr/sbin/php-fpm8.2^myblog
Execute:  /usr/bin/dash
Severity: unknown

(I)nherit / (N)amed / (U)nconfined / (X) ix On / (D)eny / Abo(r)t / (F)inish

So the myblog pool tries to launch the dash shell as part of this process. By pressing I you can let it inherit the www profile, so it will have exactly the same permissions and cannot read or modify any files from other web applications running in a different PHP-FPM pool, or other files on the system. I also had to do this for the cwebp executable and a few others which are used by WebP Express. Important is that you never choose unconfined, because then you allow these executables to do anything on your system without any AppArmor restrictions.

When WordPress sends e-mail, it will call sendmail:

[(S)can system log for AppArmor events] / (F)inish
Reading log entries from /var/log/audit/audit.log.
Target profile exists: /etc/apparmor.d/usr.sbin.sendmail

Profile:  /usr/sbin/php-fpm8.2^myblog
Execute:  /usr/sbin/sendmail
Severity: unknown

(I)nherit / (P)rofile / (N)amed / (U)nconfined / (X) ix On / (D)eny / Abo(r)t / (F)inish

Sendmail should run in its own profile, so you can choose P here.

Creating a child profile for dash

In our example our PHP-FPM worker calls dash and cwebp to process images. We have chosen to let dash and cwebp inherit all permissions of the worker. While this already is a great security improvement because it will prevent dash from executing random binaries and reading files unrelated to this web application it is still too broad. There is no reason why dash and cwebp should be able to read our PHP files for example and any other files than webp files. So we can improve our security even more by running these commands in a separate child profile.

To do so, I edit /etc/apparmor.d/usr.sbin.php-fpm8.2 in a text editor, and I remove all these lines from the www hat:

    /usr/bin/cwebp mrix,
    /usr/bin/dash mrix,
    /usr/bin/nice mrix,
    /usr/bin/cwebp mrix,
    /usr/bin/which.debianutils mrix,

and I replace it by this rule which will force dash to run in a child profile named myblogdash:

/usr/bin/dash Px -> /usr/sbin/php-fpm8.2//myblogdash,

Then in the usr/sbin/php-fpm8.2 profile, but outside the myblog hat, I create this empty child profile:

  profile myblogdash flags=(complain) {
  }

reload the profile, restart PHP-FPM and start aa-genprof again:

# /etc/apparmor.d/usr.sbin.php-fpm8.2
# systemctl restart php8.2-fpm
# aa-genprof /usr/sbin/php8.2-fpm

Now I upload again an image in order to trigger the execution of dash and and I process the generated events with aa-genprof. Finally I end up with this child profile:

  profile myblogdash {
    include <abstractions/base>
    include <abstractions/bash>

    deny /var/www/example.com/wordpress/wp-admin/admin-ajax.php r,
    deny /var/www/example.com/wordpress/wp-admin/async-upload.php r,
    deny /var/www/example.com/wordpress/wp-admin/options-general.php r,
    deny /var/www/blog.frehi.be/wordpress/xmlrpc.php r,

    /usr/bin/cwebp mrix,
    /usr/bin/dash mr,
    /usr/bin/nice mrix,
    /usr/bin/which.debianutils mrix,
    owner /var/www/example.com/wordpress/**.{jpg,jpeg,png} r,
    owner /var/www/example.com/wordpress/wp-content/uploads/webp-express-test-conversion.webp.lossless.webp w,
    owner /var/www/example.com/wordpress/wp-content/uploads/webp-express-test-conversion.webp.lossy.webp w,
    owner /var/www/example.com/wordpress/wp-content/webp-express/webp-images/doc-root/**.webp w,

  }

As you can see our dash process and all other processes which inherit this profile, including cwepb, now have much more limited permissions and can only read JPEG and PNG files from our website and write WEBP images. I had some events of the sh process reading files like wp-admin/options-general.php, but I have no idea why this process would do that. I chose to deny this, and even when this child profile is in enforce mode, everything appeared to be working OK. Anyway, these files don’t contain anything sensitive, so it would not be a real problem if you accept this.

Write permissions on PHP files

A topic which is worth thinking about: which permissions are you giving your PHP worker process on the PHP files? Obviously PHP-FPM will need read access to these files in order to execute them, but do you also want to give them write access? There is a strong argument not too: it will prevent hackers who manage to exploit your PHP application to write a new PHP file with their own code (such as a PHP backdoor) and then execute it. Or modify your existing PHP files and insert malicious code in them. So confining your PHP process to prevent it from writing PHP files is a huge advantage for security.

However applications like WordPress will try to update themselves and in order to do so, it needs write access to its own files. Also installing new plug-ins via the web interface requires the permissions to write PHP files. Automatic updates is also a big advantage for security. As an alternative to WordPress updating itself automatically, you could easily write a shell script which uses wp-cli to update WordPress and all modules and themes and regularly call this from a cron job or systemd timer.

On my system, I noticed that the wp-supercache module was writing some PHP files. If I understand this correctly, it does this in order to deal with pages which contain dynamic content. So in order not to break this, I need to allow writing PHP files in wp-content/cache/supercache anyway.

You will have to decide for each web application whether you want it to allow writing PHP files. At least run your different PHP applications in different PHP-FPM workers with different AppArmor hats, so that it’s impossible for one PHP applications to affect other PHP applications, but also strongly consider not giving write access to its own PHP files because that would be a huge win security wise.

If you decide to give a web application write access to its own PHP files, I recommend even more setting up Modsecurity with the CoreRuleSet, blocking outgoing network connections by default for that user account with your firewall (such as Foomuuri) and installing Snuffleupagus to further limit the things PHP applications can do on a PHP level.

The usr.sbin.php-fpm8.2 profile

Finally I ended up with the following profile.

abi <abi/3.0>,

include <tunables/global>

/usr/sbin/php-fpm8.2 {
  include <abstractions/base>

  capability chown,
  capability dac_override,
  capability kill,
  capability net_admin,
  capability setgid,
  capability setuid,

  signal send set=kill peer=/usr/sbin/php-fpm8.2//*,
  signal send set=quit peer=/usr/sbin/php-fpm8.2//*,
  signal send set=term peer=/usr/sbin/php-fpm8.2//*,

  /run/php/*.sock w,
  /usr/sbin/php-fpm8.2 mr,
  @{PROC}/@{pid}/attr/{apparmor/,}current rw,
  owner /etc/group r,
  owner /etc/host.conf r,
  owner /etc/hosts r,
  owner /etc/nsswitch.conf r,
  owner /etc/passwd r,
  owner /etc/php/8.2/fpm/conf.d/ r,
  owner /etc/php/8.2/fpm/php-fpm.conf r,
  owner /etc/php/8.2/fpm/php.ini r,
  owner /etc/php/8.2/fpm/pool.d/ r,
  owner /etc/php/8.2/fpm/pool.d/*.conf r,
  owner /etc/php/8.2/mods-available/*.ini r,
  owner /etc/resolv.conf r,
  owner /etc/ssl/openssl.cnf r,
  owner /proc/sys/kernel/random/boot_id r,
  owner /run/php/php8.2-fpm.pid w,
  owner /run/systemd/userdb/ r,
  owner /sys/devices/system/node/ r,
  owner /sys/devices/system/node/node0/meminfo r,
  owner /tmp/.ZendSem.* rwk,
  owner /var/log/php8.2-fpm.log w,

  change_profile -> /usr/sbin/php-fpm8.2//*,


  ^myblog {
    include <abstractions/base>
    include <abstractions/ssl_certs>

    network inet dgram,
    network inet stream,
    network inet6 dgram,
    network inet6 stream,
    network netlink raw,
    network unix stream,

    signal receive set=kill peer=/usr/sbin/php-fpm8.2,
    signal receive set=quit peer=/usr/sbin/php-fpm8.2,
    signal receive set=term peer=/usr/sbin/php-fpm8.2,

    /etc/gai.conf r,
    /etc/hosts r,
    /tmp/.ZendSem.* k,
    /usr/bin/dash Px -> /usr/sbin/php-fpm8.2//blogdash,
    /usr/sbin/postdrop Px,
    /usr/sbin/sendmail Px,
    /var/www/example.com/wordpress/**.php r,
    /var/www/example.com/wordpress/**.{css,js,svg,po} r,
    /var/www/example.com/wordpress/**/ r,
    /var/www/example.com/wordpress/.htaccess r,
    /var/www/example.com/wordpress/wp-content/uploads/wpcf7_uploads/.htaccess r,
    /var/www/example.com/wordpress/wp-includes/*.json r,
    /var/www/example.com/wordpress/wp-includes/certificates/ca-bundle.crt r,
    owner /var/www/example.com/tmp/* rw,
    owner /var/www/example.com/wordpress/.maintenance rw,
    owner /var/www/example.com/wordpress/wp-content/autoptimize_404_handler.php rw,
    owner /var/www/example.com/wordpress/wp-content/cache/**/ rw,
    owner /var/www/example.com/wordpress/wp-content/cache/**/index.html w,
    owner /var/www/example.com/wordpress/wp-content/cache/*.tmp rw,
    owner /var/www/example.com/wordpress/wp-content/cache/autoptimize/.htaccess w,
    owner /var/www/example.com/wordpress/wp-content/cache/autoptimize/css/*.css rw,
    owner /var/www/example.com/wordpress/wp-content/cache/autoptimize/js/*.js rw,
    owner /var/www/example.com/wordpress/wp-content/cache/example.com_wp_cache_gc.txt w,
    owner /var/www/example.com/wordpress/wp-content/cache/lyteCache/*.jpg rw,
    owner /var/www/example.com/wordpress/wp-content/cache/preload_permalink.txt rw,
    owner /var/www/example.com/wordpress/wp-content/cache/supercache/**.html w,
    owner /var/www/example.com/wordpress/wp-content/cache/supercache/**.html.gz rw,
    owner /var/www/example.com/wordpress/wp-content/cache/supercache/**.php rw,
    owner /var/www/example.com/wordpress/wp-content/cache/supercache/**.tmp rw,
    owner /var/www/example.com/wordpress/wp-content/cache/supercache/**.tmp.gz rw,
    owner /var/www/example.com/wordpress/wp-content/cache/supercache/*/feed/*.php rw,
    owner /var/www/example.com/wordpress/wp-content/cache/taxonomy_category.txt rw,
    owner /var/www/example.com/wordpress/wp-content/cache/taxonomy_post_tag.txt rw,
    owner /var/www/example.com/wordpress/wp-content/languages/**.{po,mo} rw,
    owner /var/www/example.com/wordpress/wp-content/languages/plugins/*.json w,
    owner /var/www/example.com/wordpress/wp-content/plugins/**.json r,
    owner /var/www/example.com/wordpress/wp-content/plugins/*/ rw,
    owner /var/www/example.com/wordpress/wp-content/plugins/webp-express/lib/options/options/**.inc r,
    owner /var/www/example.com/wordpress/wp-content/plugins/webp-express/lib/options/options/conversion-options/*.inc r,
    owner /var/www/example.com/wordpress/wp-content/plugins/webp-express/test/*.jpg r,
    owner /var/www/example.com/wordpress/wp-content/plugins/webp-express/test/*.png r,
    owner /var/www/example.com/wordpress/wp-content/themes/.htaccess rwk,
    owner /var/www/example.com/wordpress/wp-content/themes/webp-express-test-images/*.JPEG rw,
    owner /var/www/example.com/wordpress/wp-content/themes/webp-express-test-images/*.PNG rw,
    owner /var/www/example.com/wordpress/wp-content/upgrade-temp-backup/**/ rw,
    owner /var/www/example.com/wordpress/wp-content/upgrade-temp-backup/plugins/** rw,
    owner /var/www/example.com/wordpress/wp-content/upgrade/** rw,
    owner /var/www/example.com/wordpress/wp-content/uploads/**.{jpg,jpeg,png,webp} rw,
    owner /var/www/example.com/wordpress/wp-content/uploads/**/ rw,
    owner /var/www/example.com/wordpress/wp-content/uploads/.htaccess rwk,
    owner /var/www/example.com/wordpress/wp-content/uploads/webp-express-test-conversion.webp w,
    owner /var/www/example.com/wordpress/wp-content/uploads/webp-express-test-conversion.webp.lossless.webp w,
    owner /var/www/example.com/wordpress/wp-content/uploads/webp-express-test-conversion.webp.lossy.webp rw,
    owner /var/www/example.com/wordpress/wp-content/uploads/webp-express-test-images/*.JPEG rw,
    owner /var/www/example.com/wordpress/wp-content/uploads/webp-express-test-images/*.PNG rw,
    owner /var/www/example.com/wordpress/wp-content/uploads/wp-statistics/GeoLite2-Country.mmdb r,
    owner /var/www/example.com/wordpress/wp-content/webp-express/config/*.json rw,
    owner /var/www/example.com/wordpress/wp-content/webp-express/webp-images/.htaccess rwk,
    owner /var/www/example.com/wordpress/wp-content/webp-express/webp-images/doc-root/**.webp rw,

  }

  profile myblogdash {
    include <abstractions/base>
    include <abstractions/bash>

    deny /var/www/example.com/wordpress/wp-admin/admin-ajax.php r,
    deny /var/www/example.com/wordpress/wp-admin/async-upload.php r,
    deny /var/www/example.com/wordpress/wp-admin/options-general.php r,
    deny /var/www/example.com/wordpress/xmlrpc.php r,

    /usr/bin/cwebp mrix,
    /usr/bin/dash mr,
    /usr/bin/nice mrix,
    /usr/bin/which.debianutils mrix,
    owner /var/www/example.com/wordpress/**.{jpg,jpeg,png} r,
    owner /var/www/example.com/wordpress/wp-content/uploads/webp-express-test-conversion.webp.lossless.webp w,
    owner /var/www/example.com/wordpress/wp-content/uploads/webp-express-test-conversion.webp.lossy.webp w,
    owner /var/www/example.com/wordpress/wp-content/webp-express/webp-images/doc-root/**.webp w,

  }
}

Some advice

Creating an AppAmor profile for more complicated applications like this can be challenge. You will need to retest different things a lot and be prepared to do some hand editing of the profile. Some useful advice that I learnt along the way:

• Often check /var/log/audit.log yourself to see what is happening. At a certain moment I ended up in a loop where my audit log was constantly being filled because the process tried to switch to a not yet existing profile, so you want to intervene immediately if this happens to prevent the logs from filling up your disk.
• After making modifications to an AppArmor profile, it’s not a bad idea to restart your process. I am not sure, but I think some problems I encountered were fixed by restarting the process.
• Stop aa-genprof/aa-logprof when you manually edit a profile and start hem again after the editing and reloading it with apparmor_parser, otherwise these processes won’t be aware of the modification you have made to the profile file.
• When adding hats or child profiles, revise the permissions of your main profile: maybe some things can be moved to the new hat or child profile and can thus be removed completely from the main profile.
• Keep your profile long enough in complain mode and only switch to enforce mode after enough time without events (I would say: a couple of days). If you encounter problems with your PHP applications later on, don’t forget to check your audit.log.