Buffer Overflows

1. Buffer Overflows James Walden Northern Kentucky University

2. Topics • What is a Buffer Overflow? • Buffer Overflow Examples • Program Stacks • Smashing the Stack • Shellcode • Mitigations CSC 666: Secure Software Engineering

3. Buffer Overflows A program accepts too much input and stores it in a fixed length buffer that’s too small. char A[8]; short B; gets(A); CSC 666: Secure Software Engineering

4. Buffer Overflow Examples Morris Worm Took down most of Internet in 1988. Exploited a buffer overflow in fingerd. Subsequent worms used overflow attacks too. MS07-004: Internet Explorer Buffer overflow in VML. Allows remote code execution. Not the first overflow in IE or other browsers. CSC 666: Secure Software Engineering

5. Buffer Overflow Example #1 What’s the mistake in this program? int main() { int array[5] = {1, 2, 3, 4, 5}; printf("%d\n", array[5]); } Program output: > gcc -o buffer buffer.c > ./buffer 7077876 CSC 666: Secure Software Engineering

6. Buffer Overflow Example #2 Writing beyond the buffer: int main() { int array[5] = {1, 2, 3, 4, 5}; int i; for( i=0; i <= 255; ++i ) array[i] = 41; } Program output: > gcc -o bufferw bufferw.c > ./bufferw Segmentation fault (core dumped) CSC 666: Secure Software Engineering

7. What happened to our program? The buffer overflow: Overwrote memory beyond buffer with 41. Memory page was not writable by program. OS terminated prog with segmentation fault. Do overflows always produce a crash? Most of the time, yes. Careful attacker can access valid memory. CSC 666: Secure Software Engineering

8. Why do programmers keep making the same mistake? C/C++ inherently unsafe. No bounds checking. Unsafe library functions: strcpy(), sprintf(),gets(), scanf(), etc. D, Java, Python, Ruby largely immune. C/C++ gains performance by not checking. CSC 666: Secure Software Engineering

9. Process Memory Regions Argv/Env: CLI arguments and environment Stack: generally grows downwards Heap: generally grows upwards BSS: unitialized global data Data: initialized global data Text: read-only program code CSC 666: Secure Software Engineering

10. Process Memory Layout high mem low mem CSC 666: Secure Software Engineering

11. Memory Layout Example /* data segment: initialized global data */ int a[] = { 1, 2, 3, 4, 5 }; /* bss segment: uninitialized global data */ int b; /* text segment: contains program code */ int main(int argc, char **argv) /* ptr to argv */ { /* stack: local variables */ int *c; /* heap: dynamic allocation by new or malloc */ c = (int *)malloc(5 * sizeof(int)); } CSC 666: Secure Software Engineering

12. Program Stack Layout Low Memory b() { … } a() { b(); } main() { a(); } High Memory CSC 666: Secure Software Engineering

13. Stack Frames • A function call produces a stack frame. • Return address of the caller. • Actual arguments used in function call. • Local variables. • Register EBP points to current frame. • Stack frame is deallocated on return. CSC 666: Secure Software Engineering

14. C Calling Convention • Push params on stack in reverse order. Parameter #N … Parameter #1 • Issue a call instruction. • Pushes address of next instruction (the return address) onto stack. • Modifies EIP to point to start of function. CSC 666: Secure Software Engineering

15. Stack before Function Executes Frame Pointer Stack Pointer CSC 666: Secure Software Engineering

16. Function Initialization • Function pushes FP (%ebp) onto stack. pushl %ebp • Sets FP to current SP. Allows function to access params as fixed indexes from base pointer. movl %esp, %ebp • Reserves stack space for local vars. subl $12, %esp CSC 666: Secure Software Engineering

17. Stack at Function Start Frame Pointer Stack Pointer CSC 666: Secure Software Engineering

18. Function Return • Stores return value in %eax. movl $1, %eax • Restores stack and frame pointers. movl %esp, %ebp popl %ebp • Pops return address off stack and transfers control to that address. ret CSC 666: Secure Software Engineering

19. Controlling Execution Let’s make the program an infinite loop by changing the return value to start of main(). (gdb) disassemble main Dump of assembler code for main: 0x080483c1 <main+0>: push %ebp 0x080483c2 <main+1>: mov %esp,%ebp 0x080483c4 <main+3>: call 0x804839c <printInput> 0x080483c9 <main+8>: leave 0x080483ca <main+9>: ret CSC 666: Secure Software Engineering

20. Sending a non-ASCII Value void main() { char addr[44]; int i; for( i=0; i<=40; i+=4 ) *(long *) &addr[i]=0x080483c1; puts(addr); } CSC 666: Secure Software Engineering

21. Does it work? (./address; cat) | ./overflow input1 input1 input2 input2 input3 Segmentation fault (core dumped) CSC 666: Secure Software Engineering

22. Code and Data What’s the difference between code and data? CSC 666: Secure Software Engineering

23. Code Injection Use a two step process: • Use buffer overflow to write machine code (shellcode) onto stack. • Rewrite return address to point to machine code on the stack. Program will do whatever we tell it to do in those machine instructions. CSC 666: Secure Software Engineering

24. Shellcode Shellcode is machine code that starts a command shell. With a shell, you can run any command. CSC 666: Secure Software Engineering

25. Shellcode Shellcode in C. int main() { char *name[2]; name[0] = "/bin/sh"; name[1] = 0x0; execve(name[0], name, 0x0); } Running the program. > gcc –ggdb –static –o shell shellcode.c > ./shell sh-3.00$ exit CSC 666: Secure Software Engineering

26. From C to Machine Language char shellcode[] = "\xeb\x1f\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89\x46\x0c\xb0\x0b" "\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\x31\xdb\x89\xd8\x40\xcd" "\x80\xe8\xdc\xff\xff\xff/bin/sh"; void main() { int *ret; ret = (int *)&ret + 2; (*ret) = (int)shellcode; } > gcc -o testsc2 testsc2.c > ./testsc2 sh-3.00$ exit CSC 666: Secure Software Engineering

27. Writing an Exploit • Construct shellcode to inject. • Find exploitable buffer in a program. • Estimate address of buffer. • Run program with an input that: • Injects shellcode into stack memory. • Overwrites return address with address of your shellcode. CSC 666: Secure Software Engineering

28. Improving the Odds Determining the correct address of your shellcode is difficult. What if you could use multiple addrs? Pad buffer with NOP instructions preceding the shellcode. If function returns anywhere in NOP pad, it will continue executing until it executes the shellcode. CSC 666: Secure Software Engineering

29. Overflow Exploits w/o the Stack Overflows can exist in other segments. Only the stack has return addresses. Must rewrite other addresses Function pointers Longjmp buffers Or alter security critical variables. CSC 666: Secure Software Engineering

30. Buffer Overflow Mitigations • Use language with bounds checking. • Do your own bounds checking. • Avoid unsafe functions. • Use safe functions securely. • Operating system defenses. • Compiler-based defenses. CSC 666: Secure Software Engineering

31. Languages with Bounds Checking High-level languages do bounds checks • Java • OCaml • Python • Ruby • Scheme • Smalltalk C variants • CCured • Cyclone • D CSC 666: Secure Software Engineering

32. Bounds Checking Check input length before copying into buffer. int myfunction(const char *str) { char buf[1024]; /* strlen() doesn’t count NULL */ if (strlen(str) >= sizeof(buf)) { exit(1); } } CSC 666: Secure Software Engineering

33. Unsafe Functions: Input gets(char *s) Always an overflow. Cannot be checked. Use fgets(char *s, int size, FILE *stream); scanf(const char *fmt, …) Similar: _tscanf, wscanf, sscanf, fscanf, … Use of “%s” format allows overflow. Use “%Ns” to limit length of input. CSC 666: Secure Software Engineering

34. Unsafe Functions: strcat strcpy(char *dst, char *src) Similar: wscpy, wcscpy, mbscpy Overflow if dst smaller than src. strncpy(char *dst, const char *src, size_t n) Similar: _tcsncpy, wcscpyn, _mbsncpy, … No NULL termination if src >= dst. CSC 666: Secure Software Engineering

35. Bounded Function Pitfalls • Destination buffer overflows because bound depends on size of source data, not destination buffer. • Destination buffer left without null terminator, often as result of off-by-one error. • Destination buffer overflows because its bound is specified as the total size of the buffer, rather than space remaining. • Programs writes to arbitrary location in memory as destination buffer is not null-terminated and function begins writing at location of first null in destination buffer. CSC 666: Secure Software Engineering

36. UNIX Libraries C Bstrlib MT-Safe SafeStr strlcpy(), strlcat() Vstr C++ std::string (STL) Windows Libraries C Safe CRT strlcpy(), strlcat() StrSafe C++ CString (MFC) Safe C++ std::string (STL) Safe String Libraries CSC 666: Secure Software Engineering

37. strlcpy() and strlcat() size_t strlcpy (char *dst, const char *src, size_t size); size_t strlcat (char *dst, const char *src, size_t size); • Size is max size of dest buffer (not maximum number of chars to copy), including NULL. • Destination buffer always NULL terminated • Return how much space would be required in destination buffer to perform operation. • BSD-style open source license. CSC 666: Secure Software Engineering

38. Character Sets • Chars represented using encoding forms that map code points to printable chars. • Fixed Width • ISO-8859-1 • UTF-32 • Variable Width • UTF-8 • UTF-16 (Java, .NET) CSC 666: Secure Software Engineering

39. Wide Characters C/C++ • char contains 1-byte characters • wchar_t is 2-byte, 4-byte on some platforms Java and .NET strings • UTF-16 Buffer Overflow issues • Mixing up different character-set string types. • Are sizes measured in bytes or characters? CSC 666: Secure Software Engineering

40. C++ String Dangers Using C-style strings with cin char username[16]; cin >> username; The [] operator does no bounds checking. Converting from C++ to C-style strings: string::data() output is not NULL terminated. string::c_str() output is NULL terminated. CSC 666: Secure Software Engineering

41. Dealing with Overly Long Inputs • Prevent operation with an error message. • Possibly exit program too. • Truncate input to allowed length. • Can lead to null-termination errors. • Can cause input to be misinterpreted later. • Resize the buffer to accomodate input. • Limit resizing to avoid memory DoS. CSC 666: Secure Software Engineering

42. Static Simple Easy to track buffer size for bounds checks. Inflexible. Limited options if size is too small. Waste memory if size is too large. Dynamic Complex Manually tracking buffer sizes can lead to overflows due to stale size data. Flexible Can lead to memory exhaustion if no limits. Possibility of use after free and double free errors. Dynamic vs. Static Allocation CSC 666: Secure Software Engineering

43. Non-executable Stack Memory protection prevents exploit code from being executed. • Some applications execute code on the stack/heap. • x86 arch doesn’t have exec bit in page tables. • Segment limits can divide memory into two parts: executable and non-executable. • Keep program code in low memory. • Keep data and stack in high memory. • Coarse-grained. • NX Technology • Exec bit for page tables. • Added in AMD64 and newer Intel P4 processors. • Only works in PAE 64-bit page table format. CSC 666: Secure Software Engineering

44. Arc Injection (return-into-libc) Arc injection transfers control to code that already exists in memory. Adds a new arc into program’s CFG. Change return address to existing function. libc has functions to start a shell. Allows exploit even if stack non-executable. Sophisticated arc injection attacks create multiple stack frames to run multiple functions that are in memory. CSC 666: Secure Software Engineering

45. Memory Layout Randomization Randomize layout of memory space • Stack location. • Shared library locations. • Heap location. PIE: Position Independent Executable • Default format: binary compiled to work at an address selected when program was compiled. • Gcc can compile binaries to be freely relocatable throughout address space. • gcc flags: -fpie –pie • Program loaded at different address for each invocation. CSC 666: Secure Software Engineering

46. Defence: Stackguard Compiler extension for gcc • code must be compiled w/ Stackguard Detects altered return address • before function returns • adds “canary” word to stack • must overwrite canary to change return addr • use random canary words for each function to avoid guessing attacks CSC 666: Secure Software Engineering

47. Stackguard Stack Layout Frame Pointer Stack Pointer CSC 666: Secure Software Engineering

48. Stackguard Effectiveness • Code dependencies • are dynamic libraries stackguarded? • Compatibility • Recompiled entire RedHat Linux system. • Small performance cost • canary insert and check overhead on each call • Protects against future stack attacks. • Similar tools: • gcc -fstack-protector flag • Visual Studio 2005 CSC 666: Secure Software Engineering

49. Buffer Overflow: Key Points Buffer overflow attacks. • C/C++ perform no bounds checking. • There is no difference btw code and data. • Smashing the stack. Mitigating buffer overflows. • Use a language with bounds checking. • Check your own bounds in C/C++. • Use safe functions, string libraries. CSC 666: Secure Software Engineering

50. References • Aleph Null, “Smashing the Stack for Fun and Profit,” Phrack 49, 1996. • Brian Chess and Jacob West, Secure Programming with Static Analysis, Addison-Wesley, 2007. • Johnathan Bartlett, Programming from the Ground Up, Bartlett Publishing, 2004. • Matt Conover & w00w00 Security Team, “w00w00 on Heap Overflows,” http://www.w00w00.org/files/articles/heaptut.txt • Mark Graff and Kenneth van Wyk, Secure Coding: Principles & Practices, O’Reilly, 2003. • Horizon, “Bypassing Non-executable Stack Protection on Solaris,” http://packetstormsecurity.nl/groups/horizon/stack.txt • Greg Hoglund and Gary McGraw, Exploiting Software: How to Break Code, Addison-Wesley, 2004. • Michael Howard and David LeBlanc, Writing Secure Code, 2nd edition, Microsoft Press, 2003. • Koziol, et. al, The Shellcoder’s Handbook: Discovering and Exploiting Security Holes, Wiley, 2004. • Robert C. Seacord, Secure Coding in C and C++, Addison-Wesley, 2006. • John Viega and Gary McGraw, Building Secure Software, Addison-Wesley, 2002. • David Wheeler, Secure Programming for UNIX and Linux HOWTO, http://www.dwheeler.com/secure-programs/Secure-Programs-HOWTO/index.html, 2003.