1. Introduction to Information Security , Spring Lecture 1: Introduction, Control Hijacking (1/2)
Avishai Wool

2. Administration
Lecturers: Avishai Wool Zvi Ostfeld
Teaching assistants: Nir Krakowski, Dan Gittik
Exercise checker: Yuval Lewi
Course web site:
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3. Exercises
Practical emphasis, challenging hands-on exercises.
Final grade: 65% exam, 35% exercises
Individual work – no sharing / copying!
Attend recitations for necessary context.

4. Information Security Computer and Network Security Cyber Security
Motivation
Information Security Computer and Network Security Cyber Security

5. Responsible disclosure
We will learn and practice attack techniques
With knowledge – you may discover real security vulnerabilities in real systems
Do no harm!
[Wikipedia]
Responsible disclosure is a ... vulnerability disclosure model. … stakeholders agree to allow time for the vulnerability to be patched before publishing:
Tell the vendor (disclose)
Wait for vendor to patch
Then tell the world (publish)

6. A brief history of cyber
1948
1960
1980
1988
1998
2007
2010
Cybernetic = regulatory/control systems
such as computers
Cybernetic = using computers
Morris worm
on the Internet
Data Breach at Security Firm Linked to Attack on Lockheed
May 27, 2011
Million-node botnets
Cybersex
Finance: untold $billions
National (Estonia, Georgia, RSA+LM)
Scientific American background review article about “health”. Graphic: doctor+patient, acoustic
“Cybernetic” coined 1948 by U.S. mathematician Norbert Wiener
1998: “cyber” = “cybersex”
In the beginning was the word
And it was good
It is not good for a computer to be alone
So computers went forth and multiplied
Tree of knowledge
Wrecking havoc upon the fledgling Internet
1981: first wild virus written by 9-th grader for Apple II (footnote in SecEng)
Extorting: Gpcode.ak (show asm)
“Cyber attack”, “cyber war” headlines
Fallen from grace
National: Estonia 2007, Georgia 2008, RSA+LM 2011
Physical: DHS generator (2007), Stuxnet (2010)
A computer scientist would never use the word “cyber”, unless he’s writing a grant proposal
Cyber -> Journalist / politician / writing a grant proposal / getting you to attend his session
“cyber-resilient computers” - Shimon Peres, Cyber conference at TAU,
For timeline, see: SecEng 2Nd Ch21
Physical (Stuxnet)
Cybersecurity ~ harming computers and things they control
Information / computer / network / control-system security, cryptography

7. Security goals and threats
Data confidentiality
Data integrity
System availability
User authentication
Privilege separation
THREAT
Data exposure
Data modification
Denial of service
Masquerading
Privilege elevation
Prerequisite to many attacks: getting malicious code to run on victim’s computer system

8. Basic Control Hijacking Attacks

9. Control hijacking attacks
Attacker’s goal:
Take over target machine (e.g. web server)
Execute arbitrary code on target by hijacking application control flow
Examples.
Buffer overflow attacks
Integer overflow attacks
Format string vulnerabilities

10. Example 1: buffer overflows
Extremely common bug in C/C++ programs.
First major exploit: Internet Worm, fingerd.
20% of all vuln.
:  10%
Source: NVD/CVE

11. What is needed Understanding C functions, the stack, and the heap.
Know how system calls are made
The exec() system call
Attacker needs to know which CPU and OS used on the target machine:
Our examples are for x86 running Linux or Windows
Details vary slightly between CPUs and OSs:
Little endian vs. big endian (x86 vs. Motorola)
Stack Frame structure (Unix vs. Windows)

12. Linux process memory layout
0xC
user stack
%esp
shared libraries 0x
brk
run time heap
Loaded from exec 0x
unused

13. callee saved registers
Stack Frame
high
arguments
return address
stack frame pointer
exception handlers
local variables
Stack
Growth
callee saved registers SP
low

14. What are buffer overflows?
void func(char *str) { char buf[128];
strcpy(buf, str); do-something(buf); }
Suppose a web server contains a function:
When func() is called stack looks like:
argument: str
return address
stack frame pointer
char buf[128] SP

15. What are buffer overflows?
void func(char *str) { char buf[128];
strcpy(buf, str); do-something(buf); }
What if *str is 136 bytes long?
[this is a software bug]
After strcpy:
argument: str
return address
stack frame pointer
*str
Problem: no length checking in strcpy()
char buf[128] SP

16. What are buffer overflows?
void func(char *str) { char buf[128];
strcpy(buf, str); do-something(buf); }
What if *str is 136 bytes long?
After strcpy:
argument: str
When func returns, usually:
Some junk in return address
CPU tries to jump to junk address
“invalid instruction at 0x….” or “segmentation fault”
Result: Web server crash (OS kills the process)
return address
stack frame pointer
*str
char buf[128] SP

17. Now let’s think like an attacker
Attacker discovers the software bug
Attacker has control over the content & length of str
E.g., maybe str is sent in the HTTP request that the attacker can fill
Result: attacker can place binary values in the end of str to select the address that the CPU will jump to upon return
This is Control Hijacking

18. Where to jump to?
Attacker goal: jump to an address that runs attacker-written code
But the process memory only has legitimate user & OS code
Need to inject his own code somehow
Solution: make the buffer overflow even longer
Place binary machine instructions at the end of str

19. Basic stack exploit
high
Program P
Suppose *str is such that after strcpy stack looks like:
Program P: exec(“/bin/sh”)
Attack code P runs in stack.
When func() exits, the shell runs with its stdin and stdout (hopefully) redirected to the TCP connection.
The attacker, on the other side of the TCP connection, gets full shell access.
Attacker gains full privileges of the user (UID, GID) running the server.
(exact shell code by Aleph One)
return address
char buf[128]
low

20. Problem: how does attacker determine the return address?
The NOP slide
high
Program P
Problem: how does attacker determine the return address?
Solution: NOP slide
Guess approximate stack state when func() is called
Insert many NOPs before program P:
nop ,
xor eax,eax ,
inc ax
...
NOP Slide
return address
char buf[128]
low

21. Details and examples Some complications:
Program P should not contain the ‘\0’ character.
Overflow should not crash program before func() exits.
Sample remote stack smashing overflows:
(2007) Overflow in Windows animated cursors (ANI). LoadAniIcon()
(2005) Overflow in Symantec Virus Detection
test.GetPrivateProfileString "file", [long string]
Animated cursor on web page or message would result in arbitrary code execution. Used for rendering cursors, animated cursors, and icons.

22. Many unsafe libc functions
strcpy (char *dest, const char *src)
strcat (char *dest, const char *src)
gets (char *s)
scanf ( const char *format, … ) and many more.
“Safe” libc versions strncpy(), strncat() are misleading
E.g.,. strncpy() may leave string unterminated.
Windows C run time (CRT):
strcpy_s (*dest, DestSize, *src) ensures proper termination.

23. Buffer overflow opportunities
Exception handlers: (Windows SEH attacks)
Overwrite the address of an exception handler in stack frame.
Function pointers: (e.g. PHP 4.0.2, MS MediaPlayer Bitmaps)
Overflowing buf will override function pointer.
Longjmp buffers: longjmp(pos) (e.g. Perl 5.003)
Overflowing buf next to pos overrides value of pos.
Heap or stack
buf[128]
FuncPtr
Windows media player bitmaps (skins) – heap overflow, Feb. 2006
setjmp – used for exception handling (jump to global error handling code in case of error)

24. Corrupting method pointers http://phrack.org/issues/56/8.html
Compiler generated function pointers (e.g. virtual methods in C++ code)
After overflow of buf :
method1 code
FP1
FP2
method2 code
ptr
FP3
method3 code
vtable
data
Object T
NOP slide
shell code
buf[256]
vtable
ptr
data
object T

25. Finding buffer overflows
To find overflow:
Run web server on local machine
Issue malformed requests (ending with “$$$$$” )
Many automated tools exist (called fuzzers)
If web server crashes, search core dump for “$$$$$” to find overflow location
Construct exploit (not easy given latest defenses)

26. More Control Hijacking Attacks

27. More Hijacking Opportunities
Integer overflows: (e.g. MS DirectX MIDI Lib)
Double free: double free space on heap.
Can cause memory manager to write data to
specific location
Examples: CVS server
Format string vulnerabilities

28. Will two programs output the same?
Integer Overflow
Integer overflow: an arithmetic operation attempts to create a numeric value that is larger than can be represented within the available storage space.
Example:
Test 2:
short x = 30000;
short y = 30000;
short z = x + y;
printf(“%d\n”, z);
Test 1:
short x = 30000;
short y = 30000;
printf(“%d\n”, x+y);
Will two programs output the same?

29. C Data Types short int 6bits [-32,768; 32,767]
unsigned short int 16bits [0; 65,535]
unsigned int 32bits [0; 4,294,967,295]
int 32bits [-2,147,483,648; 2,147,483,647]
long 32bits [-2,147,483,648; 2,147,483,647]
char 8bits [-128; 127]
unsigned char 8bits [0; 255]
long long 64bits
unsigned long long 64bits

30. When does casting occur in C?
When assigning to a different data type
For binary operators +, -, *, /, %, &, |, ^,
if either operand is an unsigned long, both are cast to an unsigned long
in all other cases where both operands are 32-bits or less, the arguments are both upcast to int, and the result is an int
For unary operators
~ changes type, e.g., ~((unsigned short)0) is int
++ and -- does not change type

31. Where Does Integer Overflow Matter?
Allocating spaces using calculation.
Calculating indexes into arrays
Checking whether an overflow could occur
Direct causes:
Truncation; Integer casting

32. Integer Overflow: Example 1
const long MAX_LEN = 20000;
char buf[MAX_LEN];
short len = strlen(input);
if (len < MAX_LEN)
strcpy(buf, input);
Can a buffer overflow attack occur?
If so, how long does input needs to be?
When the length of input overflows

33. Integer Overflows Example 2 (see Phrack 60)
Problem: what happens when int exceeds max value? int m;(32 bits) short s;(16 bits) char c; (8 bits) c = 0x80 + 0x80 = ⇒ c = 0 s = 0xff80 + 0x80 ⇒ s = 0 m = 0xffffff80 + 0x80 ⇒ m = 0 Can this be exploited?

34. What if len1 = 0x80, len2 = 0xffffff80 ? ⇒ len1+len2 = 0
void func( char *buf1, *buf2, unsigned int len1, len2) { char temp[256]; if (len1 + len2 > 256) {return -1} // length check memcpy(temp, buf1, len1); // concatenate buffers memcpy(temp+len1, buf2, len2); do-something(temp); // do stuff }
What if len1 = 0x80, len2 = 0xffffff80 ?
⇒ len1+len2 = 0
Second memcpy() will overflow stack (or heap) !!

35. Integer overflow exploit stats
Source: NVD/CVE

36. Format string bugs

37. Format string problem
Want to print a message (without arguments) - via fprintf
Correct form:
fprintf( stdout, “%s”, input);
Bug:
int func(char *input) {
fprintf( stderr, input); }
“input” is treated as a format string – missing arguments
If “input” includes % formats – access places on stack
Problem: what if *input = “%s%s%s%s%s%s%s” ?
Most likely program will crash (Denial of Service, violates Availability)
If not, program will print memory contents. Privacy?

38. Format string attacks (“%n”)
“%n” format writes the number of bytes formatted so far into a variable!
printf(“abc %n”, &x) will change the value of the variable x (to 4)
in other words, the parameter value on the stack is interpreted as a pointer to an integer value, and the place pointed by the pointer is overwritten

39. Exploit Dumping arbitrary memory: Writing to arbitrary memory:
Walk up stack until desired pointer is found.
printf("%08x.%08x.%08x.%08x|%s|")
Writing to arbitrary memory:
printf("hello %n", &temp) writes “6” into temp.
printf("%08x.%08x.%08x.%08x.%n")

40. History First exploit discovered in June 2000. Examples:
wu-ftpd 2.* : remote root
Linux rpc.statd: remote root
IRIX telnetd: remote root
BSD chpass: local root

41. Vulnerable functions Any function using a format string. Printing:
printf, fprintf, sprintf, …
vprintf, vfprintf, vsprintf, …
Logging:
syslog, err, warn