Computer Security - Stanford University

Computer Security - Stanford University

Control Hijacking Basic Control Hijacking Attacks Dan Boneh Control hijacking attacks Attackers goal: Take over target machine (e.g. web server) Execute arbitrary code on target by hijacking application control flow Examples: Buffer overflow and integer overflow attacks

Format string vulnerabilities Use after free Dan Boneh First example: buffer overflows Extremely common bug in C/C++ programs. First major exploit: 1988 Internet Worm. fingerd. Source: Dan Boneh 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) Dan Boneh Linux process memory layout user stack 0xC0000000 %esp shared libraries

brk Loaded from exec 0x40000000 run time heap unused 0x08048000 0 Dan Boneh Stack Frame high

arguments return address stack frame pointer exception handlers local variables SP callee saved registers Stack Growth low Dan Boneh What are buffer overflows?

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

Dan Boneh What are buffer overflows? What if *str is 136 bytes long? After strcpy: void func(char *str) { char buf[128]; strcpy(buf, str); do-something(buf); argument: str return address stack frame pointer *str char buf[128]

SP } Problem: no length checking in strcpy() Dan Boneh Basic stack exploit Suppose *str is such that after strcpy stack looks like: high Program P

Program P: exec(/bin/sh) (exact shell code by Aleph One) return address When func() exits, the user gets shell ! Note: attack code P runs in stack. char buf[128] low Dan Boneh The NOP slide high

Program P Problem: how does attacker determine ret-address? NOP Slide Solution: NOP slide Guess approximate stack state when func() is called

Insert many NOPs before program P: nop , xor eax,eax , inc ax return address char buf[128] low Dan Boneh Details and examples Some complications: Program P should not contain the \0 character. Overflow should not crash program before func() exits. (in)Famous remote stack smashing overflows: Overflow in Windows animated cursors (ANI).

LoadAniIcon() Buffer overflow in Symantec virus detection (May 2016) overflow when parsing PE headers kernel vuln. Dan Boneh 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 Dan Boneh Buffer overflow opportunities Exception handlers: (Windows SEH attacks more on this later) Overwrite the address of an exception handler in stack frame. Function pointers: (e.g. PHP 4.0.2, MS MediaPlayer Bitmaps) buf[128] FuncPtr Heap or

stack Overflowing buf will override function pointer. Longjmp buffers: longjmp(pos) (e.g. Perl 5.003) Overflowing buf next to pos overrides value of pos. Dan Boneh Heap exploits: corrupting virtual tables Compiler generated function pointers (e.g. C++ code) vptr FP1 FP2 FP3

data vtable method #1 method #2 method #3 NOP slide Object T shell code

vtable data buf[256] vptr After overflow of buf : object T Dan Boneh An example: exploiting the browser heap

Request web page Web page with exploit victim browser malicious web server Attackers goal is to infect browsers visiting the web site How: send javascript to browser that exploits a heap overflow Dan Boneh A reliable exploit? // overflow buf[ ] Problem: attacker does not know where browser places shellcode on the heap buf[256] vtable data ptr ??? shellcode

Dan Boneh Heap Spraying [SkyLined 2004] Idea: 1. use Javascript to spray heap with shellcode (and NOP slides) 2. then point vtable ptr anywhere in spray area NOP slide shellcode heap

vtable heap spray area Dan Boneh Javascript heap spraying var nop = unescape(%u9090%u9090) while (nop.length < 0x100000) nop += nop; var shellcode = unescape("%u4343%u4343%..."); var x = new Array () for (i=0; i<1000; i++) { x[i] = nop + shellcode; } Pointing function-ptr almost anywhere in heap will cause shellcode to execute.

Dan Boneh Ad-hoc heap overflow mitigations Better browser architecture: Store JavaScript strings in a separate heap from browser heap OpenBSD and Windows 8 heap overflow protection: prevents cross-page overflows guard pages (non-writable pages) Nozzle [RLZ08] : detect sprays by prevalence of code on heap Dan Boneh

Finding overflows by fuzzing To find overflow: Run web server on local machine Issue malformed requests (ending with $$$$$ ) Many automated tools exist (called fuzzers next week) If web server crashes, search core dump for $$$$$ to find overflow location Construct exploit (not easy given latest defenses) Dan Boneh Control Hijacking More Control Hijacking Attacks Dan Boneh

More Hijacking Opportunities Integer overflows: (e.g. MS DirectX MIDI Lib) Double free: double free space on heap Can cause memory mgr to write data to specific location Examples: CVS server Use after free: using memory after it is freed Format string vulnerabilities Dan Boneh Integer Overflows (see Phrack 60) Problem: what happens when int exceeds max value? int m; (32 bits)

short s; (16 bits) c = 0x80 + 0x80 = 128 + 128 c = 0 c = 0 s=0 s = 0xff80 + 0x80 m = 0xffffff80 + 0x80 c = 0

char c; (8 bits) c=0 m=0 Can this be exploited? Dan Boneh An example void func( char *buf1, *buf2, unsigned int len1, len2) { char temp[256]; if (len1 + len2 > 256) {return -1} // length check memcpy(temp, buf1, len1); // cat buffers memcpy(temp+len1, buf2, len2);

do-something(temp); // do stuff } What if len1 = 0x80, len2 = 0xffffff80 ? c = 0 len1+len2 = 0 Second memcpy() will overflow heap !! Dan Boneh Integer overflow exploit stats 140 120 100 80 60

40 20 0 Source: NVD/CVE 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2015 2016 Dan Boneh Format string bugs

Dan Boneh Format string problem int func(char *user) } { fprintf( stderr, user); Problem: what if *user = %s%s%s%s%s%s%s ?? Most likely program will crash: DoS. If not, program will print memory contents. Privacy? Full exploit using user = %n Correct form:

fprintf( stdout, %s, user); Dan Boneh Vulnerable functions Any function using a format string. Printing: printf, fprintf, sprintf, vprintf, vfprintf, vsprintf, Logging: syslog, err, warn Dan Boneh Exploit Dumping 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) Dan Boneh Use after free exploits Dan Boneh IE11 Example: CVE-2014-0282 (simplified)

Loop on form elements: c1.DoReset() Dan Boneh

What just happened? c1.doReset() causes changer() to be called and free object c2 object c2 vptr FP1 FP2 FP3 data vtable DoSomething DoReset

DoSomethingElse Dan Boneh What just happened? c1.doReset() causes changer() to be called and free object c2 object c2 vptr FP1 FP2 FP3 data vtable

ShellCode Use after free ! Suppose attacker allocates a string of same size as vtable When c2.DoReset() is called, attacker gets shell Dan Boneh The exploit Lesson: use after free can be a serious security vulnerability !! Dan Boneh Next lecture DEFENSES Dan Boneh THE END Dan Boneh

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