Cis Apache 12.2

Ensure the Apache AppArmor Profile Is Configured Properly

Description

AppArmor includes customizable profiles that may be used to confine the Apache web server to enforce least privileges so the server has only the minimal access to specified directories, files, and network ports. Access is controlled by a profile defined for the apache2 process. The default AppArmor profile is typically a very permissive profile that allows read-write access to all system files. Therefore, it's important that the default profile be customized to enforce least privilege. AppArmor utilities such as aa-autodep, aa-complain, and aa-logprof can be used to generate an initial profile based on actual usage. However, thorough testing, review, and customization will be necessary to ensure the Apache profile restrictions allow the necessary functionality while implementing least privilege.

Rationale

With the proper implementation of an AppArmor profile, vulnerabilities in the web application may be prevented from being exploited due to the additional restrictions. For example, a vulnerability that allows an attacker to read an inappropriate system files may be prevented from execution by AppArmor because the inappropriate files are not allowed by the profile. Likewise, writing to an unexpected directory or executing unexpected content can be prevented by similar mandatory security controls enforced by AppArmor.

Impact

None documented

Audit Procedure

Perform the following steps to determine if the recommended state is implemented:

1. Find the Apache AppArmor profile typically found in /etc/apparmor.d/usr.sbin.apache2 with any files included by the profile such as /etc/apparmor.d/apache2.d/* and files in the /etc/apparmor.d/abstractions/ directory.

2. Review the capabilities and permissions granted to ensure that the profile implements least privileges for the web application. Wild-card paths such as /** which grant access to all files and directories starting with the root level directory should not be present in the profile. Instead, read-only access to specific necessary system files such as /etc/group and to web content files such as /var/www/html/** should be given. Refer to the apparmor.d man page for additional details. Shown below are some possible example capabilities and path permissions.

capability dac_override,
capability dac_read_search,
capability net_bind_service,
capability setgid,
capability setuid,
capability kill,
capability sys_tty_config,
. . .

/usr/sbin/apache2 mr,
/etc/gai.conf r,
/etc/group r,
/etc/apache2/** r,
/var/www/html/** r,
/run/apache2/** rw,
/run/lock/apache2/** rw,
/var/log/apache2/** rw,
/etc/mime.types r,

Remediation

Perform the following to implement the recommended state:

1. Stop the Apache server.

# service apache2 stop

2. Create a mostly empty apache2 profile based on program dependencies.

# aa-autodep apache2
Writing updated profile for /usr/sbin/apache2.

3. Set the apache2 profile in complain mode so access violations will be allowed and will be logged.

# aa-complain apache2
Setting /usr/sbin/apache2 to complain mode.

4. Start the apache2 service.

# service apache2 start

5. Thoroughly test the web application, attempting to exercise all intended functionality so AppArmor will generate the necessary logs of all resources accessed. The logs are sent via the system syslog utility and are typically found in either the /var/log/syslog or /var/log/messages files. Also stop and restart the web server as part of the testing process.

6. Use aa-logprof to update the profile based on logs generated during the testing. The tool will prompt for suggested modifications to the profile, based on the logs. The logs may also be reviewed manually in order to update the profile.

# aa-logprof

7. Review and edit the profile, removing any inappropriate content and adding appropriate access rules. Directories with multiple files accessed with the same permission can be simplified with the usage of wild-cards when appropriate. Reload the updated profile using the apparmor_parser command.

# apparmor_parser -r /etc/apparmor.d/usr.sbin.apache2

8. Test the new updated profile again and check for any new AppArmor denied logs generated. Update and reload the profile as necessary. Repeat the application to tests until no new AppArmor deny logs are created, except for access which should be prohibited.

# tail -f /var/log/syslog

9. Set the apache2 profile to enforce mode, reload AppArmor, and test the web site functionality again.

# aa-enforce /usr/sbin/apache2
# /etc/init.d/apparmor reload

Default Value

The default Apache profile is very permissive.

References

1. https://wiki.ubuntu.com/AppArmor

CIS Controls

Version 6

2 Inventory of Authorized and Unauthorized Software Inventory of Authorized and Unauthorized Software

Version 7

14.7 Enforce Access Control to Data through Automated Tools Use an automated tool, such as host-based Data Loss Prevention, to enforce access controls to data even when data is copied off a system.

Profile

Level 2 | Not Scored