Control Hijacking Attacks (PDF)

Announcements: Project 1 is out: part I due Apr. 13. Please come to section on Friday at 11:30am Control Hijacking Basic Control Hijacking Attacks Dan Boneh  Control hijacking attacks Attacker’s goal: Tak...

Announcements: Project 1 is out: part I due Apr. 13. Please come to section on Friday at 11:30am Control Hijacking Basic Control Hijacking Attacks Dan Boneh  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 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. Whenever possible avoid C/C++ Often cannot avoid C/C++ : Need to understand attacks and defenses Feb. 2024: White House support for memory safety Source: web.nvd.nist.gov 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 x86-64 running Linux or Windows – Details vary slightly between CPUs and OSs: Stack Frame structure (Unix vs. Windows, x86 vs. ARM) Little endian vs. big endian Dan Boneh  Linux process memory layout (x86-64) 0x0000 7FFF FFFF FFFF (128 TB) user stack %rsp (stack pointer) (esp in 32-bit mode) shared libraries 0x0000 7F1F6 XXXX XXXX run time heap Loaded text and data from executable 0x0000 0000 0040 0040 unused 0 Dan Boneh  Stack Frame high arguments return address rbp stack base pointer exception handlers Stack local variables Growth rsp callee saved registers low (esp in 32-bit mode) Dan Boneh  What are buffer overflows? Suppose a web server contains a function: void func(char *str) { char buf; After func() is called stack looks like: strcpy(buf, str); do-something(buf); argument: str } return address stack base pointer char buf rsp Dan Boneh  What are buffer overflows? What if *url is 144 bytes long? void func(char *url) { After strcpy: char buf; strcpy(buf, url); do-something(buf); argument: str } return address stack base pointer Poisoned return address! *str Problem: char buf no bounds checking in strcpy() rsp Dan Boneh  Stack Basic stack exploit high Suppose *url is such that Program P after strcpy stack looks like: Program P: exec(“/bin/sh”) (exact shell code by Aleph One) return address When func() exits, the user gets shell ! char buf Note: attack code P runs in stack. low Dan Boneh  Stack The NOP slide high Program P Problem: how does attacker determine ret-address? NOP Slide Solution: NOP slide return address Guess approximate stack state when func() is called Insert many NOPs before program P: char buf nop (0x90) , xor eax,eax , inc ax 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: (… more on this in a bit) – Overwrite the address of an exception handler in stack frame. Function pointers: (e.g. PHP 4.0.2, MS MediaPlayer Bitmaps) Heap buf FuncPtr 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) FP1 method #1 vptr FP2 method #2 FP3 method #3 data vtable Object T NOP shell slide code After overflow of buf : vptr data buf vtable object T Dan Boneh An example: exploiting the browser heap Request web page Web page with exploit victim browser malicious web server Attacker’s goal is to infect browsers visiting the web site How: send javascript to browser that exploits a heap overflow Dan Boneh  A reliable exploit? shellcode = unescape("%u4343%u4343%..."); // allocate in heap overflow-string = unescape(“%u2332%u4276%...”); cause-overflow(overflow-string ); // overflow buf[ ] Problem: attacker does not know where browser places shellcode on the heap ??? data buf vtable ptr shellcode Dan Boneh  Heap Spraying [SkyLined] 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 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 ? ⇒ len1+len2 = 0 Second memcpy() will overflow heap !! Dan Boneh An example: a better length check void func( char *buf1, *buf2, unsigned int len1, len2) { char temp; // length check if (len1 > 256) || (len2 > 256) || (len1+ len2 > 256) return -1; memcpy(temp, buf1, len1); // cat buffers memcpy(temp+len1, buf2, len2); do-something(temp); // do stuff } Dan Boneh  Integer overflow exploit stats 700 600 500 400 300 200 100 0 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 ⋯ 2015 2016 2017 2018 2019 2020 Dec. 2020: integer underflow in F5 Big IP if (8190 − nlen