Apache Security

Apache Security

Chapter 1. Apache Security Principles

This book contains 12 chapters. Of those, 11 cover the technical issues of securing Apache and web applications. Looking at the number of pages alone it may seem the technical issues represent the most important part of security. But wars are seldom won on tactics alone, and technical issues are just tactics. To win, you need a good overall strategy, and that is the purpose of this chapter. It has the following goals:

• Define security
  
• Introduce essential security principles
  
• Establish a common security vocabulary
  
• Present web application architecture blueprints

The Web Application Architecture Blueprints section offers several different views (user, network, and Apache) of the same problem, with a goal of increasing understanding of the underlying issues.

Security Definitions

Security can be defined in various ways. One school of thought defines it as reaching the three goals known as the CIA triad:

Confidentiality

Information is not disclosed to unauthorized parties.

Integrity

Information remains unchanged in transit or in storage until it is changed by an authorized party.

Availability

Authorized parties are given timely and uninterrupted access to resources and information.

Another goal, accountability, defined as being able to hold users accountable (by maintaining their identity and recording their actions), is sometimes added to the list as a fourth element.
The other main school of thought views security as a continuous process, consisting of phases. Though different people may name and describe the phases in different ways, here is an example of common phases:

Assessment

Analysis of the environment and the system security requirements. During this phase, you create and document a security policy and plans for implementing that policy.

Protection

Implementation of the security plan (e.g., secure configuration, resource protection, maintenance).

Detection

Identification of attacks and policy violations by use of techniques such as monitoring, log analysis, and intrusion detection.

Response

Handling of detected intrusions, in the ways specified by the security plan.

Both lines of thought are correct: one views the static aspects of security and the other views the dynamics. In this chapter, I look at security as a process; the rest of the book covers its static aspects.
Another way of looking at security is as a state of mind. Keeping systems secure is an ongoing battle where one needs be alert and vigilant at all times, and remain one step ahead of adversaries. But you need to come to terms that being 100 percent secure is impossible. Sometimes, we cannot control circumstances, though we do the best we can. Sometimes we slip. Or we may have encountered a smarter adversary. I have found that being humble increases security. If you think you are invincible, chances are you won't be alert to lurking dangers. But if you are aware of your own limitations, you are likely to work hard to overcome them and ensure all angles are covered.
Knowing that absolute security is impossible, we must accept occasional failure as certainty and design and build defensible systems. Richard Bejtlich (http://taosecurity.blogspot.com) coined this term (in a slightly different form: defensible networks). Richard's interests are networks but the same principles apply here. Defensible systems are the ones that can give you a chance in a fight in spite of temporary losses. They can be defended. Defensible systems are built by following the essential security principles presented in the following section.

Essential Security Principles

In this section, I present principles every security professional should know. These principles have evolved over time and are part of the information security body of knowledge. If you make a habit of reading the information security literature, you will find the same security principles recommended at various places, but usually not all in one place. Some resources cover them in detail, such as the excellent book Secrets & Lies: Digital Security in a Networked World by Bruce Schneier (Wiley). Here are the essential security principles:

Compartmentalize

Compartmentalization is a concept well understood by submarine builders and by the captain of the Starship Enterprise. On a submarine, a leak that is not contained to the quarter in which it originated will cause the whole submarine to be filled with water and lead to the death of the entire crew. That's why submarines have systems in place to isolate one part of the submarine from another. This concept also benefits computer security. Compartmentalization is all about damage control. The idea is to design the whole to consist of smaller connected parts. This principle goes well together with the next one.

Utilize the principle of least privilege

Each part of the system (a program or a user) should be given the privileges it needs to perform its normal duties and nothing more. That way, if one part of the system is compromised, the damage will be limited.

Perform defense in depth

Defense in depth is about having multiple independent layers of security. If there is only one security layer, the compromise of that layer compromises the entire system. Multiple layers are preferable. For example, if you have a firewall in place, an independent intrusion detection system can serve to control its operation. Having two firewalls to defend the same entry point, each from a different vendor, increases security further.

Do not volunteer information

Attackers commonly work in the dark and perform reconnaissance to uncover as much information about the target as possible. We should not help them. Keep information private whenever you can. But keeping information private is not a big security tool on its own. Unless the system is secure, obscurity will not help much.

Fail safely

Make sure that whenever a system component fails, it fails in such a way as to change into a more secure state. Using an obvious example, if the login procedure cannot complete because of some internal problem, the software should reject all login requests until the internal problem is resolved.

Secure the weakest link

The whole system is as secure as its weakest link. Take the time to understand all system parts and focus your efforts on the weak parts.

Practice simplicity

Humans do not cope with complexity well. A study has found we can only hold up to around seven concepts in our heads at any one time. Anything more complex than that will be hard to understand. A simple system is easy to configure, verify, and use. (This was demonstrated in a recent paper, "A Quantitative Study of Firewall Configuration Errors" by Avishai Wool: http://www.eng.tau.ac.il/~yash/computer2004.pdf.)

Common Security Vocabulary

At this point, a short vocabulary of frequently used security terms would be useful. You may know some of these terms, but some are specific to the security industry.

Weakness

A less-than-ideal aspect of a system, which can be used by attackers in some way to bring them closer to achieving their goals. A weakness may be used to gain more information or as a stepping-stone to other system parts.

Vulnerability

Usually a programming error with security consequences.

Exploit

A method (but it can be a tool as well) of exploiting a vulnerability. This can be used to break in or to increase user privileges (known as privilege elevation).

Attack vector

An entry point an adversary could use to attempt to break in. A popular technique for reducing risk is to close the entry point completely for the attacker. Apache running on port 80 is one example of an entry point.

Attack surface

The area within an entry point that can be used for an attack. This term is usually used in discussions related to the reduction of attack surface. For example, moving an e-commerce administration area to another IP address where it cannot be accessed by the public reduces the part of the application accessible by the attacker and reduces the attack surface and the risk.

Security Process Steps

Expanding on the four generic phases of the security process mentioned earlier (assessment, protection, detection, and response), we arrive at seven practical steps that cover one iteration of a continuous process:

1. Understand the environment and the security requirements of the project.
   
2. Establish a security policy and design the system.
   
3. Develop operational procedures.
   
4. Configure carefully.
   
5. Perform maintenance and patch regularly.
   
6. Monitor.
   
7. Handle attacks.

The first three steps of this process, referred to as threat modeling, are covered in the next section. The remaining steps are covered throughout the book.

Threat Modeling

Threat modeling is a fancy name for rational and methodical thinking about what you have, who is out there to get you, and how. Armed with that knowledge, you decide what you want to do about the threats. It is genuinely useful and fun to do, provided you do not overdo it. It is a loose methodology that revolves around the following questions:

1. What do you have that is valuable (assets)?
   
2. Why would attackers want to disrupt your operation (motivation)?
   
3. Where can they attack (entry points)?
   
4. How would they attack (threats)?
   
5. How much would it cost to protect from threats (threat ranking)?
   
6. Which threats will you fight against and how (mitigation)?

The best time to start is at the very beginning, and use threat modeling for system design. But since the methodology is attack-oriented, it is never too late to start. It is especially useful for security assessment or as part of penetration testing (an exercise in which an attempt is made to break into the system as a real attacker would). One of my favorite uses for threat modeling is system administrator training. After designing several threat models, you will see the recurring patterns. Keeping the previous threat models is, therefore, an excellent way to document the evolution of the system and preserves that little bit of history. At the same time, existing models can be used as starting points in new threat modeling efforts to save time.
Table 1-1 gives a list of reasons someone may attack you. This list (and the one that follows it) is somewhat optimized. Compiling a complete list of all the possibilities would result in a multipage document. Though the document would have significant value, it would be of little practical use to you. I prefer to keep it short, simple, and manageable.

Table 1-1. Major reasons why attacks take place

Reason	Description
To grab an asset	Attackers often want to acquire something valuable, such as a customer database with credit cards or some other confidential or private information.
To steal a service	This is a special form of the previous category. The servers you have with their bandwidth, CPU, and hard disk space are assets. Some attackers will want to use them to send email, store pirated software, use them as proxies and starting points for attacks on other systems, or use them as zombies in automated distributed denial of service attacks.
Recognition	Attacks, especially web site defacement attacks, are frequently performed to elevate one's status in the underground.
Thrill	Some people love the thrill of breaking in. For them, the more secure a system, the bigger the thrill and desire to break in.
Mistake	Well, this is not really a reason, but attacks happen by chance, too.

Table 1-2 gives a list of typical attacks on web systems and some ways to handle them.

Attack type	Description	Mitigation
Denial of service	Any of the network, web-server, or application-based attacks that result in denial of service, a condition in which a system is overloaded and can no longer respond normally.	Prepare for attacks (as discussed in Chapter 5). Inspect the application to remove application-based attack points.
Exploitation of configuration errors	These errors are our own fault. Surprisingly, they happen more often than you might think.	Create a secure initial installation (as described in Chapter 2-Chapter 4). Plan changes, and assess the impact of changes before you make them. Implement independent assessment of the configuration on a regular basis.
Exploitation of Apache vulnerabilities	Unpatched or unknown problems in the Apache web server.	Patch promptly.
Exploitation of application vulnerabilities	Unpatched or unknown problems in deployed web applications.	Assess web application security before each application is deployed. (See Chapter 10 and Chapter 11.)
Attacks through other services	This is a "catch-all" category for all other unmitigated problems on the same network as the web server. For example, a vulnerable MySQL database server running on the same machine and open to the public.	Do not expose unneeded services, and compartmentalize, as discussed in Chapter 9.

In addition to the mitigation techniques listed in Table 1-2, certain mitigation procedures should always be practiced:

• Implement monitoring and consider implementing intrusion detection so you know when you are attacked.
  
• Have procedures for disaster recovery in place and make sure they work so you can recover from the worst possible turn of events.
  
• Perform regular backups and store them off-site so you have the data you need for your disaster recovery procedures.

To continue your study of threat modeling, I recommend the following resources:

• For a view of threat modeling through the eyes of a programmer, read Threat Modeling by Frank Swiderski and Window Snyder (Microsoft Press). A threat-modeling tool developed for the book is available as a free download at http://www.microsoft.com/downloads/details.aspx?FamilyID=62830f95-0e61-4f87-88a6-e7c663444ac1.
  
• Writing Secure Code by Michael Howard and David LeBlanc (Microsoft Press) is one of the first books to cover threat modeling. It is still the most useful one I am aware of.
  
• Improving Web Application Security: Threats and Countermeasures (Microsoft Press) is provided as a free download (http://www.microsoft.com/downloads/details.aspx?familyid=E9C4BFAA-AF88-4AA5-88D4-0DEA898C31B9) and includes very good coverage of threat modeling.
  
• Attack trees, as introduced in the article "Attack trees" by Bruce Schneier (http://www.schneier.com/paper-attacktrees-ddj-ft.html), are a methodical approach to describing ways security can be compromised.
  
• "A Preliminary Classification Scheme for Information System Threats, Attacks, and Defenses; A Cause and Effect Model; and Some Analysis Based on That Model" by Fred Cohen et al. can be found at http://www.all.net/journal/ntb/cause-and-effect.html.
  
• "Attack Modeling for Information Security and Survivability" by Andrew P. Moore, Robert J. Ellison, and Richard C. Linger can be found at http://www.cert.org/archive/pdf/01tn001.pdf.
  
• A talk I gave at OSCOM4, "Threat Modelling for Web Applications" (http://www.thinkingstone.com/talks/Threat_Modelling.pdf), includes an example that demonstrates some of the concepts behind threat modeling.

System-Hardening Matrix

One problem I frequently had in the past was deciding which of the possible protection methods to use when initially planning for installation. How do you decide which method is justifiable and which is not? In the ideal world, security would have a price tag attached and you could compare the price tags of protection methods. The solution I came to, in the end, was to use a system-hardening matrix.
First, I made a list of all possible protection methods and ranked each in terms of complexity. I separated all systems into four categories:

1. Mission critical (most important)
   
2. Production
   
3. Development
   
4. Test (least important)

Then I made a decision as to which protection method was justifiable for which system category. Such a system-hardening matrix should be used as a list of minimum methods used to protect a system, or otherwise contribute to its security. Should circumstances require increased security in a certain area, use additional methods. An example of a system-hardening matrix is provided in Table 1-3. A single matrix cannot be used for all organizations. I recommend you customize the example matrix to suit your needs.

Table 1-3. System-hardening matrix example

Technique	Category 4: Test	Category 3: Development	Category 2: Production	Category 1: Mission critical
Install kernel patches	Â	Â	Â
Compile Apache from source	Â	Â
Tighten configuration (remove default modules, write configuration from scratch, restrict every module)	Â	Â
Change web server identity	Â	Â
Increase logging (e.g., use audit logging)	Â	Â
Implement SSL	Â	Â
Deploy certificates from a well-known CA	Â	Â
Deploy private certificates (where appropriate)	Â	Â	Â
Centralize logs
Jail Apache	Â
Use mod_security lightly	Â	Â
Use mod_security heavily	Â	Â	Â
Do server monitoring	Â
Do external availability monitoring	Â	Â
Do periodic log monitoring or inspection
Do real-time log monitoring	Â	Â	Â
Do periodic manual log analysis	Â	Â
Do event correlation	Â	Â	Â
Deploy host firewalls	Â
Validate file integrity	Â	Â
Install network-based web application firewall	Â	Â	Â
Schedule regular assessments	Â	Â
Arrange external vulnerability assessment or penetration testing	Â	Â	Â
Separate application components	Â	Â	Â

System classification comes in handy when the time comes to decide when to patch a system after a problem is discovered. I usually decide on the following plan:

Category 1

Patch immediately.

Category 2

Patch the next working day.

Categories 3 and 4

Patch when the vendor patch becomes available or, if the web server was installed from source, within seven days of publication of the vulnerability.

Calculating Risk

A simple patching plan, such as in the previous section, assumes you will have sufficient resources to deal with problems, and you will deal with them quickly. This only works for problems that are easy and fast to fix. But what happens if there are not sufficient resources to patch everything within the required timeline? Some application-level and, especially, architectural vulnerabilities may require a serious resource investment. At this point, you will need to make a decision as to which problems to fix now and which to fix later. To do this, you will need to assign perceived risk to each individual problem, and fix the biggest problem first.
To calculate risk in practice means to make an educated guess, usually supported by a simple mathematical calculation. For example, you could assign numeric values to the following three factors for every problem discovered

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