Selinux

Blueprints First Steps with Security-Enhanced Linux (SELinux): Hardening the Apache Web Server Blueprints First Steps with Security-Enhanced Linux (SELinux): Hardening the Apache Web Server Note Before using this information and the product it supports, read the information in “Notices” on page 17. First Edition (August 2009) © Copyright IBM Corporation 2009. US Government Users Restricted Rights – Use, duplication or disclosure restricted by GSA ADP Schedule Contract with IBM Corp. Contents Introduction . . . . . . . . . . . . . v First Steps with Security-Enhanced Linux (SELinux): Hardening the Apache Web Server . . . . . . . . . . . . 1 Scope, requirements, and support Security-Enhanced Linux overview Access control: MAC and DAC SELinux basics. . . . . . SELinux and Apache . . . . Installing and running HTTPD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 1 1 2 5 5 HTTPD and context types . . . . . . . . . 5 HTTPD and SELinux Booleans . . . . . . . 8 Configuring HTTPD security using SELinux . . . . 9 Securing Apache (static content only) . . . . . 9 Hardening CGI scripts with SELinux . . . . . 12 Appendix. Related information and downloads . . . . . . . . . . . . . 15 Notices . . . . . . . . . . . . . . 17 Trademarks . . . . . . . . . . . . . 18 © Copyright IBM Corp. 2009 iii iv Blueprints: First Steps with Security-Enhanced Linux (SELinux): Hardening the Apache Web Server Introduction This blueprint provides a brief introduction to basic Security-Enhanced Linux (SELinux) commands and concepts, including Boolean variables. In addition, the paper shows you how to increase the security of the Apache Web server with SELinux by using these concepts. Key tools and technologies discussed in this demonstration include security-enhanced Linux (SELinux), mandatory access control (MAC), getenforce, sestatus, getsebool, and setsebool. Intended audience
This blueprint is intended for Linux system or network administrators who want to learn more about securing their systems with SELinux. You should be familiar with installing and configuring Linux distributions, networks, and the Apache Web server. Scope and purpose This paper provides a basic overview of SELinux, SELinux Boolean variables, and hardening Apache on Red Hat Enterprise Linux (RHEL) 5. 3. For more information about configuring RHEL 5. 3, see the documentation supplied with your installation media or the distribution Web site. For more information about SELinux, see “Related information and downloads,” on page 15.
Software requirements This blueprint is written and tested using Red Hat Enterprise Linux (RHEL) 5. 3. Hardware requirements The information contained in this blueprint is tested on different models of IBM System x and System p hardware. For a list of hardware supported by RHEL 5. 3, see the documentation supplied with your Linux distribution. Author names Robert Sisk Other contributors Monza Lui Kersten Richter Robb Romans IBM Services Linux offers flexibility, options, and competitive total cost of ownership with a world class enterprise operating system.

Community innovation integrates leading-edge technologies and best practices into Linux. IBM® is a leader in the Linux community with over 600 developers in the IBM Linux Technology Center working on over 100 open source projects in the community. IBM supports Linux on all IBM servers, storage, and middleware, offering the broadest flexibility to match your business needs. © Copyright IBM Corp. 2009 v For more information about IBM and Linux, go to ibm. com/linux (https://www. ibm. com/linux) IBM Support Questions and comments regarding this documentation can be posted on the developerWorks Security Blueprint Community Forum: http://www. bm. com/developerworks/forums/forum. jspa? forumID=1271 The IBM developerWorks® discussion forums let you ask questions, share knowledge, ideas, and opinions about technologies and programming techniques with other developerWorks users. Use the forum content at your own risk. While IBM will attempt to provide a timely response to all postings, the use of this developerWorks forum does not guarantee a response to every question that is posted, nor do we validate the answers or the code that are offered. Typographic conventions
The following typographic conventions are used in this Blueprint: Bold Identifies commands, subroutines, keywords, files, structures, directories, and other items whose names are predefined by the system. Also identifies graphical objects such as buttons, labels, and icons that the user selects. Identifies parameters whose actual names or values are to be supplied by the user. Identifies examples of specific data values, examples of text like what you might see displayed, examples of portions of program code like what you might write as a programmer, messages from the system, or information you should actually type.
Italics Monospace Related reference: “Scope, requirements, and support” on page 1 This blueprint applies to System x® running Linux and PowerLinux. You can learn more about the systems to which this information applies. vi Blueprints: First Steps with Security-Enhanced Linux (SELinux): Hardening the Apache Web Server First Steps with Security-Enhanced Linux (SELinux): Hardening the Apache Web Server Scope, requirements, and support This blueprint applies to System x running Linux and PowerLinux. You can learn more about the systems to which this information applies.
Systems to which this information applies System x running Linux and PowerLinux Security-Enhanced Linux overview Security-Enhanced Linux (SELinux) is a component of the Linux operating system developed primarily by the United States National Security Agency. SELinux provides a method for creation and enforcement of mandatory access control (MAC) policies. These policies confine users and processes to the minimal amount of privilege required to perform assigned tasks. For more information about the history of SELinux, see http://en. wikipedia. org/wiki/Selinux.
Since its release to the open source community in December 2000, the SELinux project has gained improvements such as predefined Boolean variables that make it easier to use. This paper helps you understand how to use these variables to configure SELinux policies on your system and to secure the Apache httpd daemon. Related reference: “Scope, requirements, and support” This blueprint applies to System x running Linux and PowerLinux. You can learn more about the systems to which this information applies. Access control: MAC and DAC Access level is important to computer system security.
To compromise a system, attackers try to gain any possible level of access and then try to escalate that level until they are able to obtain restricted data or make unapproved system modifications. Because each user has some level of system access, every user account on your system increases the potential for abuse. System security has historically relied on trusting users not to abuse their access, but this trust has proven to be problematic. Today, server consolidation leads to more users per system. Outsourcing of Systems Management gives legitimate access, often at the system administrator level, to unknown users.
Because server consolidation and outsourcing can be financially advantageous, what can you do to prevent abuse on Linux systems? To begin to answer that question, let’s take a look at discretionary access control (DAC) and mandatory access control (MAC) and their differences. Discretionary access control (DAC), commonly known as file permissions, is the predominant access control mechanism in traditional UNIX and Linux systems. You may recognize the drwxr-xr-x or the ugo abbreviations for owner, group, and other permissions seen in a directory listing. In DAC, generally the resource owner (a user) controls who has access to a resource.
For convenience, some users commonly set dangerous DAC file permissions that allow every user on the system to read, write, and execute many files that they own. In addition, a process started by a user can modify or delete any file to which the user has access. Processes that elevate their privileges high enough could therefore modify or delete system files. These instances are some of the disadvantages of DAC. © Copyright IBM Corp. 2009 1 In contrast to DAC, mandatory access control (MAC) regulates user and process access to resources based upon an organizational (higher-level) security policy.
This policy is a collection of rules that specify what types of access are allowed on a system. System policy is related to MAC in the same way that firewall rules are related to firewalls. SELinux is a Linux kernel implementation of a flexible MAC mechanism called type enforcement. In type enforcement, a type identifier is assigned to every user and object. An object can be a file or a process. To access an object, a user must be authorized for that object type. These authorizations are defined in a SELinux policy. Let’s work through some examples and you will develop a better understanding of MAC and how it relates to SELinux.
Related reference: “Scope, requirements, and support” on page 1 This blueprint applies to System x running Linux and PowerLinux. You can learn more about the systems to which this information applies. SELinux basics It is a good practice not to use the root user unless necessary. However for demonstrating how to use SELinux, the root user is used in the examples in this blueprint. Some of the commands shown require root privileges to run them; for example, running getenforce and editing the /etc/selinux/config file. Related reference: “Scope, requirements, and support” on page 1 This blueprint applies to System x running Linux and PowerLinux.
You can learn more about the systems to which this information applies. Run modes You can enable or disable SELinux policy enforcement on a Red Hat Enterprise Linux system during or after operating system installation. When disabled, SELinux has no effect on the system. When enabled, SELinux runs in one of two modes: v Enforcing: SELinux is enabled and SELinux policy is enforced v Permissive: SELinux is enabled but it only logs warnings instead of enforcing the policy When prompted during operating system installation, if you choose to enable SELinux, it is installed with a default security policy and set to run in the enforcing mode.
Confirm the status of SELinux on your system. Like in many UNIX or Linux operating systems, there is more than one way to perform a task. To check the current mode, run one of the following commands: getenforce, sestatus, or cat /etc/selinux/config. v The getenorce command returns the current SELinux run mode, or Disabled if SELinux is not enabled. In the following example, getenforce shows that SELinux is enabled and enforcing the current SELinux policy: [[email protected] ~]$ getenforce Enforcing
If your system is displaying Permissive or Disabled and you want to follow along with the instructions, change the /etc/selinux/config file to run in Enforcing mode before continuing with the demonstration. Remember that if you are in Disabled mode, you should change first to Permissive and then to Enforcing. v The setstatus command returns the current run mode, along with information about the SELinux policy if SELinux is enabled. In the following example, setstatus shows that SELinux is enabled and enforcing the current SELinux policy: [[email protected] ~]$ sestatus SELinux status: SELinuxfs mount: enabled /selinux Blueprints: First Steps with Security-Enhanced Linux (SELinux): Hardening the Apache Web Server Current mode: Mode from config file: Policy version: Policy from config file: enforcing enforcing 21 targeted v The /etc/selinux/config file configures SELinux and controls the mode as well as the active policy. Changes to the /etc/selinux/config file become effective only after you reboot the system. In the following example, the file shows that the mode is set to enforcing and the current policy type is targeted. [[email protected] ~]$ cat /etc/selinux/config # This file controls the state of SELinux on the system. SELINUX= can take one of these three values: # enforcing – SELinux security policy is enforced. # permissive – SELinux prints warnings instead of enforcing. # disabled – SELinux is fully disabled. SELINUX=enforcing # SELINUXTYPE= type of policy in use. Possible values are: # targeted – Only targeted network daemons are protected. # strict – Full SELinux protection. SELINUXTYPE=targeted To enable SELinux, you need to set the value of the SELINUX parameter in the /etc/selinux/config file to either enforcing or permissive. If you enable SELinux in the config file, you must reboot your system to start SELinux.
We recommend that you set SELINUX=permissive if the file system has never been labeled, has not been labeled recently, or you are not sure when it was last labeled. Note that file system labeling is the process of assigning a label containing security-relevant information to each file. In SELinux a file label is composed of the user, role, and type such as system_u:object_r:httpd_sys_content_t. Permissive mode ensures that SELinux does not interfere with the boot sequence if a command in the sequence occurs before the file system relabel is completed. Once the system is up and running, you can change the SELinux mode to enforcing.
If you want to change the mode of SELinux on a running system, use the setenforce command. Entering setenforce enforcing changes the mode to enforcing and setenforce permissive changes the mode to permissive. To disable SELinux, edit the /etc/selinux/config file as described previously and reboot. You cannot disable or enable SELinux on a running system from the command line; you can only switch between enforcing and permissive when SELinux is enabled. Change the mode of SELinux to permissive by entering the following command: [[email protected] ~]$ setenforce permissive
Recheck the output from getenforce, sestatus, and cat /etc/selinux/config. v The getenforce command returns Permissive, confirming the mode change: [[email protected] ~]$ getenforce Permissive v The sestatus command also returns a Permissive mode value: [[email protected] ~]$sestatus SELinux status: SELinuxfs mount: Current mode: Mode from config file: Policy version: Policy from config file: enabled /selinux permissive enforcing 21 targeted v After changing the mode to permissive, both the getenforce and sestatus commands return the correct permissive mode.
However, look carefully at the output from the sestatus command: [[email protected] ~]$ cat /etc/selinux/config # This file controls the state of SELinux on the system. # SELINUX= can take one of these three values: # enforcing – SELinux security policy is enforced. # permissive – SELinux prints warnings instead of enforcing. First Steps with Security-Enhanced Linux (SELinux) 3 # disabled – SELinux is fully disabled. SELINUX=enforcing # SELINUXTYPE= type of policy in use. Possible values are: # targeted – Only targeted network daemons are protected. # strict – Full SELinux protection.
SELINUXTYPE=targeted [[email protected] ~]$ The Mode from config file parameter is enforcing. This setting is consistent with the cat /etc/selinux/config output because the config file was not changed. This status implies that the changes made by the setenforce command does not carry over to the next boot. If you reboot, SELinux returns to run state as configured in /etc/selinux/conf in enforcing mode. Change the running mode back to enforcing by entering the following command: [[email protected] ~]$ setenforce enforcing The following output confirms the mode change: [[email protected] ~]$ getenforce Enforcing
Related reference: “Scope, requirements, and support” on page 1 This blueprint applies to System x running Linux and PowerLinux. You can learn more about the systems to which this information applies. Security contexts The concept of type enforcement and the SELinux type identifier were discussed in the Overview. Let’s explore these concepts in more detail. The SELinux implementation of MAC employs a type enforcement mechanism that requires every subject and object to be assigned a type identifier. The terms subject and object are defined in the Bell-La Padula multilevel security model (see http://en. wikipedia. rg/wiki/Bell-La_Padula_model for more information). Think of the subject as a user or a process and the object as a file or a process. Typically, a subject accesses an object; for example, a user modifies a file. When SELinux runs in enforcing mode, a subject cannot access an object unless the type identifier assigned to the subject is authorized to access the object. The default policy is to deny all access not specifically allowed. Authorization is determined by rules defined in the SELinux policy. An example of a rule granting access may be as simple as: allow httpd_t httpd_sys_content_t : file {ioctol read getattr lock};
In this rule, the subject http daemon, assigned the type identifier of httpd_t, is given the permissions ioctol, read, getattr, and lock for any file object assigned the type identifier httpd_sys_content_t. In simple terms, the http daemon is allowed to read a file that is assigned the type identifier httpd_sys_content_t. This is a basic example of an allow rule type. There are many types of allow rules and some are very complex. There are also many type identifiers for use with subjects and objects. For more information about rule definitions, see: SELinux by Example in the “Related information and downloads,” on page 15 section.
SELinux adds type enforcement to standard Linux distributions. To access an object, the user must have both the appropriate file permissions (DAC) and the correct SELinux access. An SELinux security context contains three parts: the user, the role, and the type identifier. Running the ls command with the –Z switch displays the typical file information as well as the security context for each item in the subdirectory. In the following example, the security context for the index. html file is composed of user_u as the user, object_r as the role, and httpd_sys_content_t as the type identifier [[email protected] html]$ ls -Z index. tml -rw-r–r– web_admin web_admin user_u:object_r:httpd_sys_content_t index. html 4 Blueprints: First Steps with Security-Enhanced Linux (SELinux): Hardening the Apache Web Server Related reference: “Scope, requirements, and support” on page 1 This blueprint applies to System x running Linux and PowerLinux. You can learn more about the systems to which this information applies. SELinux and Apache Related reference: “Scope, requirements, and support” on page 1 This blueprint applies to System x running Linux and PowerLinux. You can learn more about the systems to which this information applies.
Installing and running HTTPD Now that you have a general understanding of the SELinux security context, you can secure an Apache Web server using SELinux. To follow along, you must have Apache installed on your system. You can install Apache on Red Hat Linux by entering the following command: [[email protected] html]$ yum install httpd Next, start the Apache http daemon by entering service httpd start, as follows: [[email protected] html]$ service httpd start Starting httpd: Related reference: “Scope, requirements, and support” on page 1 This blueprint applies to System x running Linux and PowerLinux.
You can learn more about the systems to which this information applies. HTTPD and context types Red Hat Enterprise Linux 5. 3, at the time of this writing, uses selinux-policy-2. 4. 6-203. el5. This policy defines the security context for the http daemon object as httpd_t. Because SELinux is running in enforcing mode, entering /bin/ps axZ | grep httpd produces the following output: [[email protected] html]$ ps axZ | grep http rootroot:system_r:httpd_t 2555 ? Ss 0:00 /usr/sbin/httpd rootroot:system_r:httpd_t 2593 ? S 0:00 /usr/sbin/httpd rootroot:system_r:httpd_t 2594 ? S 0:00 /usr/sbin/httpd root:system_r:httpd_t 2595 ?
S 0:00 /usr/sbin/httpd root:system_r:httpd_t 2596 ? S 0:00 /usr/sbin/httpd root:system_r:httpd_t 2597 ? S 0:00 /usr/sbin/httpd root:system_r:httpd_t 2598 ? S 0:00 /usr/sbin/httpd root:system_r:httpd_t 2599 ? S 0:00 /usr/sbin/httpd root:system_r:httpd_t 2600 ? S 0:00 /usr/sbin/httpd The Z option to ps shows the security context for the httpd processes as root:system_r:httpd_t, confirming that httpd is running as the security type httpd_t. The selinux-policy-2. 4. 6-203. el5 also defines several file security context types to be used with the http daemon. For a listing, see the man page for httpd_selinux.
The httpd_sys_content_t context type is used for files and subdirectories containing content to be accessible by the http daemon and all httpd scripts. Entering ls –Z displays the security context for items in the default http directory (/var/www/), as follows: [[email protected] ~]$ ls -Z /var/www/ | grep html drwxr-xr-x root root system_u:object_r:httpd_sys_content_t html First Steps with Security-Enhanced Linux (SELinux) 5 The /var/www/html directory is the default location for all Web server content (defined by the variable setting of DocumentRoot /var/www/html in the /etc/httpd/conf/httpd. conf http configuration file).
This directory is assigned the type httpd_sys_content_t as part of its security context which allows the http daemon to access its contents. Any file or subdirectory inherits the security context of the directory in which it is created; therefore a file created in the html subdirectory inherits the httpd_sys_content_t type. In the following example, the root user creates the index. html file in the /root directory. The index. html inherits the security root:object_r:user_home_t context which is the expected security context for root in RHEL 5. 3. [[email protected] ~]$ touch /root/index. html [[email protected] ~]$ ls -Z /root/index. tml -rw-r–r– root root root:object_r:user_home_t /root/index. html If the root user copies the newly created index. html file to the /var/www/html/ directory, the file inherits the security context (httpd_sys_content_t) of the html subdirectory because a new copy of the file is created in the html subdirectory: [[email protected] ~]$ cp /root/index. html /var/www/html [[email protected] ~]$ ls -Z /var/www/html/index. html -rw-r–r– root root user_u:object_r:httpd_sys_content_t /var/www/html/index. html If you move the index. html file instead of copying it, a new file is not created in the html subdirectory and index. tml retains the user_home_t type: [[email protected] ~]$ mv -f /root/index. html /var/www/html [[email protected] ~]$ ls -Z /var/www/html/index. html -rw-r–r– root root user_u:object_r:user_home_t /var/www/html/index. html When a Web browser or network download agent like wget makes a request to the http daemon for the moved index. html file, with user_home_t context, the browser is denied access because SELinux is running in enforcing mode. [[email protected] ~]# wget localhost/index. html –21:10:00– http://localhost/index. html Resolving localhost… 127. 0. 0. 1 Connecting to localhost|127. 0. 0. 1|:80… onnected. HTTP request sent, awaiting response… 403 Forbidden 21:10:00 ERROR 403: Forbidden. SELinux generates error messages in both /var/log/messages and /var/log/httpd/error_log. The following message in /var/log/httpd/error_log is not very helpful because it tells you only that access is being denied: [Wed May 20 12:47:57 2009] [error] [client 172. 16. 1. 100] (13) Permission denied: access to /index. html denied The following error message in /var/log/messages is more helpful because it tells you why SELinux is preventing access to the /var/www/html/index. html file – a potentially mislabeled file.
Furthermore, it provides a command that you can use to produce a detailed summary of the issue. May 20 12:22:48 localhost setroubleshoot: SELinux is preventing the httpd from using potentially mislabeled files (/var/www/html/index. html). For complete SELinux messages. run sealert -l 9e568d42-4b20-471c-9214-b98020c4d97a Entering sealert –l 9e568d42-4b20-471c-9214-b98020c4d97 as suggested in the previous error message returns the following detailed error message: [[email protected] ~]$ sealert –l 9e568d42-4b20-471c-9214-b98020c4d97 Summary: SELinux is preventing the httpd from using potentially mislabeled files (/var/www/html/index. html).
Detailed Description: SELinux has denied httpd access to potentially mislabeled file(s) (/var/www/html/index. html). This means that SELinux will not allow httpd to use these files. It is common for users to edit files in their home directory or tmp directories and then 6 Blueprints: First Steps with Security-Enhanced Linux (SELinux): Hardening the Apache Web Server move (mv) them to system directories. The problem is that the files end up with the wrong file context which confined applications are not allowed to access. Allowing Access: If you want httpd to access this files, you need to relabel them using restorecon -v ’/var/www/html/index. tml’. You might want to relabel the entire directory using restorecon -R -v ’/var/www/html’. Additional Information: Source Context root:system_r:httpd_t Target Context root:object_r:user_home_t Target Objects /var/www/html/index. html [ file ] Source httpd Source Path /usr/sbin/httpd Port Host localhost. localdomain Source RPM Packages httpd-2. 2. 3-22. el5 Target RPM Packages Policy RPM selinux-policy-2. 4. 6-203. el5 Selinux Enabled True Policy Type targeted MLS Enabled True Enforcing Mode Enforcing Plugin Name home_tmp_bad_labels Host Name localhost. localdomain Platform Linux localhost. ocaldomain 2. 6. 18-128. 1. 10. el5 #1 SMP Wed Apr 29 13:55:17 EDT 2009 i686 i686 Alert Count 24 First Seen Fri May 15 13:36:32 2009 Last Seen Wed May 20 12:47:56 2009 Local ID 9e568d42-4b20-471c-9214-b98020c4d97a Line Numbers Raw Audit Messages host=localhost. localdomain type=AVC msg=audit(1242838076. 937:1141): avc: denied { getattr } for pid=3197 comm=”httpd” path=”/var/www/html/index. html” dev=dm-0 ino=3827354 scontext=root:system_r:httpd_t:s0 context=root:object_r:user_home_t:s0 tclass=file host=localhost. localdomain type=SYSCALL msg=audit(1242838076. 37:1141): arch=40000003 syscall=196 success=no exit=-13 a0=8eaa788 a1=bfc8d49c a2=419ff4 a3=2008171 items=0 ppid=3273 pid=3197 auid=500 uid=48 gid=48 euid=48 suid=48 fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=4 comm=”httpd” exe=”/usr/sbin/httpd” subj=root:system_r:httpd_t:s0 key=(null) Although called a summary, this output is a very detailed report that provides the necessary commands to resolve the issue. As shown below, entering /sbin/restorecon -v ’/var/www/html/index. html as suggested not only resolves the problem, but also explains how you should change the security context for the /var/www/html/index. tml file. [[email protected] ~]$ restorecon -v ’/var/www/html/index. html’ /sbin/restorecon reset /var/www/html/index. html context root:object_r:user_home_t:s0-; root:object_r:httpd_sys_content_t:s0 The previous restorecon -v command changed the security context of /var/www/html/index. html from root:object_r:user_home_t to root:object_r:httpd_sys_content_t. With a root:object_r:httpd_sys_content_t security context, the http daemon can now access /var/www/html/index. html. Use a Web browser or wget to make another request to the httpd daemon for the index. html file with a restored security context.
This time, the request is permitted: [[email protected] ~]# wget localhost/index. html –21:09:21– http://localhost/index. html Resolving localhost… 127. 0. 0. 1 Connecting to localhost|127. 0. 0. 1|:80… connected. HTTP request sent, awaiting response… 200 OK Length: 0 [text/html] Saving to: ’index. html’ First Steps with Security-Enhanced Linux (SELinux) 7 [ ] 0 –. -K/s in 0s 21:09:21 (0. 00 B/s) – ’index. html’ saved [0/0] Related reference: “Scope, requirements, and support” on page 1 This blueprint applies to System x running Linux and PowerLinux. You can learn more about the systems to which this information applies.
HTTPD and SELinux Booleans SELinux has a set of built-in switches named Booleans or conditional policies that you can use to turn specific SELinux features on or off. Entering the getsebool -a | grep http command lists the 23 Booleans related to the http daemon, which are a subset of the 234 Booleans currently defined in the selinux-policy-2. 4. 6-203. el5 policy. These 23 Booleans allow you to customize SELinux policy for the http daemon during runtime without modifying, compiling, or loading a new policy. You can customize the level of http security by setting the relevant Boolean values or toggling between on and off values. [email protected] ~]$ getsebool -a | grep http allow_httpd_anon_write –> off allow_httpd_bugzilla_script_anon_write –> off allow_httpd_mod_auth_pam –> off allow_httpd_nagios_script_anon_write –> off allow_httpd_prewikka_script_anon_write –> off allow_httpd_squid_script_anon_write –> off allow_httpd_sys_script_anon_write –> off httpd_builtin_scripting –> on httpd_can_network_connect –> off httpd_can_network_connect_db –> off httpd_can_network_relay –> off httpd_can_sendmail –> on httpd_disable_trans –> off httpd_enable_cgi –> on httpd_enable_ftp_server –> off httpd_enable_homedirs –> on httpd_rotatelogs_disable_trans –> off httpd_ssi_exec –> off httpd_suexec_disable_trans –> off httpd_tty_comm –> on httpd_unified –> on httpd_use_cifs –> off httpd_use_nfs –> off SELinux provides three command-line tools for working with Booleans: getsebool, setsebool, and togglesebool. The getsebool –a command returns the current state of all the SELinux Booleans defined by the policy.
You can also use the command without the –a option to return settings for one or more specific Booleans entered on the command line, as follows: [[email protected] ~]$ getsebool httpd_enable_cgi httpd_enable_cgi –> on Use setsebool to set the current state of one or more Booleans by specifying the Boolean and its value. Acceptable values to enable a Boolean are 1, true, and on. Acceptable values to disable a Boolean are 0, false, and off. See the following cases for examples. You can use the -P option with the setsebool command to write the specified changes to the SELinux policy file. These changes are persistent across reboots; unwritten changes remain in effect until you change them or the system is rebooted. Use setsebool to change status of the httpd_enable_cgi Boolean to off: [[email protected] ~]$ setsebool httpd_enable_cgi 0 8
Blueprints: First Steps with Security-Enhanced Linux (SELinux): Hardening the Apache Web Server Confirm status change of the httpd_enable_cgi Boolean: [[email protected] ~]$ getsebool httpd_enable_cgi httpd_enable_cgi –> off The togglesebool tool flips the current value of one or more Booleans. This tool does not have an option that writes the changes to the policy file. Changes remain in effect until changed or the system is rebooted. Use the togglesebool tool to switch the status of the httpd_enable_cgi Boolean, as follows: [[email protected] ~]$ togglesebool httpd_enable_cgi httpd_enable_cgi: active Confirm the status change of the httpd_enable_cgi Boolean: [[email protected] ~]$ getsebool httpd_enable_cgi httpd_enable_cgi –> on
Related reference: “Scope, requirements, and support” on page 1 This blueprint applies to System x running Linux and PowerLinux. You can learn more about the systems to which this information applies. Configuring HTTPD security using SELinux Related reference: “Scope, requirements, and support” on page 1 This blueprint applies to System x running Linux and PowerLinux. You can learn more about the systems to which this information applies. Securing Apache (static content only) The default Red Hat Enterprise Linux 5. 3 installation with SELinux running in enforcing mode provides a basic level of Web server security. You can increase that security level with a little effort.
Because security is related to the function of the system, let’s start with a Web server that only serves static content from the /var/www/html directory. 1. Ensure that SELinux is enabled and running in enforcing mode: [[email protected] ~]$ sestatus SELinux status: SELinuxfs mount: Current mode: Mode from config file: Policy version: Policy from config file: enabled /selinux enforcing enforcing 21 2. Confirm that httpd is running as type httpd_t: [[email protected] html]$ /bin/ps axZ root:system_r:httpd_t 2555 ? root:system_r:httpd_t 2593 ? root:system_r:httpd_t 2594 ? root:system_r:httpd_t 2595 ? root:system_r:httpd_t 2596 ? root:system_r:httpd_t 2597 ? root:system_r:httpd_t 2598 ? root:system_r:httpd_t 2599 ? root:system_r:httpd_t 2600 ? grep http Ss 0:00 httpd S 0:00 httpd S 0:00 httpd S 0:00 httpd S 0:00 httpd S 0:00 httpd S 0:00 httpd S 0:00 httpd S 0:00 httpd 3. Confirm that the /var/www/html directory is assigned the httpd_sys_content_t context type: [[email protected] ~]$ ls -Z /var/www/ drwxr-xr-x root root root:object_r:httpd_sys_script_exec_t cgi-bin drwxr-xr-x root root root:object_r:httpd_sys_content_t error drwxr-xr-x root root root:object_r:httpd_sys_content_t html First Steps with Security-Enhanced Linux (SELinux) 9 drwxr-xr-x drwxr-xr-x drwxr-xr-x root root root:object_r:httpd_sys_content_t icons root root root:object_r:httpd_sys_content_t manual webalizer root root:object_r:httpd_sys_content_t usage 4.
Confirm that the content to be served is assigned the httpd_sys_content_t context type. For example: [[email protected] ~]$ ls -Z /var/www/html/index. html -rw-r–r– root root root:object_r:httpd_sys_content_t /var/www/html/index. html Use a Web browser or wget to make a request to the httpd daemon for the index. html file and you should see that permission is granted. To increase the level of protection provided by SELinux, disable any httpd-related features that you do not want by turning off their corresponding Boolean. By default, the following six Boolean are set to on. If you do not need these features, turn them off by setting their Boolean variables to off. [email protected] ~]# getsebool -a|grep http|grep “–> on” httpd_builtin_scripting –> on httpd_can_sendmail –> on httpd_enable_cgi –> on httpd_enable_homedirs –> on httpd_tty_comm –> on httpd_unified –> on httpd_can_sendmail If the Web server does not use Sendmail, turn this Boolean to off. This action prevents unauthorized users from sending e-mail spam from this system. httpd_enable_homedirs When this Boolean is set to on, it allows httpd to read content from subdirectories located under user home directories. If the Web server is not configured to serve content from user home directories, set this Boolean to off. httpd_tty_comm By default, httpd is allowed to access the controlling terminal.
This action is necessary in certain situations where httpd must prompt the user for a password. If the Web server does not require this feature, set the Boolean to off. httpd_unified This Boolean affects the transition of the http daemon to security domains defined in SELinux policy. Enabling this Boolean creates a single security domain for all http-labeled content. For more information, see SELinux by Example listed under the “Related information and downloads,” on page 15 section. httpd_enable_cgi If your content does not use the Common Gateway Interface (CGI) protocol, set this Boolean to off. If you are unsure about using CGI in the Web server, try setting it to off and examine the log entries in the /var/log/messages file.
The following example shows an error message from /var/log/messages resulting from SELinux blocking httpd execution of a CGI script: May 28 15:48:37 localhost setroubleshoot: SELinux is preventing the http daemon from executing cgi scripts. For complete SELinux messages. run sealert -l 0fdf4649-60df-47b5-bfd5-a72772207adc Entering sealert -l 0fdf4649-60df-47b5-bfd5-a72772207adc produces the following output: Summary: SELinux is preventing the http daemon from executing cgi scripts. Detailed Description: SELinux has denied the http daemon from executing a cgi script. httpd can be setup in a locked down mode where cgi scripts are not allowed to be executed. If the httpd server has been setup to not execute cgi scripts, this could signal a intrusion attempt.
Allowing Access: If you want httpd to be able to run cgi scripts, you need to turn on the httpd_enable_cgi Boolean: “setsebool -P httpd_enable_cgi=1″ 10 Blueprints: First Steps with Security-Enhanced Linux (SELinux): Hardening the Apache Web Server The following command will allow this access: setsebool -P httpd_enable_cgi=1 Additional Information: Source Context root:system_r:httpd_t Target Context root:object_r:httpd_sys_script_exec_t Target Objects /var/www/cgi-bin [ dir ] Source httpd Source Path httpd Port Host localhost. localdomain Source RPM Packages httpd-2. 2. 3-22. el5 Target RPM Packages httpd-2. 2. 3-22. el5 Policy RPM selinux-policy-2. 4. 6-203. l5 Selinux Enabled True Policy Type targeted MLS Enabled True Enforcing Mode Enforcing Plugin Name httpd_enable_cgi Host Name localhost. localdomain Platform Linux localhost. localdomain 2. 6. 18-128. 1. 10. el5 #1 SMP Wed Apr 29 13:55:17 EDT 2009 i686 i686 Alert Count 1 First Seen Thu May 28 15:48:36 2009 Last Seen Thu May 28 15:48:36 2009 Local ID 0fdf4649-60df-47b5-bfd5-a72772207adc Line Numbers Raw Audit Messages host=localhost. localdomain type=AVC msg=audit(1243540116. 963:248): avc: denied { getattr } for pid=2595 comm=”httpd” path=”/var/www/cgi-bin” dev=dm-0 ino=5527166 scontext=root:system_r:httpd_t:s0 tcontext=root:object_r:httpd_sys_script_exec_t:s0 tclass=dir host=localhost. localdomain type=SYSCALL msg=audit(1243540116. 63:248): arch=40000003 syscall=196 success=no exit=-13 a0=8bd0a88 a1=bfc790bc a2=4d0ff4 a3=2008171 items=0 ppid=2555 pid=2595 auid=4294967295 uid=48 gid=48 euid=48 suid=48 fsuid=48 egid=48 sgid=48 fsgid=48 tty=(none) ses=4294967295 comm=”httpd” exe=”httpd” subj=root:system_r:httpd_t:s0 key=(null) At the end of the previous output, listed under the Raw Audit Messages are these lines: “scontext=root:system_r:httpd_t:s0 tcontext=root:object_r:httpd_sys_script_exec_t:s0 tclass=dir” This output shows you that httpd attempted to access a subdirectory with the httpd_sys_script_exec_t context type. This type is the context type of /var/www/cgi-bin, the directory where httpd looks for CGI scripts. The httpd daemon, with a httpd_t context type, was unable to access this subdirectory because the httpd_enable_cgi variable is set to off.
With this configuration, SELinux does not allow a user or process of type httpd_t to access a directory, file, or process of type httpd_sys_script_exec_t. Therefore, the http daemon was denied access to the CGI script located in /var/www/cgi-bin. If you find similar messages in your log file, set the httpd_enable_cgi Boolean to on. httpd_builtin_scripting If you did not configure Apache to load scripting modules by changing the /etc/httpd/conf/ httpd. conf configuration file, set this Boolean to off. If you are unsure, turn httpd_builtin_scripting to off and check the /var/log/messages file for any httpd-related SELinux warnings. See the description of httpd_enable_cgi for an example. PHP and other scripting modules run with the same level of access as the http daemon.
Therefore, turning httpd_builtin_scripting to off reduces the amount of access available if the Web server is compromised. To turn off all six of these Booleans and write the values to the policy file by using the setsebool -P command follow these steps: 1. Enter the setsebool -P command: First Steps with Security-Enhanced Linux (SELinux) 11 [[email protected] ~]# setsebool -P httpd_can_sendmail=0 httpd_enable_homedirs=0 httpd_tty_comm=0 httpd_unified=0 httpd_enable_cgi=0 httpd_builtin_scripting=0 2. Check all the Boolean settings related to httpd by entering getsebool –a | grep httpd. The following output shows that all Boolean are set to off, including the six previously described variables which default to on. [email protected] ~]$ getsebool -a | grep httpd allow_httpd_anon_write –> off allow_httpd_bugzilla_script_anon_write –> off allow_httpd_mod_auth_pam –> off allow_httpd_nagios_script_anon_write –> off allow_httpd_prewikka_script_anon_write –> off allow_httpd_squid_script_anon_write –> off allow_httpd_sys_script_anon_write –> off httpd_builtin_scripting –> off httpd_can_network_connect –> off httpd_can_network_connect_db –> off httpd_can_network_relay –> off httpd_can_sendmail –> off httpd_disable_trans –> off httpd_enable_cgi –> off httpd_enable_ftp_server –> off httpd_enable_homedirs –> off httpd_rotatelogs_disable_trans –> off httpd_ssi_exec –> off httpd_suexec_disable_trans –> off httpd_tty_comm –> off httpd_unified –> off httpd_use_cifs –> off httpd_use_nfs –> off 3. Use a Web browser or wget to make another request to the httpd daemon for the index. html file and you should succeed. Rebooting your machine does not change this configuration. This completes the necessary basic SELinux settings for hardening a Web server with static content. Next, look at hardening scripts accessed by the http daemon. Related reference: “Scope, requirements, and support” on page 1 This blueprint applies to System x running Linux and PowerLinux. You can learn more about the systems to which this information applies.
Hardening CGI scripts with SELinux In the previous section, you used SELinux Booleans to disable scripting because the Web server used only static content. Beginning with that configuration, you can enable CGI scripting and use SELinux to secure the scripts. 1. Confirm that your Web server is configured as described in section “Securing Apache (static content only)” on page 9. 2. Red Hat Enterprise Linux provides a CGI script that you can use for testing. You can find the script at /usr/lib/perl5/5. 8. 8/CGI/eg/tryit. cgi. Copy this script to the /var/www/cgi-bin/ directory, as follows: [[email protected] ~]$ cp /usr/lib/perl5/5. 8. 8/CGI/eg/tryit. gi /var/www/cgi-bin/ 3. Make sure that the first line of the tryit. cgi script contains the correct path to the perl binary. From the which perl output shown below, the path should be changed to ! #/usr/bin/perl. [[email protected] ~]# which perl /usr/bin/perl [[email protected] ~]# head -1 /var/www/cgi-bin/tryit. cgi #! /usr/local/bin/perl 4. Confirm that /var/www/cgi-bin is assigned the httpd_sys_script_exec_t context type as follows: [[email protected] ~]$ ls -Z /var/www/ | grep cgi-bin drwxr-xr-x root root root:object_r:httpd_sys_script_exec_t cgi-bin 12 Blueprints: First Steps with Security-Enhanced Linux (SELinux): Hardening the Apache Web Server 5.
Allow and confirm read and execute permission for the tryit. cgi script to all users: [[email protected] cgi-bin]# chmod 555 /var/www/cgi-bin/tryit. cgi [[email protected] cgi-bin]# ls -Z -r-xr-xr-x root root root:object_r:httpd_sys_script_exec_t tryit. cgi 6. Confirm that /var/www/cgi-bin/tryit. cgi is assigned the httpd_sys_script_exec_t context type: [[email protected] ~]$ ls -Z /var/www/cgi-bin/tryit. cgi -r-xr-xr-x root root root:object_r:httpd_sys_script_exec_t /var/www/cgi-bin/tryit. cgi 7. Enable CGI scripting in SELinux and confirm that it is enabled: [[email protected] cgi-bin]$ setsebool httpd_enable_cgi=1 [[email protected] cgi-bin]$ getsebool httpd_enable_cgi httpd_enable_cgi –> on 8.
Open a Web browser and type the Web server address into the location bar. Include the /cgi-bin/tryit. cgi in the URL. For example, type http://192. 168. 1. 100/cgi-bin/tryit. cgi. The tryit. cgi script should return output similar to Figure 1: Figure 1. Figure 1: A Simple Example 9. Provide test answers to the form fields and click Submit Query. The tryit. cgi script should return output similar to Figure 2: First Steps with Security-Enhanced Linux (SELinux) 13 Figure 2. Figure 2: A Simple Example with results Related reference: “Scope, requirements, and support” on page 1 This blueprint applies to System x running Linux and PowerLinux. You can learn more about the systems to which this information applies. 14
Blueprints: First Steps with Security-Enhanced Linux (SELinux): Hardening the Apache Web Server Appendix. Related information and downloads Related information v Wikipedia: Security-Enhanced Linux http://en. wikipedia. org/wiki/Selinux v Bell-La Padula model http://en. wikipedia. org/wiki/Bell-La_Padula_model v NSA Security-Enhanced Linux http://www. nsa. gov/research/selinux/index. shtml v Managing Red Hat Enterprise Linux 5 presentation http://people. redhat. com/dwalsh/SELinux/Presentations/ManageRHEL5. pdf v developerWorks Security Blueprint Community Forum http://www. ibm. com/developerworks/forums/forum. jspa? forumID=1271 v Red Hat Enterprise Linux 4: Red Hat SELinux Guide http://www. linuxtopia. rg/online_books/redhat_selinux_guide/rhlcommon-section-0055. html v F. Mayer, K. MacMillan, D. Caplan, “SELinux By Example – Using Security Enhanced Linux” Prentice Hall, 2007 Related reference: “Scope, requirements, and support” on page 1 This blueprint applies to System x running Linux and PowerLinux. You can learn more about the systems to which this information applies. © Copyright IBM Corp. 2009 15 16 Blueprints: First Steps with Security-Enhanced Linux (SELinux): Hardening the Apache Web Server Notices This information was developed for products and services offered in the U. S. A. IBM may not offer the products, services, or features discussed in this document in other countries.
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