# Mandatory Access Control A *mandatory access control* (MAC) policy is a means of assigning access rights based on regulations by a central authority. This class of policies includes examples from both industry and government. The philosophy underlying these policies is that information belongs to an organization (rather than individual members of it), and it is that organization which should control the security policy. ## Multi-level security (MLS) In national security and military environments, documents are labeled according to their *sensitivity* levels. In the US, these range from *Unclassified* (anyone can see this) to *Confidential* to *Secret* and finally (we believe) to *Top Secret*; other countries use similar classifications. These levels correspond to the risk associated with release of the information. But it is not sufficient to use only sensitivity levels to classify objects if one wants to comply with the *Need to Know* principle: access to information should only be granted if it is necessary to perform one's duties. (Need to Know is an instance of Least Privilege.) *Compartments* are used to handle this decomposition of information. Every object is associated with a set of compartments (e.g. crypto, nuclear, biological, reconnaissance, etc.). An object associated with {crypto, nuclear} may be accessed only by subjects who need to know about both cryptography and nuclear weapons. An object associated with the empty set {} simply doesn't pertain to any need-to-know compartments. A *label* is a pair of a sensitivity level and a set of compartments. A document might have the label (Top Secret, {crypto,nuclear}) if it contained extremely sensitive information regarding cryptography and nuclear weapons. In practice, each paragraph in a document is assigned a set of compartments and a sensitivity. The classification of the entire document would then be the most restrictive classification given to a paragraph in that document. Users are also labelled according to their security *clearance*. A user's clearance, just like a document's label, is a pair of a sensitivity level and a set of compartments. Typical DAC mechanisms, like access control lists and capabilities, aren't adequate to enforce confidentiality (or integrity) in the MLS setting. Consider the following example. **Leakage through Trojan Horse.** <i>A subject G is cleared at (Top Secret, {}) because it runs on behalf of an army general. G has access to object BP containing battle plans. BP is labeled (Top Secret, {}). An attacker, who does not have access to BP, creates a Trojan Horse program T that if executed does the following: - Create a new object O. - Grant G write access to O. - Grant A read access to O, where A is subject that is running on behalf of attacker. - Copy BP into O. Obviously, it doesn't do any good for the attacker to run T. But suppose T also has other functionality&mdash;it's word processor, or a game (Minesweeper?), or some other application that G might be interested in running. Then A might be able to trick G into running T. When that happens, A will get access to the contents of BP through O. DAC can't stop that.</i> How can we prevent this kind of attack from violating MLS? We need a different kind of mechanism than DAC. One famous solution comes from Bell and LaPadula (1973). They gave a formal, mathematical model of multi-level security. This model enforces the *BLP policy*: **Information cannot leak to subjects who are not cleared for the information.** Given two labels L1 = (S1, C1) and L2 = (S2, C2), we write that L1 &#8849; L2&mdash;meaning that L1 is no more restrictive than L2&mdash;when - S1 &leq; S2, where Unclassified &leq; Confidential &leq; Secret &leq; Top Secret, and - C1 &sube; C2. Notice that &#8849; is a partial order: it is possible to have two labels that are incomparable (e.g. (secret, {crypto}) vs. (top secret, {nuclear})) according to &#8849;. Let L(X) denote the label of an *entity* X, where an entity is either a subject or an object. The *BLP security conditions* are: - A subject S may read object O only if L(O) &#8849; L(S). In other words, subjects are not allowed to "read up." - A subject S may write object O only if L(S) &#8849; L(O). In other words, a subject may not "write down." Do the BLP security conditions enforce the BLP policy? Yes. First, note that a subject can never directly read an object for which it is not cleared. The first condition guarantees this. Second, a subject must never be able to learn information about some highly-labeled object O by reading another low-labeled object O'. Note that this is only possible if some other subject first reads O then writes O'. By the two conditions, a read then write by S entails L(O) &#8849; L(S) &#8849; L(O'). But then O actually has a lower label than O', so no information can have leaked. The above was considered a significant result when it first was proved. But there are several problems with the BLP formulation of MLS. These include: - It is possible that the security level for an entity could be changed in mid-operation. This change could violate the information-flow constraints we wish to preserve. - The model is concerned with only confidentiality, not integrity. For example, subjects can "write up". Thus, a subject that cannot read an object is permitted to make changes to that object; this is called a "blind write". Yet it makes little sense to trust a subject to modify the information contained in an object, if that subject is not trusted to read the information contained in the object. Biba (1977) shows how to handle integrity as a mathematical dual of confidentiality. - A subject S that wants to write object O is not allowed to do so if L(O) &#8816; L(S). The subject must login at a lower level than his or her clearance. It is annoying for principals to be forced to decide, at the time they login, what rights they will need. But it is also a nice application of the Principle of Least Privilege. - Some processes must be allowed to violate the BLP security conditions. For example, an encryption program takes secret information and outputs encrypted but unclassified information. Similarly, to do accounting, programs that may access confidential data produce summary (billing) information that is not confidential. Such programs would seem to be violating the no "write down" condition. The traditional solution is to create *trusted subjects*, which are not restricted by the BLP security conditions. This introduces a potential vulnerability into the system, however. Some real-world systems, including SELinux and TrustedBSD, combine MAC and DAC policies. In such cases, an operation is allowed only if both the MAC policy and the DAC policy both permit the operation. RBAC and groups are also employed alongside MAC. ## Brewer-Nash MLS is appropriate for national security confidentiality policies, and it is sometimes appropriate for business confidentiality policies. Consider a microprocessor company's plans for its next-generation chip. The company might consider these plans Top Secret and desire an access control mechanism that can prevent leakage of this sensitive information. Other business confidentiality policies do not exhibit such close correspondence to MLS. Consider an investment bank. It employs consultants who both advise and analyze companies. When advising, such consultants learn secret information about a company's finances that should not be shared with the public. The consultant could exploit this *insider information* while performing analysis, to profit either himself or other clients. Such abuse is prohibited by law. Brewer and Nash (1989) developed a MAC policy for this scenario, calling it *Chinese Wall* by analogy to the Great Wall of China. The intuition is that an unbreachable wall is erected between different parts of the same company; no information may pass over or through the wall. In the Brewer-Nash policy, we (as usual) have have objects, subjects, and users. However, objects are now grouped into *company datasets* (CDs). For example, an object might be a file, and a company dataset would then be all of the files related to a single company. Company datasets are themselves grouped into *conflict of interest classes* (COIs). For example, one COI might be the set of all companies in the banking industry, and another COI might be all the companies in the oil industry. The Brewer and Nash security conditions are as follows. Note that these conditions require tracking the set of read objects for each user and subject. 1. A subject S may read object O only if S has never read any object O' such that: * COI(O) = COI(O'), and * CD(O) &ne; CD(O'). 2. A subject S may write object O only if S has never read an object O' such that CD(O) &ne; CD(O'). The first condition guarantees that a single user never breaches the wall by reading information from two different CDs within the same COI. The second condition guarantees that two or more subjects never cooperatively breach the wall by performing a series of read and write operations. How would a cooperative breach work? **Cooperative breach:** <i>Suppose S1 has read from CD1, and S2 has read from CD2, where CD1 and CD2 are both in COI1. Then: 1. S1 reads information from an object in CD1. 2. S1 writes that information to object O in CD3 in COI2. 3. S2 reads that information from O. S2 now has read information about both CD1 and CD2, which violates the Brewer-Nash policy.</i> Condition 2a prevents the write operation in step (ii) above by restricting when a subject may write: once a subject reads two objects from different CDs, that subject may never write any object. There's still a problem&mdash;all this assumes the user can't store information outside the system (e.g., in their brain). In that case, even 2a wouldn't suffice. So this policy really isn't used to constrain the activity of humans so much as the activity of potentially malicious programs. ## Exercises 1. A fictitious microprocessor company called Mintel, Inc., is implementing a MLS model for its computer systems. The security officer of Mintel proposes the following labels for users and objects: * Alice is the CEO of Mintel. L(Alice) = (Top Secret, {NewCPU,HR}) * Bob is a manager in the HR office. L(Bob) = (Secret, {HR}) * Cindy is a scientist working on a part of a new CPU product. L(Cindy) = (Secret, {NewCPU}) * Dave is a receptionist in the main lobby. L(Dave) = (Unclassified, {}) * payroll.xlsx is a spreadsheet containing salary information. L(payroll.xlsx) = (Confidential, {HR}) * strategy.pptx is a briefing on Mintel’s strategy for its new CPU product. L(strategy.pptx) = (TopSecret, {NewCPU}) * index.php is the homepage of Mintel’s website. L(index.php) = (Unclassified, {}) Using the BLP security conditions, enter an 'r' in the following table if a subject may read an object, and a 'w' if a subject may write an object. Assume that each user logs in with their full clearance enabled, hence the subject executing on the user’s behalf has the full clearance of the user. <table> <tr><td></td><td>payroll.xlsx</td><td>strategy.pptx</td><td>index.php</td></tr> <tr><td>Alice/Etrade.exe</td><td></td><td></td><td></td></tr> <tr><td>Bob/Solitaire.app</td><td></td><td></td><td></td></tr> <tr><td>Cindy/emacs</td><td></td><td></td><td></td></tr> <tr><td>Dave/IE.exe</td><td></td><td></td><td></td></tr> </table> 2. Consider the database of an investment firm Invy. Analysts employed by Invy use the database to guide the investments of companies and individuals. The Invy database contains records for the following companies: * Banks: - Bank of America - Citibank - PNC * Oil companies: - ExxonMobil - BP - Chevron Invy uses a Brewer-Nash policy in which the banks are one COI and the oil companies are another COI. Anthony is a new analyst at Invy. He is assigned to counsel Citibank on its investments. He logs in and begins to familiarize himself by reading from the database records pertaining to Citibank. * After doing so, what companies can Anthony read from according to the Brewer-Nash security conditions? * What companies can Anthony write to according to the Brewer-Nash security conditions? * In the same session, Anthony also reads some records pertaining to BP. Now what companies can he read and write? * What is the minimum number of analysts Invy must employ, assuming that the database contains only those companies listed above, and that every company is assigned to at least one analyst?