CS363Week 6 - Monday

Last time

What did we talk about last time? Quantum cryptography Non-malicious program errors

Questions?

Assignment 2

Project 2

Non-malicious program errors

Incomplete mediation

Incomplete mediation happens with a system does not have complete control over the data that it processes

Example URL: http://www.security.com/query.php?date=2012March20

Wrong URL: http://www.security.com/query.php?date=2000Hyenas

The HTML generates the URL, but the URL can be entered manually

Incomplete mediation security For a website, a carelessly altered

URL might just mean a 404 error For a program, bad data could cause

any number of faults and failures Malicious attackers could change

data or mount an SQL injection attack to destroy or reveal database internals

Values should always be checked and sanitized

Bobby Tables

File CommandMyFile.txt Change byte 4 to

'A'

File CommandYourFile.txt Delete file

Time-of-check to time-to-use It seems like things happen instantly in a computer Many operations, especially those on files, may be

put into a queue of work Imagine you give the OS a data structure with this

command:

After it is authorized but before it can be executed, you change it to:

Undocumented access point A program might have a way to access

its private internal data These access points are called

backdoors or trapdoors During development, these backdoors

can be really useful for debugging In production, they cause a security risk,

either because the developers can have control they shouldn't or because other attackers can exploit the backdoor

General programming errors Integer overflow and underflow

Someone ordered -2 billion oranges? Unterminated C-style string

A C-style string ends with the null character ('\0') Without the null character, string processing

functions might keep reading (or writing) into memory

Race conditions In multi-threaded environments, data can be

updated by multiple threads, leading to inconsistent (and unpredictable) results

Case Study: Therac-25

Therac-25 background Therac-25 was a radiation therapy machine built by

the Atomic Energy of Canada Limited It was the successor to the Therac-6 and Therac-20

machines The machine had low power and high power modes The low power mode shot a beam directly at the

patient The high power mode created X-rays by shooting

the beam at a target, spread these X-rays with a flattening filter, shaped the beam with movable blocks, and tested the strength of the beam with an X-ray ion chamber

Tragedies

In some situations, the high power beam was activated without the spreader in place

The software and hardware systems did not catch this particular problem

Over 100 times the intended dose was given

At least 2 people died and there were at least 6 overdoses total

Software bugs actually kill people!

Direct causes A certain unusual combination of keystrokes

had to happen within 8 seconds There were no hardware interlocks to prevent

the problem if the user overrode the error code

Error codes were not well-documented and were displayed as a number

Software was reused from previous models that did have hardware interlocks

Arithmetic overflow caused safety checks to fail in some cases

Indirect causes

The software/hardware combination had never been tested before use

Personnel did not believe complaints due to confidence in the system

Code was not independently reviewed

Errors were easily overridden

Malicious Code

Malicious code Obviously, it’s a problem It’s very difficult to stop

You never really know what’s getting installed on your computer

You’re downloading thousands of files from the Internet every day

Even if you had the source code for every program, could you catch all the dangerous stuff?

Malicious code has been around since at least 1970

Terminology Malicious code (or a rogue program) is our

blanket term for any code that has undesirable effects that were intentionally designed

The agent is the person who writes the code A virus is a program that can replicate itself

and add malicious code to nonmalicious programs A transient virus runs when its host program is

running A resident virus lives in memory and can be

active anytime

Viruses Terminology is inconsistent Popular culture tends to call everything a virus Sometimes we will too, but here are some other terms:

Almost all of these are, by definition, Trojan horses Worms differ from viruses primarily because they spread across networks

Type Characteristics

Virus Attaches itself to a program and propagates copies of itself to other programs

Trojan horse Contains unexpected, additional functionalityLogic bomb Triggers action when condition occursTime bomb Triggers action when specified time occursTrapdoor Allows unauthorized access to functionality

Worm Propagates copies of itself through a networkRabbit Replicates itself without limit to exhaust resources

How viruses attach

A virus is not dangerous unless it is active

Just having an infected file on your hard drive won’t cause a problem unless it is accessed

But files get opened all the time Programs call other programs Just previewing files can be dangerous E-mail programs open attachments

automatically How do these viruses infect code?

Appended viruses A virus can be designed so that it

starts running before the real program does Machine code for the virus is inserted

before the machine code for the beginning of the program

After the virus runs, it transfers control to the real program

The real program runs as if nothing happened

This kind of virus is easy to write It is also relatively easy to catch for

antivirus software

Program Code

Virus Code

+

Program CodeVirus Code

Surrounding or integrating Another possibility is viruses that surround a program,

gaining control before and after execution The code may not be at the beginning and end of the executable,

but that’s how the control flow works Viruses can also be spread throughout the code

Program Code

Virus Code

+

Program CodeVirus Code

A

Virus Code

B

Program Code

Virus Code

+

Infected Program Code

Document viruses

According to the book, the most common form of virus is a document virus

A document virus is an infected document (instead of an executable file)

Nevertheless, the macros that can be stored in Word, Excel, Access, and other similar complex documents are powerful enough to cause just as much damage as any other virus

Where Viruses Live

The perfect virus If you are making a virus, the following

characteristics are ideal: Hard to detect Difficult to destroy or deactivate Spreads infection widely Capable of reinfecting its host or other programs Easy to create Machine and OS independent

It’s difficult to make a virus that meets all these criteria

One-time execution

Many viruses will be executed just once

This could be on running a pirated (and infected) file

One of the most common avenues of attack is through an e-mail attachment

Boot sector viruses The boot sector is the part of a hard drive

that says what code to load to start your OS The details are technical, but a boot sector

virus is one that is stored in the chain of code that starts up your whole computer

A virus that can start this early can circumvent or disable antivirus

It has complete control over your system It is also not obvious from the file system

Memory resident viruses Some programs start up and then never

really die They are low level parts of the OS that need

to keep running Sometimes called TSR (terminate and stay

resident) Because these programs are always

running, they are an attractive home for a virus

Even if you delete the original infected file, the memory resident virus can replace it

Somewhere else… As with everything in security, the

assumption is that attackers do not play by the rules

A virus does not have to live where we expect it to

A few other places that are sensible: Applications Libraries Compilers (infect programs as you create them) Antivirus software

Virus Signatures

A fundamental problem with looking for Trojan horses Ken Thompson's seminal paper Reflections on

Trusting Trust: He added a backdoor to the Unix login program Too easy to trace, so he added a backdoor to the C

compiler to insert the backdoor in any program called login

Too easy to trace, so he added a backdoor in the compiler compiler to insert code that would insert the backdoor in any program called login

And so on, and so on… You can’t trust anything you didn’t completely

create yourself Some amount of trust is necessary

Virus signatures

Viruses are difficult to detect, but we can still classify them by the way they change code or the way they execute

We call these tell-tale signs a signature

Antivirus programs work by searching for certain signatures in code

Storage patterns At simplest, this is just a particular string of code in

the binary Often this code is at the beginning of a program so

that it gets control immediately Craftier viruses will put themselves other places

that get jumped to early in execution An antivirus program can check:

The size of a file The functioning of the code compared to some standard It can look for suspicious execution patterns (weird JUMP

commands) The program against a hash digest for the program

Execution patterns

Viruses are also suspicious because of the way they execute

The virus should: Spread infection Avoid detection Cause harm

How do these behaviors look like normal programs?

How do they look abnormal? It’s not easy to tell…

Polymorphic viruses Because virus scanners try to match strings in machine

code, virus writers design polymorphic viruses that change their appearances

No-ops, code that doesn’t have an impact on execution, can be used for simple disguises

Clever viruses can break themselves apart and hide different parts in randomly chosen parts of code Similar to code obfuscation

Advanced polymorphic viruses called encrypting viruses encrypt parts of themselves with randomly chosen keys A scanner would have to know to decrypt the virus to detect it

Virus scanners cannot catch everything

Virus effects and causesVirus Effect Virus Cause

Attach to executable program Modify file directory Write to executable program

fileAttach to data or

control file Modify directory Rewrite data

Append to data Append data to self

Remain in memory Intercept interrupt

by modifying interrupt handler address table

Load self in nontransient memory area

Infect disks Intercept interrupt Intercept OS system

call Modify system file Modify ordinary executables

Conceal self Intercept system calls Classify self as hidden file

Spread infection Infect boot sector Infect system

program

Infect ordinary program Infect data ordinary program

reads

Prevent deactivation Activate before

deactivating program

Store copy to reinfect after deactivation

Prevention of infection It is impossible to prevent infection entirely Some guidelines:

Use only commercial software acquired from reliable, well-established vendors

Test all new software on an isolated computer Open attachments only when you know them to be

safe Make a recoverable system image and store it safely Make and retain backup copies of executable system

files Use virus detectors regularly and update them daily

Truths and misconceptions Viruses can infect only Microsoft

Windows systems Viruses can modify hidden and read-

only files Viruses can appear only in data files

or only in programs Viruses spread only on disks or only

through e-mail Viruses cannot remain in memory

after a power cycle Viruses can be malevolent, benign, or

benevolent

False

True

False

False

True, but so what?True

Virus Case Studies

Brain virus The Brain virus is one of the oldest known

It changed the label of disks it attacked to “BRAIN” It was written by two brothers from Pakistan

It copies itself to the boot sector in MS-DOS It rewrites the system interrupt for disk

reading so that it controls reads If you try to look at the boot sector, it will lie to you

about what’s there Anytime it sees an uninfected disk, it infects it It doesn’t otherwise do anything malicious

The Internet Worm In 1988 Robert Morris, a Cornell graduate

student, wrote an worm that infected a lot of the Internet that existed at that time

Serious connectivity issues happened because of the worm and because people disconnected uninfected system

He claimed the point was the measure the size of the Internet

The worm’s goal:1. Determine where it could spread to2. Spread its infection3. Remain undiscovered

Determining where to spread It tried to find user accounts on the host

machine It tried 432 common passwords and compared

their hash to the list of password hashes Ideally, this list should not have been visible

It tried to exploit a bug in the fingerd program (using a buffer overflow) and a trapdoor in the sendmail mail program Both were known vulnerabilities that should

have been patched

Spreading infection Once a target was found, the worm would

send a short loader program to the target machine

The program (99 lines of C) would compile and then get the rest of the virus

It would use a one-time password to talk to the host

If the host got the wrong password, it would break connection

This mechanism was to prevent outsiders from gaining access to the worm’s code

Remain undiscovered

Any errors in transmission would cause the loader to delete any code and exit

As soon as the code was successfully transmitted, the worm would run, encrypt itself, and delete all disk copies

It periodically changed its name and process identifier so that it would be harder to spot

What happened The worm would ask machines if they were already

infected Because of a flaw in the code, it would reinfect

machines 1 out of 7 times Huge numbers of copies of the worm started filling

infected machines System and network performance dropped

Estimates of the damage are between $100,000 and $97 million Morris was fined $10,000 and sentenced to 400 hours of

community service The CERT was formed to deal with similar problems

Code Red

Code Red appeared in 2001 It infected a quarter of a million systems

in 9 hours It is estimated that it infected 1/8 of the

systems that were vulnerable It exploited a vulnerability by

creating a buffer overflow in a DLL in the Microsoft Internet Information Server software

It only worked on systems running an MS web server, but many machines did by default

Versions

The original version of Code Red defaced the website that was being run

Then, it tried to spread to other machines on days 1-19 of a month

Then, it did a distributed denial of service attack on whitehouse.gov on days 20-27

Later versions attacked random IP addresses

It also installed a trap door so that infected systems could be controlled from the outside

Upcoming

Next time…

Review for exam

Reminders

Read section 3.2 Finish Assignment 2

Due tonight! Start on Project 2