Design Principles for Secure Mechanisms

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Design Principles for Secure Mechanisms
C. Edward Chow
CS591 Chapter 5.4 Trusted OS DesignCS691
Chapter 13 of Matt Bishop
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Design Principles for Security Mechanisms

• Based on the ideas of simplicity and restriction.
• J. Saltzer and M. Schroeder Proceeding of IEEE
  1975 describes 8 principles for security
  mechanism
• Least Privileges
• Fail-Safe Defaults
• Economy of Mechanism
• Complete Mediation
• Open Design
• Separation of Privilege
• Least Common Mechanism
• Psychological Acceptability

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Overview

• Simplicity makes designs and mechanisms easy to
  understand.
• Simplicity reduces the potential for
  inconsistencies within a policy or set of
  policies.
• Minimizing the interaction of system components
  minimizes the number of sanity checks on data
  being transmitted among components.
• Restriction minimizes the power of an entity.
• The entity can access only information it needs.
• Only communicates with other entities when
  necessary, and in as few and narrow ways as
  possible.

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Examples

• Sendmail reads configuration data from a binary
  file, compiled (freezing) from a text version of
  the configuration file.
• 3 interfaces
• The mechanism that edits the text configuration
  file.
• The mechanism that comples (freezes) the text
  file.
• The mechanism sendmail used to read the binary
  (frozen) file.
• Version control problem. What if text
  configuration file is newer than the binary file.
  Sendmail warns the user?
• Should sendmail rechecks the parameters in
  configuration file?
• If the compiler allows the string name as default
  UID (daemon) while the sendmail accepts only
  integer as UID, the input routine of sendmail
  will read daemon and return error value 0. 0 as
  UID is root!

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Example for Avoiding Inconsistency in Policies

• Policy rule1 TA needs any cheating.
• Policy rule2 ensure the privacy of student
  files.
• Case
• TA reminds student file not submitted.
• Student ask TA to look for files in students
  directory.
• TA finds two files. Unsure which files.
• TA reads the first file, it turns out to be
  written by other
• TA reads the 2nd file, it turns out identical
  except names.
• TA reports the cheating.
• Student charges TA violating his privacy by
  reading the first set of files.

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Examples of Restrictions

• Government officials are denied access to info
  for which they have no need (the need to know
  policy). They cannot communicate that which they
  do not know.
• Case Imparting information by not
  communicatingBernstein and Woodward, Watergate
  reporters, ask the source to hang up if
  information was inaccurate, remain on the line if
  the information was accurate.

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Principle of Least Privileges

• A subject should be given only those privileges
  that it needs in order to complete its task.
• Exception case? for certain action, subjects
  access right can be augmented but relinquished
  immediately on completion of the action.
• Append right? vs. write right
• In practice, most system does not have the
  granularity of privileges and permission required
  to apply this principle precisely.
• The designers of mechanisms try their best?
• Does the root/administrator user concept violate
  the above rule?

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Example of Tomcat User Access Control Files

• lt?xml version'1.0' encoding'utf-8'?gt
• lttomcat-usersgt
• ltrole rolename"cs526stu"/gt
• ltrole rolename"softwareRequester"/gt
• ltrole rolename"tomcat"/gt
• ltrole rolename"cs526prof"/gt
• ltrole rolename"role1"/gt
• ltrole rolename"manager"/gt
• ltrole rolename"admin"/gt
• ltuser username"cs526stu" password"xxxx"
  roles"cs526stu,manager"/gt
• ltuser username"softwareRequester"
  password"sesame" roles"softwareRequester"/gt
• ltuser username"tomcat" password"xxxx"
  roles"tomcat"/gt
• ltuser username"cs526prof" password"xxxx"
  roles"tomcat,cs526prof,manager,admin"/gt
• ltuser username"role1" password"xxxx"
  roles"role1"/gt
• ltuser username"both" password"xxxx"
  roles"tomcat,role1"/gt
• lt/tomcat-usersgt
• User with Admin role can start/shutdown the
  Tomcat web server.
• User with Manage role can insert/delete web
  applications.
• User with cs526stu role can read cs526 web
  pages.

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Mail Server Access Rights

• Mail server accepts mail from Internet and copies
  the msgs to a spool directory.
• A local server will complete delivery.
• Mail server needs rights
• to access network port 25,
• To create files in the spool directory
• To alter those files (copy msg to file, rewrite
  delivery address if needed, incase of aliases?)
• It should surrender the right when finishes.
• It should not access the users files.
• Local server only has the read and delete access
  to spool direcotry.
• The admin should only be able to access subjects
  and objects involved in mail queueing and
  delivery, in case it is compromised??

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Principle of Fail-Safe Defaults

• Unless a subject is given explicit access to an
  object, it should be denied access to that
  object.
• If the subject is not able to complete its
  action/task, it should undo those changes it made
  in the security state of the system before it
  terminates. If the program fails, the system is
  still safe.
• What happens if the program crashes, not fails?
• Mail server should not write msg to a different
  directory than spool (if it is full). It should
  just close the network connection, issue an error
  msg and stop.

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Principle of Economy of Mechanism

• Security mechanisms should be as simple as
  possible.
• Fewer errors less checking and testing
• Bad example Mechanism on host A allows access
  based on the ident protocol. Ident protocol
  sends the user name associated with a process
  that has a TCP connection to a remote host. A
  compromised host can send any identity.
• Interface to other modules are particular
  suspect.
• Example of DoS attack using Finger protocol. It
  returns infinite streams of characters. Client
  will crash.

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Principle of Complete Mediation

• All accesses to objects be checked to ensure that
  they are allowed.
• Unfortunately, most OS will check the access
  right when the object was open, but will not
  check access right again when the client program
  reads. The OS cached the results of the first
  check.
• If the owner disallows reading the file after the
  file descriptor is issue, the kernel will still
  allow the client process to read.
• DNS server caches Domain name-IP address entries.
  The attacker can poison the cache with
  incorrect entries, a host will be routed to a
  spoof site.

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Principle of Open Design

• The security of a mechanism should not depend on
  the secrecy of its design or implementation.
• Attack such as disassembly and analysis,
  dumpster-diving for source code,
• Isnt Security through obscurity a good
  principle?
• Example cryptograph software, algorithms.
• How about proprietary softare/trade secrets?
• The famous Content Scramble System (CSS). 1999
  break by Norway group. Plaitiffs lawyer filed
  law suit containing the source code!

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Principle of Separation of Privilege

• A system should not grant permission based on a
  single condition.
• Separation of duty principle.
• Company checks gt 75K signed by two officers.
• Berkeley Unix allows a user to change to root if
• The user knows root password.
• The user is in the wheel group (the group with
  GID 0).

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Principle of Least Common Mechanism

• Mechanisms used to access resources should not be
  shared.
• Virtual machie/memory concept follows this.
• Internet access route should not be shared?
• How to restrict the attackers access to the
  segment of Internet connected to a web site?
• Purdue SYN intermediary system.
• Secure Collective Defense Project.

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Principle of Psychological Acceptability

• Security mechanisms should not make the resource
  more difficult to access than if the security
  mechanisms were not present.
• Example SSH.
• Example not allow access after 3 tries.