Transcript ACCESS CONTROL

Summary of Lecture 4
Authentication (Review)
Authentication means
 to establish the proof of identity.
 Authentication techniques may vary
depending on the kind of resource being
accessed.
 The various kinds of access can be
classified into
– user-to-host
– host-to-host
– user(or process) –to – user (process)
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Authentication is done by
 by something you are (SYA)
 by something you know (SYK)
 by something you have (SYH)
– SYA is more reliable and accurate compared
to SYH.
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Authentication
 SYK is the most commonly used end-user
authentication (user to systems).
– e.g: user name and password.
 Can also be applied to programs that
exchange the data over the network without
human intervention.
 The strength of SYK authentication
depends on whether what is known is a
secret, and can be kept as a secret.
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User-to-Host authentication
 Typical methods are
– static passwords
– challenge and response
– one-time passwords
– trusted third parties
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Today’s lecture will be
 So far we have discussed how to
authenticate user to host.
 Once user is logged on the system, we need
a mechanisms to control the access of
objects (such as files, programs,
processes,etc.) within the system.
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ACCESS CONTROL &
SECURITY MODELS
Center of gravity of computer
security
Fundamental Model of Access
Control
subject
Access request
Reference
Monitors
object
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Controlling Access
 Access control policy: what can be used to
indicate who is allowed to do what to/with whom
on the system.
 Who is who ?
 Subject is what we call active entities
(processes, users, other computers) that want to “do
something”
 The what the subject does with the object can be
just about anything, and it may be multi-part.
 Typical manipulations include READ, MODIFY,
CREATE, CHANGE, DELETE
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Access Control Policy
 Access right or privilege:
– An indication that a SUBJECT may
legitimately use a specific type of ACCESS
or MANIPULATION with respect to a
particular OBJECT or set of OBJECTS.
 The underlying system itself determines
which primitive (or bottom level) access
rights are available for which
user/object combinations
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Levels of Access Control
 Application
 Middleware
 Operating system
 Hardware
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Operating System Access Controls
 Authenticate prinicipals/users
– Passwords
– Kerberos
 Mediate access
– Files
– Communication ports
– System resources
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Models of Security
 Need for a model
– High assurance security system
 What a model supposed to do?
– Express the security policy in a formal way
– Describe the entities governed by the policy
– State the rules that decide who gets access to
your data
 Scope and limitations of models
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Security Models : Bell-LaPadula
– The Bell-LaPadula model is about
information confidentiality, and this
model formally represents the long
tradition of attitudes to the flow of
information concerning national
secrets.
– Multi-level security (MLS)
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Security Models: Chinese Wall
– Large consultancies can easily find
there are conflicts of interest if
individual consultants are given
access to all information held by the
consultancy.
Chinese Wall models a particular
way of restricting information flow.
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Security Models : Biba
 We need models – continued
 Based on the Cold War experiences,
information integrity is also
important, and the Biba model,
complementary to Bell-LaPadula, is
based on the flow of information
where preserving integrity is critical.
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Security Models: Clarke-Wilson
 In the commercial sphere, the need is to
engage in well-formed transactions which
can only be undertaken by authorised
personnel, and the Clarke-Wilson model is
an attempt to formally model a policy based
on well-formed transactions.
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Possible Access Control
Mechanisms are
 Control Matrix
 Control lists
 Groups and Roles
 Extension to Distributed (+file) Systems
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Access Control Matrix
Object
Operating
system
Accounts Accounting
Program
Data
Audit
Trail
Users
Sam
rwx
rwx
rw
r
Alice
x
x
rw
-
Bob
rx
r
r
r
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Example Access Control Matrix for
Bookkeeping
Operating
system
Accounts Accounting
Program
Data
Audit
Trail
Sam
rwx
rwx
r
r
Alice
rx
x
-
-
Accounts
program
Bob
rx
r
rw
w
rx
r
r
r
Srini
rx
r
r
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Access Control Matrices
 2/3 dimensions used to implement
protection mechanisms and model them
 Do not scale well
– A bank with 50,000 staff & 300 objects 
15million entries
– Update and performance problem
– Prone to administrators’ mistakes
 A more compact way is required
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Groups and Roles
 Group is a list of users/principals--
categories
 Role is a fixed set of access permissions
that one or more principals may assume
 Group manager is a rank while the role of
acting manager can be taken up by an
assistant accountant standing in while the
manager, deputy manager and accountant
are all sick
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Let us look at the example once again
Operating
system
Accounts Accounting
Program
Data
Audit
Trail
Sam
rwx
rwx
r
r
Alice
rx
x
-
-
Accounts
program
Bob
rx
r
w
w
rx
r
r
r
Srini
rx
r
r
r
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ACLs per subject(Capabilities
list)
Sam
Alice
Bob
Srini
User
rx
Acc.
pgm
rx
rwx
rx
rx
OS
rwx
x
r
r
r
r
-
rw
r
r
r
-
w
r
r
A/C
Prgm
A/C
Data
Audit
trail
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Access Control Lists
User
Accounting Data
Sam
rw
Alice
rw
Bob
r
Srini
r
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Access Control Lists/Capabilities
 How do you modify the entries in the lists?
– add a new entry
– delete an existing entry
– modify the access right to an object?
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Access Control Triples
 Subject
 Object
 Access  r, w, x, ?
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Capabilities
 While ACLs are kept by the O/S,capabilities
are kept by the subject.
 Capabilities give the possessor (of the token)
certain rights to an object
 Capabilities do not require authentication of
subjects, but do require that the token be
unforgeable (encrypted or in inaccessible
storage) and that the propagation of
capabilities be controlled.
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Access Control lists (cont.)
 Users manage their own file security, Unix
 Data-oriented protection, for centrally set access
control policy
 OS checks the ACL at each file access
 Not efficient security checking at runtime, though
simple to implement
 Tedious to find all files to which a user has access
or perform system-wide checks
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Let us look at an example of ACL
implementations
UNIX
NT
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Unix Operating System Security
 Superuser account on Unix is root
– UID (user identifier) equal to ‘0’
 The superuser can effectively do anything
within the system
 Superuser password is the most valuable
password in the system
 Don’t share the superuser password outside
the administrative group.
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Basic file security
-rw-rw-r--
1 root sys
1344 Jul
2 22:57 /etc/vfstab
Others
Group
Owner
Access Control
-rwxrwxrwx
Owner permissions
-rwxrwxrwx
Group permissions
-rwxrwxrwx
Other permissions
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Basic file security
 Important system files must have appropriate file
permissions
 e.g:
-r--r--r--r--------rw-r--r-drwxr-xr-x
1
1
1
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root
root
root
root
other
sys
sys
sys
/etc/passwd
/etc/shadow
/etc/profile
/usr
 A finer granularity of file permissions can be
achieved with access control lists (ACLs), e.g.
AIX, HP-UX.
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Unix Operating System
Security(cont.)
 A common defense against root compromise
by hackers -- is system log to a printer in a
locked room or to another machine/server,
eg. Berkeley, FreeBSD
 ACLs have only names of users, not of
programs
 Indirect method => suid and sgid file
attributes
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SUID and SGID Security
 Owner of a program can mark it as suid,
enabling a user, special privileges of access
control attributes
 sgid for groups
 What is the security issue here?
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SUID and SGID Security(cont.)
 SUID root programs are particularly vulnerable to
attack.
 If it is possible to subvert the program in some
way, then root access can be gained.
 A very well known method of such subversion is
the buffer overflow.
 Buffer overflow vulnerability results from bad
coding practices on the part of the original
programmer of the SUID root program!
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