CNIT 127 Ch 4: Introduction to format string bugs
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This document discusses format string vulnerabilities. It explains that format strings can be used to read arbitrary memory locations on the stack, allowing information disclosure. It also describes how uncontrolled format strings can be exploited to write to arbitrary memory addresses, enabling denial of service attacks or potentially remote code execution. The key steps of a format string exploit are outlined: controlling a write operation, finding a target memory location like the return address or GOT, writing shellcode to memory, and changing the target location to point to the shellcode.
![Buffer Overflow
• This code is obviously stupid
char name[10];
strcpy(name, "Rumplestiltskin");
• C just does it, without complaining](https://image.slidesharecdn.com/127-ch4-210301010231/85/CNIT-127-Ch-4-Introduction-to-format-string-bugs-7-320.jpg)
CNIT 127 Ch 4: Introduction to format string bugs
- 1. CNIT 127: Exploit Development Ch 4: Introduction to Format String Bugs Updated 2-28-21
- 3. Data Interpretation • RAM contains bytes • The same byte can be interpreted as – An integer – A character – Part of an instruction – Part of an address – Part of a string – Many, many more...
- 4. Format String Controls Output
- 5. Most Important for Us • %x Hexadecimal • %8x Hexadecimal padded to 8 chars • %10x Hexadecimal padded to 10 chars • %100x Hexadecimal padded to 100 chars
- 7. Buffer Overflow • This code is obviously stupid char name[10]; strcpy(name, "Rumplestiltskin"); • C just does it, without complaining
- 8. Format String Without Arguments • printf("%x.%x.%x.%x"); – There are no arguments to print! – Should give an error message – Instead, C just pulls the next 4 values from the stack and prints them out – Can read memory on the stack – Information disclosure vulnerability
- 9. Format String Controlled by User
- 10. Explanation • %x.%x.%x.%x -- read 4 words from stack • %n.%n -- write 2 numbers to RAM addresses from the stack
- 11. %n Format String • %n writes the number of characters printed so far • To the memory location pointed to by the parameter • Can write to arbitrary RAM locations • Easy DoS • Possible remote code execution
- 12. printf Family • Format string bugs affect a whole family of functions
- 13. Countermeasures
- 14. Defenses Against Format String Vulnerabilities • Stack defenses don't stop format string exploits – Canary value • ASLR and NX – Can make exploitation more difficult • Static code analysis tools – Generally find format string bugs • gcc – Warnings, but no format string defenses
- 16. Steps for a Format String Exploit • Control a write operation • Find a target RAM location – That will control execution • Write 4 bytes to target RAM location • Insert shellcode • Find the shellcode in RAM • Write shellcode address to target RAM location
- 17. Control a Parameter • The format string is on the stack • Insert four letters before the %x fields • Controls the fourth parameter – Note: sometimes it's much further down the list, such as parameter 300
- 18. Target RAM Options • Saved return address – Like the Buffer Overflows we did previously • Global Offset Table – Used to find shared library functions • Destructors table (DTORS) – Called when a program exits • C Library Hooks
- 19. Target RAM Options • "atexit" structure (link Ch 4n) • Any function pointer • In Windows, the default unhandled exception handler is easy to find and exploit
- 20. Disassemble in gdb • gdb -q ED204 • disassemble main • First it calls printf • Later it calls exit
- 21. Dynamic Relocation (also called Global Offset Table (GOT)) • PLT and GOT are used to address shared libraries • See links Ch 4o, 4p
- 22. Writing to the GOT • We control the eip!
- 23. Python Code to Write 1 Byte
- 24. Python Code to Write Four Bytes
- 25. Write Chosen Values in 4 Bytes
- 26. Write Chosen Values in 4 Bytes
- 27. Inserting Dummy Shellcode xcc is BRK
- 28. View the Stack in gdb • Choose an address in the NOP sled
- 29. Dummy Exploit Runs to xcc
- 30. Testing for Bad Characters • Avoid these
- 31. Testing for Bad Characters • All the other characters got through
- 33. Keep Total Length of Injection Constant • Add 'A' characters after shellcode • To keep the stack frame size constant
- 34. Final Check • Address in NOP sled • Shellcode intact
- 35. Shell