1. Memory Corruption James Walden Northern Kentucky University CSC 666: Secure Software Engineering

2. Topics 1.Buffer Overflows 2.Stack Overflows 3.Heap Overflows 4.Use after Free 5.Memory Corruption Mitigations 6.Return Oriented Programming 7.Writing Secure Code

3. Memory Corruption Memory corruption occurs when contents of a memory location are modified in a way unintended by the programmer. Modifications can result from  Buffer overflows  Format string attacks (%n)  Use after free Memory corruption vulnerabilities can result in  Execution of attacker-supplied code.  Attacker hijacking of program control flow.  Alteration of program data.  Information leak. CSC 666: Secure Software Engineering

4. Buffer Overflows buffer: limited contiguously allocated set of memory.  static: char buffer[32]  dynamic: malloc(), new What happens when you attempt to access an element beyond the end of the buffer?  Bounds checking prevents such accesses in most languages like Python, Ruby, and Java.  But in C, C++, Objective C, Forth, and assembly large inputs can overflow the buffer, overwriting adjacent data in memory.

5. C Strings  C strings terminated with \0 character.  Many operating systems and software components are written in C  Interfaces inherit semantic “strings end with \0”.  Some components don’t handle \0 embedded in string gracefully, even if programming language can.  Note that UTF-16/UTF-32 include many byte 0s.  Note that \0 takes space – account for it!  Overwriting can create string doesn’t end.  Formal name is NUL character Hello\0

6. An Example Buffer Overflow char A[8]; short B=3; AAAAAAAABB 0000000003 AAAAAAAABB overflows0 gets(A); CSC 666: Secure Software Engineering

7. Out-of-Bounds Read 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

8. CSC 666: Secure Software Engineering Out of Bounds Write 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)

9. CSC 666: Secure Software Engineering What happens when a buffer overflows? What happened to our buffer overflow? 1.Overwrote memory beyond buffer with 41. 2.Program crashed with Segmentation fault. 1.Directly or indirectly accessed unmapped memory. 2.Creates a page fault exception. 3.OS does not find mapping on page table. 4.OS sends segmentation fault signal to process. Do overflows always produce a crash?  Unintentional overflows usually do, but  Attackers will restrict writes to mapped pages.

10. History of Overflows  1972: Computer Security Technology Planning Study.  1988: Morris Worm spreads with fingerd overflow. CERT formed.  1989: CERT published CA-1989-01 for BSD 4.3 passwd overflow.  1995: Thomas Lopatic posts info on httpd overflow to Bugtraq.  1996: Aleph One publishes “Smashing the Stack for Fun and Profit”  1997: Solar Designer describes return-to-libc technique.  1999: Compiler-based mitigations proposed.  2001: Code Red worm exploits overflow in IIS.  2003: Slammer worm infects every vulnerable server in an hour.  2004: AMD and Intel add non-executable bit in page tables.  2008: Twilight hack unlocks Wii consoles.  2010: Android 2.3 adds support for non-exec stack.  2013: Unicode SSIDs crash iPhones.  2015: GHOST overflow impacts glibc. CSC 666: Secure Software Engineering

11. Process Memory Map 11 Stack (procedure/ method calls) Heap (dynamically allocated) Heap grows, e.g., due to “new” or malloc() Stack grows, e.g., due to procedure call Stack pointer (SP) (current top of stack) This diagram shows how stacks grow on Intel x86s & others; some grow other way. Multi-threaded programs have multiple stacks Heap pointer Lower-numbered addresses Higher-numbered addresses Warning: Some diagrams elsewhere show lower-numbered addresses at the bottom Text (compiled program code) Often read- only Initialized global “data” Uninitialized global “data” Used for global constants & variables Set on code load

12. 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

13. Abstract data type “Stack” “Stack”: Abstract CS concept –“A stack of objects has the property that the last object placed on the stack will be the first object removed. This property is commonly referred to as last in, first out queue” (LIFO). Minimum stack operations: –PUSH: Add an element to the top of the stack –POP: Removes the last element at the top of the stack (returning it) and reduces stack size by one. CSC 666: Secure Software Engineering

14. “Stack” in process memory map Memory area set aside to implement calls to a procedure/function/method/subroutine For now we’ll use these terms interchangeably In C the term is “function” Stack is used to implement control flow When you call a procedure, where it “came from” is pushed on stack When a procedure returns, the “where I came from” is popped from stack; system starts running code there Stack also used for other data (in many cases) Parameters passed to procedures Procedure local variables Return values from procedure CSC 666: Secure Software Engineering

15. Call Stack Layout b() { … } a() { b(); } main() { a(); } Unallocated Stack Frame for b() Stack Frame for a() Stack Frame for main() High Memory Low Memory CSC 666: Secure Software Engineering

16. What is a Stack Frame? Block of stack data for one procedure call. Frame pointer (FP) points to frame: –Use offsets to find local variables. –SP continually moves with push/pops. –FP only moves on function call/return. –Intel CPUs use %ebp register for FP.

17. Why use stacks for function calls? First compiled languages (e.g., FORTRAN) did not use stacks Call data stored with fn where program “came from” Result: Procedures could not call themselves, directly or indirectly, as that would overwrite stored information. If functions can arbitrarily call other functions Need to store old state so can return back Need dynamic allocation for call (frame) sequences Stack is flexible & efficient CSC 666: Secure Software Engineering

18. CPUs track two stack values Stack pointer (SP): Value of “top” of stack –Where last data was stored on stack, possibly +/- 1 depending on architecture conventions –Modified when data pushed/popped May even be modified during expression calculation Frame pointer (FP): Value of “this frame” –Simplifies accessing parameters & local variables –Points inside stack to where “this procedure” starts –Modified on entry/exit of a procedure CSC 666: Secure Software Engineering

19. Accessing the Stack Pushing an item onto the stack. 1.Copy 4 bytes of data to stack. 2.Decrement SP by 4. Example: pushl $12 Popping data from the stack. 1.Copy 4 bytes of data from stack. 2.Increment SP by 4. Example: popl %eax Retrieve data without pop: movl %esp, %eax

20. Calling a procedure Given this C program: void main() { f(1,2,3); } The invocation of f() might generate assembly: pushl $3 ; constant 3 pushl $2 ; Most C compilers push in reverse order by default pushl $1 call f “call” instruction pushes instruction pointer (IP) on stack –In this case, the position in “main()” just after f(…) –Saved IP named the return address (RET) –CPU then jumps to start of “function” CSC 666: Secure Software Engineering

21. Stack: After push of value 3 Lower-numbered addresses Higher-numbered addresses Stack pointer (SP) (current top of stack) 3

22. Stack: After push of value 2 Lower-numbered addresses Higher-numbered addresses 2 Stack pointer (SP) (current top of stack) 3 Stack grows, e.g., due to procedure call

23. Stack: After push of value 1 Lower-numbered addresses Higher-numbered addresses 1 2 Stack pointer (SP) (current top of stack) 3 Stack grows, e.g., due to procedure call

24. Stack: Immediately after call Lower-numbered addresses Higher-numbered addresses Return address in main() 1 2 Stack pointer (SP) (current top of stack) 3 Stack grows, e.g., due to procedure call

25. Function prologue Imagine f() has local variables, e.g. in C: void f(int a, int b, int c) { char buffer1[5]; char buffer2[10]; strcpy(buffer2, "This is a very long string!!!!!!!"); } Typical x86-32 assembly on entry of f() (“prologue”): pushl %ebp ; Push old frame pointer (FP) movl %esp,%ebp ; New FP is old SP subl $20,%esp ; New SP is after local vars ; “$20” is calculated to be >= local var space In the assembly above, “;” introduces a comment to end of line CSC 666: Secure Software Engineering

26. Stack: Immediately after call Lower-numbered addresses Higher-numbered addresses Return address in main() 1 2 Stack pointer (SP) (current top of stack) 3 Stack grows, e.g., due to procedure call

27. Stack: After prologue Lower-numbered addresses Higher-numbered addresses Frame pointer (FP) – use this to access local variables & parameters Return address in main() 1 2 Saved (old) frame pointer Local array “buffer1” Local array “buffer2” Stack pointer (SP) (current top of stack) 3 Stack grows, e.g., due to procedure call

28. Stack: Overflowing buffer2 Lower-numbered addresses Higher-numbered addresses Frame pointer (FP) – use this to access local variables & parameters Return address in main() 1 2 Saved (old) frame pointer Local array “buffer1” Local array “buffer2” Stack pointer (SP) (current top of stack) 3 Stack grows, e.g., due to procedure call Overwrite

29. Effect of Overflowing Buffer2 Overwrites whatever is past buffer2!  As you go further, overwrite higher addresses Impact depends on system details In our example, can overwrite:  Local values (buffer1)  Saved frame pointer  Return value (changing what we return to)  Parameters to function  Previous frames CSC 666: Secure Software Engineering

30. Common buffer overflow attack  Send too large data as user input.  Overlarge data overwrites buffer  Modifies return value, to point to something the attacker wants us to run  Maybe with different parameters, too  On return, runs attacker-selected code  But it gets worse… CSC 666: Secure Software Engineering

31. Shellcode Injection  Attacker can also include machine code that they want us to run.  If they can set the “return” value to point to this malicious code, on return the victim will run that code  Unless something else is done  Significant portion of “Smashing the Stack” paper describes how to insert such code. CSC 666: Secure Software Engineering

32. Stack: Overflow with Shellcode Lower-numbered addresses Higher-numbered addresses Frame pointer (FP) – use this to access local variables & parameters Return address in main() 1 2 Saved (old) frame pointer Local array “buffer1” Local array “buffer2” Stack pointer (SP) (current top of stack) 3 Stack grows, e.g., due to procedure call Malicious code Ptr to malicious code

33. Stack: Shellcode + NOP Sled Lower-numbered addresses Higher-numbered addresses Frame pointer (FP) – use this to access local variables & parameters Return address in main() 1 2 Saved (old) frame pointer Local array “buffer1” Local array “buffer2” Stack pointer (SP) (current top of stack) 3 Stack grows, e.g., due to procedure call Ptr to malicious code Shellcode: \xeb\x1f\x5e\x89\x76\x08\x31\xc0\x88\x46\ x07\x89\x46\x0c\xb0\x0b\x89\xf3\x8d\x4e\x 08\x8d\x56\x0c\xcd\x80\x31\xdb\x89\xd8\x 40\xcd\x80\xe8\xdc\xff\xff\xff/bin/sh NOP sled: \x90\x90\x90\x90\x90…. NOP sleds let attacker jump anywhere to attack; real ones often more complex (to evade detection) Shellcode often has odd constraints, e.g., no byte 0

34. Shellcode  Shellcode is a small piece of machine code inserted into a program by exploiting a vulnerability.  Called shellcode since it is often used to start a command shell under control of attacker.  Example shellcode  Remote shell (like ssh)  Reverse shell (opens connection to attacker server)  Remote desktop (RDP, VNC, etc.)  Downloader (installs remote control tools) CSC 666: Secure Software Engineering

35. Off-by-One Overflows Even 1 byte overflow can allow attacker to control execution by overwriting low order byte of FP. CSC 666: Secure Software Engineering

36. Heap Overflows Programs allocate memory on heap via  C: malloc() and free()  C++: new and delete To run shellcode, attack overwrites function pointer  Function pointers in C.  C++ methods implemented as function pointers.  C++ exceptions result in following a function pointer. CSC 666: Secure Software Engineering

37. Memory Allocation  Memory allocators typically  Allocate memory in chunks.  Chunks stored in a linked list of bins.  Chunks contain header, pointers, and data.  Each bin contains chunks of a specific size.  Final chunk contains free space information.  Memory corruption attacks target  Header information, and  Pointers. CSC 666: Secure Software Engineering

38. Malloc Data Structure CSC 666: Secure Software Engineering

39. Malloc Example char *a, *b, *c; a = malloc(32) b = malloc(32); c = malloc(32); … free(c) free(b) free(a) CSC 666: Secure Software Engineering

40. Malloc Example Memory Layout CSC 666: Secure Software Engineering

41. Malloc Layout after Free CSC 666: Secure Software Engineering

42. Overflowing Dynamic Buffers CSC 666: Secure Software Engineering

43. Overwriting vtable Pointers CSC 666: Secure Software Engineering

44. free() vulnerabilities  To de-allocate memory, free(void *) must  Use pointer to find chunk to de-allocate.  Modify pointers in chunk header to remove chunk from the list of in-use chunks.  Loop: if adjacent chunk is free, combine chunks to avoid memory fragmentation.  To exploit free(), an attacker can  Overwrite adjacent chunk metadata.  Cause free() to overwrite a specific word in memory with attacker specified data.  Attacker will choice location to be that of a pointer (lib function, vtable ptr, ret addr, etc.) to control execution. CSC 666: Secure Software Engineering

45. Double free() vulnerability 1.Program free()s chunk twice, so chunk appears twice in the free list. 2.Program allocates memory, reusing part or all of chunk.  malloc() interprets first bytes as free list header.  Program interprets first bytes as data. 3.Write to allocated chunk, changing first bytes. 4.Program allocates, attempts to reuse chunk.  malloc() writes to location specified in first bytes, treating bytes as free list pointer, but those bytes now point to any location specified by attacker. CSC 666: Secure Software Engineering

46. Non-executable Stack  Memory protection prevents code on the stack from being executed.  Adds NX permission bit to page tables.  Required CPU and OS modifications.  Limitations  Some applications need to execute code on the stack (JIT compilers for Java, JavaScript).  Attackers can target other areas of memory.  Buffer overflows can result in remote code execution without running attacker generated shellcode.

47. Address Space Randomization  ASLR randomly changes code/data locations to prevent memory exploits.  Problems  Code must be compiled as Position Independent Executables (PIE) to relocate code segment at cost of a 10% performance penalty.  Memory corruption can result in information leakage of address layout.  Attackers can brute force 32- bit ASLR with many copies of shellcode (heap-spraying). CSC 666: Secure Software Engineering

48. Canary Defenses Compiler changes calling convention in two ways: –Adds a canary word to the stack (“canary in a coal mine”) –Verifies presence of canary word before executing the ret instruction to return to address on stack. Protects against stack smashing since –Overflow would have to overwrite canary to reach return value. –If canary is chosen randomly, attacker cannot know what to overwrite to that memory location. –Does not protect against indirect memory writes. CSC 666: Secure Software Engineering

49. Canary Stack Layout Frame Pointer Stack Pointer old frame param1 param2 old PC canary word old FP local vars

50. Return-Oriented Programming ROP is an exploit technique in which the attacker gains control of the call stack and uses it to execute small pieces of code, called “gadgets.” –Attacker uses existing code. –Bypasses NX defense, since no new code executed. Gadgets are typically found in shared libraries –Gadgets can be entire functions. –Gadgets can be fragments of code that end in a ret instruction (even unintentional instructions.) –Attacker controls order of execution and parameters by placing data on call stack. CSC 666: Secure Software Engineering

51. Normal Programming  Instruction pointer (%eip) determines which instruction to fetch and execute.  CPU automatically increments %eip to next instruction.  Control flow by modifying %eip. CSC 666: Secure Software Engineering

52. Return Oriented Programming Stack pointer (%esp) determines which instruction sequence to fetch and execute. Move to next sequence after ret instruction. CSC 666: Secure Software Engineering

53. Finding Gadgets  Intel x86 assembly is dense  Instructions are of variable length.  Instruction pointer can point to any location.  As a result  Any byte c3 is a ret instruction.  By pointing %eip into the middle of a long instruction, part of that instruction can be executed as a different instruction or set of instructions. CSC 666: Secure Software Engineering

54. Unintended Instructions Example CSC 666: Secure Software Engineering

55. Writing Overflow Free Code 1.Know which functions are  Unsafe: guaranteed buffer overflow.  Safe: no overflow if arguments correct. 2.Know how to use safe functions securely. 3.Do your own bounds checking.  Be careful to avoid integer overflows. 4.Check for memory errors  Security-oriented static analysis like Fortify.  Memory dynamic analysis: Valgrind, Purify. CSC 666: Secure Software Engineering

56. Many C fns don’t check bounds  gets(3) – reads input without checking. Don’t use it!  strcpy(3) – strcpy(dest, src) copies from src to dest  If src longer than dest buffer, keeps writing!  strcat(3) – strcat(dest, src) appends src to dest  If src + data in dest longer than dest buffer, keeps writing!  scanf() family of input functions – many dangerous  scanf(3), fscanf(3), sscanf(3), vscanf(3), vsscanf(3), vfscanf(3)  Many options don’t control max length (e.g., bare “%s”)  Many other dangerous functions, e.g.:  realpath(3), getopt(3), getpass(3)  streadd(3), strecpy(3), and strtrns(3)  It’s not just functions; ordinary loops can overflow.

57. CSC 666: Secure Software Engineering gets()

58. CSC 666: Secure Software Engineering strcpy()

59. CSC 666: Secure Software Engineering sprintf()

60. Bounded Function Pitfalls 1.Destination buffer overflows because bound depends on size of source data, not destination buffer. 2.Destination buffer left without null terminator, often as result of off-by-one error. 3.Destination buffer overflows because its bound is specified as the total size of the buffer, rather than space remaining. 4.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

61. C11 Annex K Bounds Checking C11 standard adds bounds-checking interfaces Creates “safer” versions of C functions Limits lengths of results Example: strcpy_s(s1, s1max, s2); Copies s2 to s1. s1max > strnlen_s(s2, s1max) Does not automatically resize. On error, calls runtime-constraint handler function, controlled by set_constraint_handler_s(). This handler can permit returns If it returns, returns 0 if ok, nonzero if a constraint failed Not universally available. Provided by slibc. CSC 666: Secure Software Engineering

62. Safe String Libraries strlcat() and strlcpy() –BSD-licensed open source routines C++ std::string library –Dynamically-sized strings SafeStr library provides safestr_t objects –Dynamically-sized –Cast to (char *) for read-only purposes only Microsoft’s strsafe.h

63. Safe String Libraries 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) CSC 666: Secure Software Engineering

64. 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

65. Character Sets Characters represented using encoding forms that map code points to printable chars.  Fixed Width  ISO-8859-1  UTF-32  Variable Width  UTF-8 (most Internet protocols, Python, Ruby)  UTF-16 (Java,.NET) CharacterEncodingCode Point sISO-8859-1 UTF-8 73 ÿISO-8859-1 UTF-8 FF C3 BF

66. 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 encoding Buffer Overflow issues  Mixing up different character-set string types.  Are sizes measured in bytes or characters? CSC 666: Secure Software Engineering

67. C++ Dangers Using C-style strings with cin char username[16]; cin >> username; The [] operator does not perform bounds checking Converting from C++ to C-style strings string::data() output is not NULL terminated string::c_str() ouput is NULL terminated

68. Key Points 1.Memory corruption vulnerabilities can result in  Execution of attacker-supplied code.  Attacker hijacking of program control flow.  Alteration of program data.  Information leak. 2. Memory Corruption Attack Types  Buffer overflows: stack and heap  Format string attacks (%n)  Use after free CSC 666: Secure Software Engineering

69. Key Points (2) 3.Mitigations  Non-executable Stack (OS, hardware)  Address Space Layout Randomization (OS)  Stack Canaries (compiler) 4.Countering Mitigations  Brute force (heap spraying)  Return Oriented Programming 5.Writing Secure Code  Avoiding unsafe functions.  Performing bounds checking. CSC 666: Secure Software Engineering

70. References 1.Aleph Null, “Smashing the Stack for Fun and Profit,” Phrack 49, 1996. 2.Erik Buchanan et al., “Return-Oriented Programming: Exploits Without Code Injection”, https://cseweb.ucsd.edu/~hovav/talks/blackhat08.html, 2008.https://cseweb.ucsd.edu/~hovav/talks/blackhat08.html 3.Johnathan Bartlett, Programming from the Ground Up, Bartlett Publishing, 2004. 4.James C. Foster et al., Buffer Overflow Attacks, Syngress, 2005. 5.Scot Hand, Heap Based Exploitation, https://csg.utdallas.edu/wp- content/uploads/2012/08/Heap-Based-Exploitation.pdf, 2012. 6.Koziol et al., The Shellcoder’s Handbook: Discovering and Exploiting Security Holes, Wiley, 2004. 7.Haroon Meer, “History of Memory Corruption Attacks”, http://media.blackhat.com/bh-us-10/whitepapers/Meer/BlackHat-USA-2010-Meer- History-of-Memory-Corruption-Attacks-wp.pdf, 2010. http://media.blackhat.com/bh-us-10/whitepapers/Meer/BlackHat-USA-2010-Meer- History-of-Memory-Corruption-Attacks-wp.pdf 8.Tom Plum, “The New C Standard Explored”, http://www.drdobbs.com/cpp/the- new-c-standard-explored/232901670, Dr. Dobb’s Journal, 2012.http://www.drdobbs.com/cpp/the- new-c-standard-explored/232901670 9.Robert C. Seacord, Secure Coding in C and C++, Addison-Wesley, 2006. 10.Laszlo Szekeres et al. "Eternal war in memory." Proceedings of the 34th IEEE Symposium on Security and Privacy (SP’13), San Francisco, CA. 2013. 11.John Viega and Gary McGraw, Building Secure Software, Addison-Wesley, 2002. CSC 666: Secure Software Engineering