Chapter-4

Software Security

Why Software? Why is software as important to security as

crypto, access control and protocols? Virtually all of information security is

implemented in software If your software is subject to attack, your

security is brokenRegardless of strength of crypto, access control or

protocols

Software is a poor foundation for security

Software Issues

Attackers Actively look for bugs

and flaws Like bad software… …and try to make it

misbehave Attack systems thru

bad software

“Normal” users Find bugs and flaws

by accident Hate bad software… …but must learn to

live with it Must make bad

software work

Complexity “Complexity is the enemy of security”

Netscape 17,000,000

Space shuttle 10,000,000

Linux 1,500,000

Windows XP 40,000,000

Boeing 777 7,000,000

system Lines of code (LOC)

Software Security Topics Program flaws (unintentional)

Buffer overflowIncomplete mediationRace conditions

Malicious software (intentional)VirusesWorms

Program Flaws

An error is a programming mistakeDone by human

An error may lead to incorrect state: faultA fault is internal to the program

A fault may lead to a failure, where a system departs from its expected behaviorA failure is externally observable

error fault failure

Examplechar array[10];for(i = 0; i < 10; ++i)

array[i] = `A`;array[10] = `B`;

This program has an error This error might cause a fault

o Incorrect internal state

If a fault occurs, it might lead to a failureo Program behaves incorrectly (external)

We use the term flaw for all of the above

Secure Software In software engineering, try to insure that a

program does what is intended Secure software engineering requires that

the software does what is intended… …and nothing more Absolutely secure software is impossible

Absolute security is almost never possible

Program Flaws Program flaws are unintentional

But still create security risks

We’ll consider 3 types of flawsBuffer overflow (smashing the stack)Incomplete mediationRace conditions

Many other flaws can occur These are most common

Buffer Overflow• In computer security and programming, a buffer overflow, or buffer overrun, is an anomaly where a program, while writing data to a buffer, overruns the buffer's boundary and overwrites adjacent memory.

• This may result in abnormal program behavior, including memory access errors, incorrect results, a crash, or a breach of system security.

Buffer Overflow

Programming languages commonly associated with buffer overflows include C and C++, which provide no built-in protection against accessing or overwriting data in any part of memory and do not automatically check that data written to an array (the built-in buffer type) is within the boundaries of that array.

Buffer Overflow

Most commonly this occurs when copying strings of characters from one buffer to another.

Buffer Overflow

a program has defined two data items which are adjacent in memory: an 8-byte-long string buffer, A, and a two-byte integer, B.

Initially, A contains nothing but zero bytes, and B contains

the number 1979. Characters are one byte wide.

Buffer Overflow Now, the program attempts to store the null-

terminated string "excessive" in the A buffer. By failing to check the length of the string, it overwrites the value of B:

Although the programmer did not intend to change B at all, B's value has now been replaced by a number formed from part of the character string. That uses ASCII "e" followed by a zero byte would become the number 25856.

Exploitation The techniques to exploit a buffer overflow

vulnerability vary per architecture, operating system and memory region. For example, exploitation on the heap (used for dynamically allocated memory) is very different from on the call stack.

Stack-based exploitation Heap-based exploitation

Buffer Overflow

Q: What happens when this is executed? A: Depending on what resides in memory at

location “buffer[20]”Might overwrite user data or codeMight overwrite system data or code

int main(){

int buffer[10];

buffer[20] = 37;}

Simple Buffer Overflow Consider Boolean flag for authentication Buffer overflow could overwrite flag allowing

anyone to authenticate!

buffer

FTF O U R S C …

Boolean flag

In some cases, attacker need not be so lucky as to have overflow overwrite flag

Memory Organization

Text == code Data == static variables Heap == dynamic data Stack == “scratch paper”

Dynamic local variablesParameters to functionsReturn address

stack

heap

data

text

high address

low address

SP

Stack-based exploitation The malicious user may exploit stack-based buffer

overflows to manipulate the program in one of the following method

By overwriting a local variable that is near the buffer in memory on the stack to change the behavior of the program which may benefit the attacker.

By overwriting the return address in a stack frame. Once the function returns, execution will resume at the return address as specified by the attacker.

By overwriting a function pointer

Heap-based exploitation A buffer overflow occurring in the heap data area is

referred to as a heap overflow and is exploitable in a different manner to that of stack-based overflows.

Memory on the heap is dynamically allocated by the application at run-time and typically contains program data.

Exploitation is performed by corrupting this data in specific ways to cause the application to overwrite internal structures such as linked list pointers.

Typical Attack Scenario Users enter data into a Web form Web form is sent to server Server writes data to buffer, without

checking length of input data Data overflows from buffer Sometimes, overflow can enable an attack Web form attack could be carried out by

anyone with an Internet connection

Simplified Stack Example

high

void func(int a, int b){

char buffer[10];

}

void main(){

func(1, 2);

}

::

buffer

ret

a

b

return address

low

SP

SP

SP

SP

Smashing the Stack

high

What happens if buffer overflows?

::

buffer

a

b

ret…

low

SP

SP

SP

SP

ret

Program “returns” to wrong location

NOT!

???

A crash is likelyoverflow

overflow

Smashing the Stack

high

Trudy has a better idea… :

:

a

b

low

SP

SP

SP

SP

ret

Code injection Trudy can run

code of her choosing!

evil code

ret

Smashing the Stack

Trudy may not knowo Address of evil codeo Location of ret on stack

Solutionso Precede evil code with

NOP “landing pad” o Insert lots of new ret

evil code

::

::

ret

ret

:

NOP

NOP

:

ret

ret

Stack Smashing Summary A buffer overflow must exist in the code Not all buffer overflows are exploitable If exploitable, attacker can inject code Trial and error likely required Stack smashing is “attack of the decade”

Example Source code of the buffer overflow

Flaw easily found by attacker

Even without the source code!

Stack Smashing Example Program asks for a serial number that the

attacker does not know Attacker does not have source code Attacker does have the executable (exe)

Program quits on incorrect serial number

Example By trial and error, attacker discovers an

apparent buffer overflow

Note that 0x41 is “A” Looks like ret overwritten by 2 bytes!

Example Next, disassemble bo.exe to find

The goal is to exploit buffer overflow to jump to address 0x401034

Example Find that 0x401034 is “@^P4” in ASCII

Example Reverse the byte order to “4^P@” and…

Success! We’ve bypassed serial number check by exploiting a buffer overflow

Overwrote the return address on the stack

Stack Smashing Prevention 1st choice: employ non-executable stack

“No execute” NX bit (if available) Memory can be flagged so that the code can’t

execute on specified location.

2nd choice: use safe languages (Java, C#)

3rd choice: use safer C functionsFor unsafe functions, there are safer versionsFor example, strncpy instead of strcpy

Incomplete mediation Inputs to programs are often specified by

untrusted usersWeb-based applications are a common example

Users sometimes mistype data in web formsPhone number: 51998884567Email: iang#cs.uwaterloo.ca

The web application needs to ensure that what the user has entered represents a meaningful request

This is called mediation

Incomplete mediation Incomplete mediation occurs when the

application accepts incorrect data from the user Sometimes this is hard to avoid

Phone number: 519-886-4567This is a reasonable entry, that happens to be wrong

We focus on catching entries that are clearly wrongNot well formed

○ DOB: 1980-04-31Unreasonable values

○ DOB: 1876-10-12Inconsistent with other entries

Why do we care? What's the security issue here?

What happens if someone fills in:DOB: 98764874236492483649247836489236492

○ Buffer overflow?DOB: '; DROP DATABASE clients --

○ SQL injection?

We need to make sure that any user-supplied input falls within well-specified values, known to be safe

Client-side mediation You've probably visited web site with forms that

do client-side mediationWhen you click “submit”, Javascript code will first run

validation checks on the data you enteredIf you enter invalid data, a popup will prevent you from

submitting it

Related issue: client-side stateMany web sites rely on the client to keep state for

themThey will put hidden fields in the form which are

passed back to the server when the user submits the form

Client-side mediation Problem: what if the user

Turns off Javascript?Edits the form before submitting it? (Greasemonkey)Writes a script that interacts with the web server

instead of using a web browser at all?Connects to the server “manually”?

(telnet server.com 80)

Note that the user can send arbitrary (unmediated) values to the server this way

The user can also modify any client-side state

Example At a bookstore website, the user orders a copy

of the course text. The server replies with a form asking the address to ship to. This form has hidden fields storing the user's order<input type=“hidden” name=“isbn” value=“0-13-239077-9”><input type=“hidden” name=“quantity” value=“1”><input type=“hidden” name=“unitprice” value=“111.00”>

What happens if the user changes the “unitprice” value to “50.00” before submitting the form?

Defences against incomplete mediation Client-side mediation is an OK method to use in order

to have a friendlier user interface, but is useless for security purposes.

You have to do server-side mediation, whether or not you also do client-side.

For values entered by the user:Always do very careful checks on the values of all fields

For state stored by the client:Make sure the client has not modified the data in any way

TOCTTOU errors / Race Condition TOCTTOU (“TOCK-too”) errors

Time-Of-Check To Time-Of-UseAlso known as “race condition” errors

These errors occur when the following happens:User requests the system to perform an actionThe system verifies the user is allowed to perform

the actionThe system performs the action

Race Condition

Security processes should be atomicOccur “all at once”

Race conditions can arise when security-critical process occurs in stages

Attacker makes change between stagesOften, between stage that gives authorization, but

before stage that transfers ownership

Example: Unix mkdir

mkdir Race Condition mkdir creates new directory How mkdir is supposed to work

1. Allocate space

mkdir

2. Transfer ownership

mkdir Attack

Not really a “race”o But attacker’s timing is critical

1. Allocate space

mkdir

3. Transfer ownership

2. Create link to password file

The mkdir race condition

Race Conditions Race conditions are common Race conditions may be more common than

buffer overflows But race conditions harder to exploit To prevent race conditions, make security-

critical processes atomicOccur all at once, not in stages