Secure Coding Against Code Injection Attacks

1. Stack-based buffer overflows, part 2 Yves Younan DistriNet, Department of Computer Science Katholieke Universiteit Leuven Belgium Yves.Younan@cs.kuleuven.ac.be

2. Overview • Code injection Yves Younan - Methodology for Designing Countermeasures against Code injection Attacks

3. Code injection • Finding the inserted code is sometimes a problem • Often an attacker will fill a buffer with nops and place the shellcode at the end • If he misses the address he may end up in the nops Yves Younan - Methodology for Designing Countermeasures against Code injection Attacks

4. Gdb intro • Compile code with -g for debugging information • Gdb program • break main -> tells the debugger to stop when main is reached • run -> run program • x buffer -> prints out the contents of buffer (and address) • If the shellcode is stored in the buffer, that address will be what to overwrite the return address with Yves Younan - Methodology for Designing Countermeasures against Code injection Attacks

5. Execve • Execve allows execution of a program • int execve(const char *filename, char *const argv [], char *const envp[]); • Must pass an array of arguments, note that the program name is argument 0, terminated with NULL • Must also pass an array of environment variables, terminated with NULL Yves Younan - Methodology for Designing Countermeasures against Code injection Attacks

6. Execve • #include <unistd.h> Int main (int argc, char **argv) { char *execargv[3] = { "/bin/ls", "--color=always", NULL }; char *env[2] = { "TEST=1", NULL }; execve(execargv[0],execargv,env); } Yves Younan - Methodology for Designing Countermeasures against Code injection Attacks

7. Finding inserted code • Generally (on kernels < 2.6) the stack will start at a static address • Finding shell code means running the program with a fixed set of arguments/fixed environment • This will result in the same address • Not very precise, small change can result in different location of code • Not mandatory to put shellcode in buffer used to overflow • Pass as environment variable Yves Younan - Methodology for Designing Countermeasures against Code injection Attacks

8. Controlling the environment Stack start: 0xBFFFFFFF High addr 0,0,0,0 Passing shellcode as environment variable: Program name Env var n • Stack start - 4 null bytes • strlen(program name) - • null byte (program name) • strlen(shellcode) Env var n-1 … Env var 0 Arg n • 0xBFFFFFFF - 4 • strlen(program name) - • 1 • strlen(shellcode) Arg n-1 … Arg 0 Low addr Yves Younan - Methodology for Designing Countermeasures against Code injection Attacks

9. Conclusion • Follow “Gera’s Insecure Programming by example”: • http://community.corest.com/~gera/InsecureProgramming/ • Login/pass for the computers: cstudy/distrinet Yves Younan - Methodology for Designing Countermeasures against Code injection Attacks