Network Security Layers

Why Covert Channels are considered a hole in Mls and La- Bella Padula ModelsBy: Adam ReedNatara FonsecaRonald Warden

Multi-Level Security (MLS)Multilevel security (abbreviated as MLS) is the application of a computer system to process information with different sensitivities (i.e., at different security levels), permit simultaneous access by users with different security clearance and needs-to-know, and prevent users from obtaining access to information for which they lack authorization.MLS allows easy access to less-sensitive information by higher-cleared individuals, and it allows higher-cleared individuals to easily share sanitized documents with less-cleared individuals. A sanitized document is one that has been edited to remove information that the less-cleared individual is not allowed to see

Why Multi-Level?The term multi-level arises from the defense community's security classifications: Confidential, Secret, and Top Secret. Individuals must be granted appropriate clearances before they can see classified information. Those with Confidential clearance are only authorized to view Confidential documents; they are not trusted to look at Secret or Top Secret information. The rules that apply to data flow operate from lower levels to higher levels, and never the reverse. This is illustrated below.

MLS and System PrivilegesMLS access rules are always combined with conventional access permissions (file permissions). For example, if a user with a security level of "Secret" uses Discretionary Access Control (DAC) to block access to a file by other users, this also blocks access by users with a security level of "Top Secret". A higher security clearance does not automatically give permission to arbitrarily browse a file system. Users with top-level clearances do not automatically acquire administrative rights on multi-level systems. While they may have access to all information on the computer, this is different from having administrative rights.

Available data flows using an MLS systemUnder such a system, users, computers, and networks use labels to indicate security levels. Data can flow between like levels, for example between "Secret" and "Secret", or from a lower level to a higher level. This means that users at level "Secret" can share data with one another, and can also retrieve information from Confidential-level (i.e., lower-level), users. However, data cannot flow from a higher level to a lower level. This prevents processes at the "Secret" level from viewing information classified as "Top Secret". It also prevents processes at a higher level from accidentally writing information to a lower level. This is referred to as the "no read up, no write down" model.

Trusted operating systemsAn MLS operating environment often requires a highly trustworthy information processing system often built on an MLS operating system, but not necessarily. Most MLS functionality can be supported by a system composed entirely from entrusted computers, although it requires multiple independent computers linked by hardware security-compliant channels. An example of hardware enforced MLS is Asymmetric Isolation. If a single computer is being used in MLS mode, then that computer must use a trusted operating system (OS). Because all information in an MLS environment is physically accessible by the OS, strong logical controls must exist to ensure that access to information is strictly controlled.

MILS architectureMILS (multiple independent levels of security) is an architecture that addresses the domain separation component of MLS. Security models such as the Bell-La Padula model(for confidentiality) allow one-way flow between certain security domains that are otherwise assumed to be isolated. MILS addresses the isolation underlying MLS without addressing the controlled interaction between the domains addressed by the above models. Trusted security-compliant channels mentioned above can link MILS domains to support more MLS functionality.The MILS approach pursues a strategy characterized by an older term, MSL (multiple single level), that isolates each level of information within its own single-level environment (System High).The rigid process communication and isolation offered by MILS may be more useful to ultra high reliability software applications than MLS. MILS notably does not address the hierarchical structure that is embodied by the notion of security levels. This requires the addition of specific import/export applications between domains each of which needs to be accredited appropriately. As such, MILS

MILS architecture cont.might be better called Multiple Independent Domains of Security (MLS emulation on MILS would require a similar set of accredited applications for the MLS applications). By declining to address out of the box interaction among levels consistent with the hierarchical relations of Bell-La Padula, MILS is (almost deceptively) simple to implement initially but needs non-trivial supplementary import/export applications to achieve the richness and flexibility expected by practical MLS applications.Any MILS/MLS comparison should consider whether the accreditation of a set of simpler export applications is more achievable than accreditation of a single, more complex MLS kernel. This question depends in part on the extent of the import/export interactions that the stakeholders require. In favor of MILS is the possibility that not all the export applications will require maximal assurance.

Multiple Single-Level is a method of separating different levels of data by using separate PCs or virtual machines for each level. It aims to give some of the benefits of Multilevel security without needing special changes to the OS or applications, but at the cost of requiring extra hardware.The drive to develop MLS operating systems was severely hampered by the dramatic fall in data processing costs in the early 1990s. Before the advent of desktop computing, users with classified processing requirements had to either spend a lot of money for a dedicated computer or use one that hosted an MLS operating system. Throughout the 1990s, however, many offices in the defense and intelligence communities took advantage of falling computing costs to deploy desktop systems classified to operate

Multiple Single-Level cont.only at the highest classification level used in their organization. These desktop computers operated in System High mode and were connected with LANs that carried traffic at the same level as the computers.MSL implementations such as these neatly avoided the complexities of MLS but traded off technical simplicity for inefficient use of space. Because most users in classified environments also needed unclassified systems, users often had at least two computers and sometimes more (one for unclassified processing and one for each classification level processed). In addition, each computer was connected to its own LAN at the appropriate classification level, meaning that multiple dedicated cabling plants were incorporated (at considerable cost in terms of both installation and maintenance).

The Bell-La Padula Model (abbreviated BLP and often misspelled Bell-LaPadula) is a state machine model used for enforcing access control in government and military applications. It was developed by David Elliott Bell and Leonard J. La Padula, subsequent to strong guidance from Roger R. Schell to formalize the Department of Defense (DoD) multi level security (MLS) policy. The model is a formal state transition model of computer security model that describes a set of access control rules which use security labels on objects and clearances for subjects. Security labels range from the most sensitive (e.g."Top Secret"), down to the least sensitive (e.g., "Unclassified" or "Public").The Bell-La Padula model is an example of a model where there is no clear distinction of computer and security.

The Bell-La Padula model focuses on data confidentiality and controlled access to classified information, in contrast to the Biba integrity model which describes rules for the protection of data integrity. In this formal model, the entities in an information systems are divided into subjects and objects. The notion of a “secure state" is defined, and it is proven that each state transition preserves security by moving from secure state to secure state, thereby inductively proving that the system satisfies the security objectives of the model. The Bell-La Padula model is built on the concept of a state machine with a set of allowable states in a computer network system. The transition from one state to another state is defined by transition functions.A system state is defined to be "secure" if the only permitted access modes of subjects to objects are in accordance with a security policy. To determine whether a specific access mode is allowed, the clearance of a subject is compared to the classification of the object (more precisely, to the combination of classification and set of compartments, making up the security level) to determine if the subject is authorized for the specific access mode. The clearance/classification scheme is expressed in terms of a lattice. The model defines two mandatory access control (MAC) rules and one discretionary access control (DAC) rule with three security properties:The Simple Security Property - a subject at a given security level may not read an object at a higher security level (no read-up).The *-property (read "star"-property) - a subject at a given security level must not write to any object at a lower security level (no write-down). The *-property is also known as the Confinement property.The Discretionary Security Property - use of an access matrix to specify the discretionary access control.

The transfer of information from a high-sensitivity document to a lower-sensitivity document may happen in the Bell-La Padula model via the concept of trusted subjects. Trusted Subjects are not restricted by the *-property. Un-trusted subjects are. Trusted Subjects must be shown to be trustworthy with regard to the security policy. This security model is directed toward access control and is characterized by the phrase: "no read up, no write down." Compare the Biba model, the Clark-Wilson model and the Chinese Wall model.With Bell-La Padula, users can create content only at or above their own security level (i.e. secret researchers can create secret or top-secret files but may not create public files; no write-down). Conversely, users can view content only at or below their own security level (i.e. secret researchers can view public or secret files, but may not view top-secret files; no read-up).The Bell-La Padula model explicitly defined its scope. It did not treat the following extensively:Covert Channels. Passing information via pre-arranged actions was described briefly.Networks of systems. Later modeling work did address this topic.Policies outside multilevel security. Work in the early 1990s showed that MLS is one version of boolean policies, as are all other published policies.

LimitationsOnly addresses confidentiality, control of writing (one form of integrity), *-property and discretionary access controlCovert channels are mentioned but are not addressed comprehensivelyThe tranquility principle limits its applicability to systems where security levels do not change dynamically. It allows controlled copying from high to low via trusted subjects.

Covert ChannelIs a type of computer security attack that creates a capability to transfer information objects between processes that are not supposed to be allowed to communicate by the computer security policy. Covert channels are those that "use entities not normally viewed as data objects to transfer information from one subject to another."

A covert channel is called this because it is hidden from the access control mechanisms of ultra high assurance secure operating systems since it does not use the legitimate data transfer mechanisms of the computer system such as read and write, and therefore cannot be detected or controlled by the hardware based security mechanisms that underlie ultra high assurance secure operating systems. Covert channels are exceedingly hard to install in real systems, and can often be detected by monitoring system performance; in addition, they suffer from a low signal to-noise ratio and low data rates (on the order of a few bits per second). They can also be removed manually with a high degree of assurance from secure systems by well established covert channel analysis strategies.Covert channels are distinct from, and often confused with legitimate channel exploitations that attack low assurance pseudo-secure systems using schemes such as steganography or even less sophisticated schemes to disguise prohibited objects inside of legitimate information objects. This legitimate channel misuse by data hiding schemes is specifically not covert channels and can be prevented by ultra high assurance secure OSs.Covert channels can tunnel through secure operating systems and require special measures to control. Covert channel analysis is the only proven way to control covert channels. By contrast, secure operating systems can easily prevent misuse of legitimate channels. Distinguishing these is important. Analysis of legitimate channels for hidden objects is often misrepresented as the only successful countermeasure for legitimate channel misuse. Because this amounts to analysis of large amounts of software, it was shown as early as 1972 to be unsuccessful. Without being informed of this, some are misled to believe an analysis will "manage the risk" of these legitimate channels.

TCSECThe Trusted Computer Security Evaluation Criteria is a set of criteria established by the National Computer Security Center, an agency managed by the United States National Security Agency.Lampson's definition of a covert channel was paraphrased in the TCSEC specifically to refer to ways of transferring information from a higher classification compartment to a lower classification. In a shared processing environment, it is difficult to completely insulate one process from the effects another process can have on the operating environment. A covert channel is created by a sender process that modulates some condition (such as free space, availability of some service, wait time to execute) that can be detected by a receiving process.The TCSEC defines two kinds of covert channels:Timing channels - Perform operations that affect the relative timing of eventsStorage channels - Communicate by modifying a stored object

Covert Timing Channel A covert channel in which one process signals information to another by modulating its own use of system resources (e.g.CPU time) in such a way that this manipulation affects the real response time observed by the second process.is usually defined in terms of a real-time clock or a timer, but temporal relationships sometimes use neither. An ordering of events implies a time-based relationship that involves neither a real-time clock nor a timer.

Covert Storage ChannelCovert storage channels occur when out-of-band data is stored in messages for the purpose of memory reuse. Covert channels are frequently classified as either storage or timing channels. Examples would include using a file intended to hold only audit information to convey user passwords--using the name of a file or perhaps status bits associated with it that can be read by all users to signal the contents of the file. Steganography, concealing information in such a manner that no one but the intended recipient knows of the existence of the message, is a good example of a covert storage channel.

Covert Storage VulnerabilityConfidentialityCovert storage channels may provide attackers with important information about the system in question.

If these messages or packets are sent with unnecessary data contained within, it may tip off malicious listeners as to the process that created the message. With this information, attackers may learn any number of things, including the hardware platform, operating system, or algorithms used by the sender. This information can be of significant value to the user in launching further attacks. Examples of Covert Channels

Data Hiding in LAN Environment by Covert ChannelsAs Girling first analyzes covert channels in a network environment. His work focuses on local area networks (LANs) in which three obvious covert cannels (two storage channel and one timing channel) are identified. This demonstrates the real examples of bandwidth possibilities for simple covert channels in LANs. For a specific LAN environment, the author introduced the notion of a wiretapper who monitors the activities of a specific transmitter on LAN. The covertly communication parties are the transmitter and the wire trapper. The covert information according to Girling can be communicated through any of following obvious ways:I. By observing the addresses as approached by the transmitter. If total number of addresses, a sender can approach is 16, then there is a possibility of secret communication having 4 bits for the secret message. The author termed this possibility as covert storage channel as it depends in what is sent (i.e.. which address is approached by the sender)II. In the same way, the other obvious storage covert channel would depend on the size of the frame sent by the sender. For the 256 possible sizes, the amount of covert information deciphered from one size of the frame would be of 8 bits. Again this scenario was termed as the covert storage channel.III. The third scenario presented is pertaining to the existence sends can be observed by the wire trappers to decipher for instance “0” for the odd time difference and “1” for the even time difference.The scenario transmits covert information through “a when-is–sent” strategy therefore termed as timing covert channel. The time to transmit a block of data is calculated as function of software processing time, network speed, network block sizes and protocol overhead. Assuming block of various sizes are transmitted on the LAN, software overhead is computed on average and novel time evaluation is used to estimate the bandwidth (capacity) of covert channels are also presented. The work paves the way for future research.

Data Hiding in TCP/IP Protocol suite by Covert channelsA more specific approach is adopted by Rowland. Focusing on the IP and TCP headers of TCP/IP Protocol suite, Rowland devises proper encoding and decoding techniques by utilizing the IP identification field, the TCP initial sequence number and acknowledge sequence number fields. These techniques are implemented in a simple utility written for Linux system running version 2.0 kernels.Rowland simply provides a proof of concept of existence as well as exploitation of covert channels in TCP/IP protocol suite. This work can, thus, be regarded as a practical breakthrough in this specific area. The adopted encoding and decoding techniques are more pragmatic as compared to previously proposed work. These techniques are analyzed considering security mechanisms like firewall network address translation.However, the non-detectabilityof these covert communication techniques is questionable. For instance, a case where sequence number field of TCP header is manipulated, the encoding scheme is adopted such that every time the same alphabet is covertly communicated, it is encoded with the same sequence number.Moreover, the usages of sequence number field as well as the acknowledgment field cannot be made specific to the ASCII coding of English language alphabet as proposed, since both fields take in to account the receipt of data bytes pertaining to specific network packet(s).The Data Hiding in TCP/IP Protocol suit by Covert channels have following important aspects:Identify the existence of covert channels in a network environment.Point to devising satisfying techniques of embedding and extraction processes at the source and destination, respectively.Do not consider the effect of employing covert communications network as a whole.

Eliminating Covert ChannelsThe possibility of covert channels cannot be completely eliminated, although it can be significantly reduced by careful design and analysis.The detection of a covert channel can be made more difficult by using characteristics of the communications medium for the legitimate channel that are never controlled or examined by legitimate users. For example, a file can be opened and closed by a program in a specific, timed pattern that can be detected by another program, and the pattern can be interpreted as a string of bits, forming a covert channel. Since it is unlikely that legitimate users will check for patterns of file opening and closing operations, this type of covert channel can remain undetected for long periods.

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