Outline for Today’s Lecture

Administrative: – If you weren’t here last time, pick up

midterm– Assignment 3 still in progress

Objective: – Security– Access Control Mechanisms

Grades*

0

1

2

3

4

5

6

7

89-100 79-88 69-78 59-68 <60

Median = 80 Mean = 75.8

A

A-/B+

B

B-/C+

C-/D*

* = remedialconcurrency problem

*histogram not updatedafter grade changes

Step 0: Basic Mechanisms

Discussed in previous lecture:

• symmetric key cryptography

• public key cryptography

• digital signatures

• one-way functions

• hashes

Step 1: User Authentication

Basic Principles. Authentication must identify:

1. Something the user knows

2. Something the user has

3. Something the user is

This is done before user can use the system for access control

Authentication Using Passwords

(a) A successful login(b) Login rejected after name entered(c) Login rejected after name and password typed

Authentication Using Passwords

• How a cracker broke into LBL– a U.S. Dept. of Energy research lab

Login Spoofing

(a) Correct login screen(b) Phony login screen

Authentication Using Passwords

The use of salt to defeat precomputation of encrypted passwords

Salt Password

,

,

,

,

One-Time PasswordsUsing 1-way function:• Function such that given formula for f(x)

– easy to evaluate y = f(x)• But given y

– computationally infeasible to find x

• One-time passwords– Choose password s and integer n– 1st time P1=f(f(f(f(s)))), 2nd time P2= f(f(f(s))), etc– Login name supplies current integer value– Server stores old password, f(newpassword)==old?

Challenge - Response

• Sets of question – answer pairs– Server picks one and asks– User knows answer

• User picks function f(x)– Server sends a value for x– User sends back f(x) as password

• Using symmetric encryption– Server sends random value r– User encrypts with secret key – e(r,k)

• Server compares

Graphical Challenge-Response

Authentication Using a Physical Object

• Magnetic cards– magnetic stripe cards– chip cards: stored value cards, smart cards

• RFIDs

Authentication Using Biometrics

• A device for measuring finger length.

• Retinal scans• Voice recognition• Surveillance tech

– Image analysis– Gait analysis

Countermeasures

• Limiting times when someone can log in• Automatic callback at number

prespecified• Limited number of login tries• A database of all logins• Simple login name/password as a trap

– security personnel notified when attacker bites

Step 2: Access Control Mechanisms

Protection Domains

Examples of three protection domains

The Access Model• Authorization problems can be represented

abstractly by of an access model.– each row represents a subject/principal/domain– each column represents an object– each cell: accesses permitted for the {subject,

object} pair• read, write, delete, execute, search, control, or any other

method

• In real systems, the access matrix is sparse and dynamic.

• need a flexible, efficient representation

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Access Control Matrix

• Processes execute in a protection domain, initially inherited from subject TA

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Two Representations• ACL - Access Control Lists

– Columns of previous matrix– Permissions attached to Objects– ACL for file hotgossip: Terry, rw; Lynn, rw

• Capabilities– Rows of previous matrix– Permissions associated with Subject– Tickets, Namespace (what it is that one can name)– Capabilities held by Lynn: luvltr, rw; hotgossip,rw

File Access Control

• Access control lists - detailed list attached to file of users allowed (denied) access, including kind of access allowed/denied.

• UNIX RWX - owner, group, everyone• Capabilities – permitted accesses associated

with subject (user), similar to an address space.– Un-forgeable object reference, like a pointer.

Trusted SystemsTrusted Computing Base

A reference monitor

Access Control Lists

• Approach: represent the access matrix by storing its columns with the objects.

• Tag each object with an access control list (ACL) of authorized subjects/principals.

• To authorize an access requested by S for O– search O’s ACL for an entry matching S– compare requested access with permitted access– access checks are often made only at bind time

Access Control Lists

Use of access control lists of manage file access

R

Access Control Lists

Two access control lists

UNIX access control

• Each file carries its access control with it.

rwx rwx rwx setuid

OwnerUID

GroupGID

Everybody else

• Owner has chmod, chgrp rights (granting, revoking)

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Access Control Matrix

• How to encode in Unix? TA

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Capabilities• Approach: represent the access matrix by

storing its rows with the subjects.• Tag each subject with a list of capabilities for the objects it

is permitted to access.

– A capability is an unforgeable object reference, like a pointer.

– It endows the holder with permission to operate on the object

• e.g., permission to invoke specific methods

– Typically, capabilities may be passed from one subject to another.

• Rights propagation and confinement problems

Capabilities

Each process has a capability list

• Cryptographically-protected capability

• Generic Rights1. Copy capability2. Copy object3. Remove capability4. Destroy object

Capabilities

Server Object Rights f(Objects, Rights, Check)

Dynamics of Protection Schemes

• How to endow software modules with appropriate privilege?– What mechanism exists to bind principals with

subjects?• e.g., setuid syscall, setuid bit

– What principals should a software module bind to?• privilege of creator: but may not be sufficient to perform

the service• privilege of owner or system: dangerous

Dynamics of Protection SchemesProblems:• Mutual Suspicion

When a caller requests a service, the callee should be limited in what stuff of the caller’s it has access to (only what it is passed). (entering domain - setuid vs. passing in single object)

• Modification When caller does pass an object as parameter, we want to control whether

the callee can modify it (I send you a photo, you remove my copyright mark)

• Conservation When caller passes an object to gain service, we want to know that the

callee doesn’t retain or even pass on the right to the object after we think they are done with it.

(our credit card number – and selling such databases)

• Confinement We want to prevent information leaks (besides the objects themselves)

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Dynamics of Protection Schemes

• How to revoke privileges?• What about adding new subjects or new

objects?• How to dynamically change the set of objects

accessible (or vulnerable) to different processes run by the same user?– Need-to-know principle / Principle of minimal

privilege– How do subjects change identity to execute a more

privileged module?• protection domain, protection domain switch (enter)

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Protection Domains• Processes execute in a

protection domain, initially inherited from subject

• Goal: to be able to change protection domains

• Introduce a level of indirection

• Domains become protected objects with operations defined on them: owner, copy, control

TA

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Domain0

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• If domain contains copy on right to some object, then it can transfer that right to the object to another domain.

• If domain is owner of some object, it can grant that right to the object, with or without copy to another domain

• If domain is owner or has ctl right to a domain, it can remove right to object from that domain

• Rights propagation.

TA

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UNIX access control

• Each file carries its access control with it.

rwx rwx rwx setuid

OwnerUID

GroupGID

Everybody else When bit set, itallows processexecuting objectto assume UID ofowner temporarily -enter owner domain(rights amplification)

• Owner has chmod, chgrp rights (granting, revoking)

Capability Calling Mechanism

• Assume a capability to execute a procedure (lpr)

• Parameter is a capability to an object of mine (read access to file foo)

• lpr’s c-list has a template capability for the parameter, requires that certain rights are passed in (if lpr can’t read the file foo, it obviously can’t do its job)

• Sometimes, need rights amplification to do the job.

Dynamics of Protection SchemesProblems:• Mutual Suspicion

enter my domain vs. me, as owner, granting right to another domain (temp)

• Modification rights for particular operations

• Conservationctl – being able to revoke rights; not granting copy rights

• Confinement remove rights to write or create anything else?

Multilevel Security

The Bell-La Padula multilevel security model

Risks

Trojan Horses: Inside Jobs• Free program made available to unsuspecting user

– Contains code to do harm– Example of tricking user, himself or herself, into running that program

• Place altered version of utility program on victim's computer, say, in some bin directory that might be in their PATHs

• Lay a trap for sysadmin to gain root privilege– Install local version of ls; then do something that causes sysadmin to

% cd yourHomeDir% ls -l

Logic Bombs

Company programmer writes program– potential to do harm– OK as long as he/she enters password daily; his/her

name stays on the paycheck database; etc.– if programmer fired, no password and bomb explodes

Trap Doors

(a) Normal code.

(b) Code with a trapdoor inserted

Generic Security AttacksTypical attacks• Request “free” memory, disk space, tapes and just read

what was left there (not zero filled on dealloc)• Try illegal system calls – if the system gets confused

enough, you may be in.• Start a login and hit DEL, RUBOUT, or BREAK to possibly

kill password checking• Try modifying complex OS structures kept in user space (if

any)• Try to do specified DO NOTs• Convince a system programmer to add a trap door• Beg admin's sec’y to help a poor user who forgot password

Buffer Overflow

(a) Situation when main program is running

(b) After procedure A is called

(c) Fixed sized buffer B - overflow shown in grayUser supplies long enough string to overwrite return address

UNIX lpr utility, option to remove file after printing; print and remove password file

Link file core in working directory to the password file. Force a core dump of a SETUID program, writes on the core file, overwriting password file

Famous Security Flaws

Famous Security Flaws

The TENEX – password problem• Page faults could be monitored by user – user func. called• One char at a time password checking – either illegal or fault

(a) (b) (c)

Design Principles for Security

1. System design should be public – open source2. Default should be no access3. Check for current authority – e.g. not just at “open”4. Give each process least privilege possible5. Protection mechanism should be

- simple- uniform- in lowest layers of system

Security is not an add-on feature6. Scheme should be psychologically acceptable

And … keep it simple

From the Outside:Network Security

• External threat– code transmitted to target machine– code executed there, doing damage

• Goals of virus writer– quickly spreading virus– difficult to detect– hard to get rid of

• Virus = program can reproduce itself by attaching its code to another program– additionally, do harm

Virus Damage Scenarios

• Blackmail• Denial of service as long as virus runs• Permanently damage hardware• Target a competitor's computer

– do harm– espionage

• Intra-corporate dirty tricks– sabotage another corporate officer's files

How Viruses Work

• Virus usually written in assembly language• Inserted into another program

– use tool called a “dropper”

• Virus dormant until program executed– then infects other programs– eventually executes its “payload”

• possibly waits for significant date

• Types: companion, executable program, memory, boot sector, device driver, macro, source code

Bait and switch – companion viruses• MS-DOS example:

run progsystem tries prog.com then prog.exeRelease a prog.com for a popular prog.exe

How Viruses Work

How Viruses Work

Recursive

procedure that

finds executable

files on a UNIX

system

Virus could

infect them all

How Viruses Work

• An executable program• with a parasitic virus at the front• at the end• spread over free space within program (cavity virus)

Boot sector viruses1st hide the real boot sector

When booted, copies virus into memory, making it a memory resident virus

Then boots the OS

Device driver infected with virus, loads it at boot time.

How Viruses Work

How Viruses Work

• After virus has captured interrupt, trap vectors– Syscall trap a good one. Can look for exec calls

• After OS has retaken printer interrupt vector• After virus has noticed loss of printer interrupt vector and recaptured it

Macros

Applications like Word or Excel allow macros that get executed via keystroke or menu

Attach a macro to open file function and you are off and running

Can be sent in email attachments

Some emailers automatically open attachments

How Viruses Work

How Viruses Spread

• Virus placed where likely to be copied

• When copied– infects programs on hard drive, floppy– may try to spread over LAN

• Attach to innocent looking email– when it runs, use mailing list to replicate

Antivirus and Anti-Antivirus Techniques

(a) A program(b) Infected program, metadata giveaways(c) Compressed infected program(d) Encrypted virus(e) Compressed virus with encrypted compression code

Antivirus and Anti-Antivirus Techniques

Examples of a polymorphic virusAll of these examples do the same thing

Mutation engine – code that morphs the signature part of the virus each time it spreads

Antivirus and Anti-Antivirus Techniques

• Integrity checkers - checksums• Behavioral checkers• Virus avoidance

– good OS– install only shrink-wrapped software– use antivirus software– do not click on attachments to email– avoid active content– frequent backups

• Recovery from virus attack– halt computer, reboot from safe disk, run antivirus

The Internet Worm

• Worm = replicating program• Nov. 1988, Robert Morris, Cornell grad student• Consisted of two programs

– bootstrap to upload worm– the worm itself

• Worm first hid its existence• Next replicated itself on new machines

– rsh– finger name@site - overflow finger daemon’s stack with long string– Bug in sendmail to mail bootstrap & exec it– Tried to break user passwords and go on

• Too aggressive – let 1 in 7 re-infects live• CERT – Computer Emergency Response Team – collects info on

system flaws that can be attacked. Fields reports of security break-ins

Mobile Code Sandboxing

(a) Memory divided into 1-MB sandboxes

(b) One way of checking an instruction for validity

Mobile Code

Applets can be interpreted by a Web browser

Mobile Code

How code signing works

Covert Channels

Client, server and collaborator processes

Encapsulated server can still leak to collaborator

via covert channels

Covert Channels

A covert channel using file locking

Covert Channels• Pictures appear the same• Picture on right has text of 5 Shakespeare plays

– encrypted, inserted into low order bits of color values

ZebrasHamlet, Macbeth, Julius CaesarMerchant of Venice, King Lear

Is it a Technical Problem?

Lots of known solution techniques

• Access control

• Crypto

• Firewalls

• Intrusion detection

So why isn’t it a solved problem?

Economics

“The party who is in a position to protect a system is not the party who would suffer the results of security failure.”

Ross Anderson

Security• For whom is it built?• Who pays for it?