authentication james walden northern kentucky university

Authentication

James Walden

Northern Kentucky University

CSC 666: Secure Software Engineering

Access Control

1. What is Access Control?2. Access Control Matrix Model

Protection State Transitions Special Rights Principle of Attenuation of Privilege

3. Groups and Roles4. Implementation of the Access Control Matrix

Access Control Lists: by column (object). Capabilities: by row (subject).

5. Access Control Flaws6. Web Access Control


What is Access Control?

“Its function is to control which principals (persons, processes, machines, …) have access to which resources in the system—which files they can read, which programs they can execute, how they share data with other principals, and so on.”

–Ross Anderson, Security Engineering


Access Control is Pervasive

Application

Middleware

Operating System

Hardware


Access Control is Pervasive

1. ApplicationComplex, custom security policy.Ex: Amazon account: wish list, reviews, CC

2. MiddlewareDatabase, system libraries, 3rd party softwareEx: Credit card authorization center

3. Operating SystemFile ACLs, IPC

4. HardwareMemory management, hardware device access.


Access Control Matrix

Precisely describes protection state of system.

Sets of system states: P: Set of all possible states. Q: Set of allowed states, according to security policy. P-Q: Set of disallowed states.

ACM describes the set of states Q.

PQ


Access Control Matrix

objects (entities)

subj

ects

s1

s2

…

sn

o1 … om s1 … sn

Objects O = { o1,…,om }

All protected entities.

Subjects S = { s1,…,sn }

Active entities, S

Rights R = { r1,…,rk }

Entries A[si, oj] R

A[si, oj] = { rx, …, ry } means the subject si has rights rx, …, ry over the object oj.


ACM Example

Processes bash(UID=1024), passwd(UID=0)

Files: .bashrc, /etc/passwd

Rights: r, w, x

.bashrc /etc/passwd bash passwd

bash rwx r rw rw

passwd rwx rw rw rw


Web ACM Example


Copy Right

Allows possessor to give rights to another Often attached to a right, so only applies

to that right r is read right that cannot be copied rc is read right that can be copied

Is copy flag copied when giving r rights? Depends on model, instantiation of model


Ownership Right

Usually allows possessor to change entries in ACM column. Owner of object can add or remove rights for

other subjects. May depend on what system allows

- Can’t give rights to specific (set of) users.- Can’t pass copy flag to specific (set of) users.


Attenuation of Privilege

Principle: Subject may not give rights it does not possess to another. Restricts addition of rights within a system. Usually ignored for owner

- Why? Owner should have ability to recover rights on object if desired.


How can we implement the ACM?

Problem: scale Thousands of subjects. Millions of objects. Yet most entries are blank or default.

Solutions Group subjects together as a single entities

- Groups and Roles

Implement by row: Capabilities Implement by column: Access Control Lists


Groups and Roles

Collect subjects together to express: Need to share objects. Security categories (e.g., admin, faculty,

student, guest.)

role: group tying membership to function. Problem: loss of granularity.


Capabilities Implement ACM by row. Access Control associated with subject. Example: Web authenticated session IDs. Example: UNIX file descriptors

System checks ACL on file open, returns fd. Process subsequently uses fd to read and write file. If ACL changes, process still has access via fd.

User ls homedir rootdir

james rx rw r


Capability Questions

1. How to prevent user from modifying capabilities?

2. How to prevent user from copying capabilities?

3. How to revoke rights to an object?


How to prevent modification?

Memory protection Capabilities are readable, but not writable.

Indirection Capability is pointer to per-process table

whose access control prevents user from touching.

Cryptography Cryptographically secure checksum

associated with capability and checked before usage.


How to prevent user from copying?

Copying capabilities allows users to grant rights to others.

Solution: Use indirection or cryptographic techniques

from prev slide to prevent direct access. Add copy flag to capability, as a specific right

given to copy capabilities in order to give rights to other users.


How to revoke rights to an object?

Direct solution Check capabilities of every process. Remove those that grant access to object. Computationally expensive.

Alternative solution Create a global object table. Capabilities reference objects indirectly via

their entries in the global object table. Invalidate entry in global object table to

revoke.


Access Control Lists (ACLs)

Implement ACM by column. Access control by object. Example: UNIX ACLs

Short “rwx” user/group/other. Long POSIX ACLs.

User audit data

root rw

james r

joe


Access Control Flaws

No access control Relying on attacker not knowing URL.

Client-side access control Relying on a URL parameter like admin=true. Relying on the Referer header.

Sequence based access control Relying on attacker accessing pages in

correct sequence.


Securing Access Controls

Requirements Create ACM based on roles, resources.

Design Single point of control for AC decisions. Base AC decisions on user’s session. Apply additional AC for admin pages, such as

IP range requirements. Use per-transaction re-authentication for high

impact transactions. Log events where sensitive data accessed.


Defense in Depth

Network Access Control Prevent access to unauthorized ports. Prevent access from unauthorized IPs.

Web Server Access Control Use web server controls to prevent access to static

resources except through application.Application Access Control

Custom AC for your application.Database Access Control

Use different DB accounts for different roles. Fine-grained AC at the row or column level.

Operating System Access Control Use least privilege accounts for application, servers.


Key Points

Access Control Matrix Implementation as capabilities or ACLs.

Common Flaws No AC or relying on client-side AC.

Defense in Depth AC at Network, Server, App, DB, and OS.


References

1. Matt Bishop, Computer Security: Art and Science, Addison-Wesley, 2005.

2. Brian Chess and Jacob West, Secure Programming with Static Analysis, Addison-Wesley, 2007.

3. PCI Security Standards Council, PCI DSS Requirements and Security Assessment Procedures, v1.2, 2008.

4. Dafydd Stuttart and Marcus Pinto, The Web Application Hacker’s Handbook, Wiley, 2008.

authentication james walden northern kentucky university

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