Buffer Overflows

1. Buffer Overflows Lesson 14

2. Example of poor programming/errors • Buffer Overflows • result of poor programming practice • use of functions such as gets and strcpy • these don’t check input for boundaries • may allow individual to gain root or admin access • Easy to do in any programming language…what is the real problem?

3. What is a buffer overflow? • “A buffer overflow attack is when an attacker tries to store too much information in an undersized receptacle.” • “A common implementation is when a user of the program gives the program more data than the developers of the program allocated to store it.”

4. Exploits • Buffer Overflows • fingerd, statd, talkd, … • result of poor programming practice • Shell Escapes • special character in input string causes escape to shell

5. Buffer Overflow Example #include <stdio.h> #include <string.h> void func(char *p) { char stack_temp[20]; strcpy(stack_temp, p); printf(stack_temp); } int main(int argc, char* argv[]) { func(“I AM MORE THAN TWENTY CHARACTERS LONG!”); return 0; }

6. Buffer Overflows Data Execute A Return Addr Subroutine A Process Stack Read Variable Return Program

7. Buffer Overflows Data New Addr Execute A Return Addr Subroutine A Process Stack Read Variable Return Program Another Routine

8. Buffer Overflows Data New Addr Execute A Machine Code Return Addr Subroutine A Process Stack Read Variable Return Program

9. Types of buffer overflow attacks • Denial of service – buffer overflow will cause the system to “crash” • Since important information needed by the OS to continue running can be located on the stack, by overflowing with enough data you can wipe out this important information. • Execution of code that the attacker chooses to run. • Overwrite just the right amount of information to overflow the stack and rewrite the return address pointer. • Do this right and you can point to your own code.

10. Buffer Overflows (cont) • “A key point to remember is that the attacker’s code will run at whatever privileges the software that is exploited is running at.” • “In most cases, an attacker tries to exploit programs that are running as a privileged account such as root or domain administrator.”

11. Protection against buffer overflow attacks • Close the port or service • Best way to protect yourself is to remove SW that is subject to an overflow. • If this SW is installed by default, close ports and remove service. • Rule of thumb: “Know what is installed on your systems and have the least amount of services running and ports open that are required for the system to operate in a specific environment.”

12. Protection • Apply the vendor’s patch or install the latest version of the software. • Usually shortly after a buffer overflow vulnerability is discovered the vendor will develop and release a patch. • This fixes the problem as opposed to just minimizing exposure.

13. Protection • Filter specific traffic at the firewall. • Block the traffic of the vulnerable software at the firewall. • This will restrict the ability of external attackers to exploit the vulnerability. • Does not prevent an insider from exploiting the vulnerability, just limits the exposure.

14. Prevention • Test key applications. • Take a proactive approach and attempt to find buffer overflow exploits yourself. • Not practical for all applications but for key ones it is.

15. Prevention • Run Software at the Least Privilege Required • Often system administrators will install and configure applications as root. • Quick an easy to ensure they have access to what they need. • Also easy way to guarantee system is vulnerable if buffer overflow exploit is discovered in one of the applications since it will execute code as root.