CAP6135: Malware and Software
Vulnerability Analysis
Fuzzing Test Example
Cliff Zou
Spring 2013
Objective

Explain basic fuzzing with concrete coding
example

Explain how the vulnerable code in
programming project 2 is derived

Introduce several useful techniques in
doing the fuzzing test on project 2
2
Example Code

$ fuzzTest-target 200 “what is this?” 2

Example code needs three inputs

Int, string, Int
int inputInteger; /* global variable */
if (argc != 4){
fprintf(stderr, "fuzzTest needs 3 input parameters: int string int!\n");
exit(0);
}
sscanf(argv[1], "%d", &inputInteger);
my_func(inputInteger, argv[2], argv[3]);

Subfun my_func() introduces 3 man-made bugs
3
Bug # 1: Integer Overflow
int my_func(short argLen, char *str, char *divStr)
{
int denominator;
float x;
char buf[bufLen];
if (argLen != inputInteger) {
fprintf(stderr, "Bug #1: integer overflow triggered\n");
foo = (void *)0xbfffffff;
foo(argLen); /* trigger illegal instruction fault */
exit(1);

Int variable inputInteger changes to short


Overflow happens when inputInteger>32767
foo() is a function pointer

Give it an arbitrary address will cause illegal memory reference
for executing code
4
Bug # 2: buffer Overflow
char buf[10];
if (strlen(str) > 10){
fprintf(stderr, "Bug #2: buffer overflow triggered. strlen=%d\n", strlen(str));
strcpy(buf, str); /* trigger segmentation fault or stack smashing , */
return 2; /*if overwriting return address, it will cause segmentation fault */
}
5
Bug #3: divide by zero
int denominator; float x;
sscanf(divStr, "%d", &denominator);
if (denominator == 0){
fprintf(stderr, "Bug #3: division by zero triggered\n");
x = argLen / denominator;
foo = (void *)0xbffbffff;
foo(argLen); /* trigger illegal instruction fault */
}else
x = argLen / denominator;
return 0;
6
Fuzzer Outline

Generate inputs (random or follow rules)
firstInt = rand()%50000; secondInt = rand() % 2;
arraySize = rand() % 20;
charArray = (char *) malloc(arraySize);
for (j=0; j< arraySize; j++)
charArray[j] = 'A';
charArray[arraySize-1] = NULL;

Generate execution command line
sprintf(buffer, "./fuzzTest-target %d \"%s\" %d\n", firstInt,
charArray, secondInt);
free(charArray); /* must free memory for repeat testing! */
7
Fuzzer Outline



Execute target code
ret = system(buffer);
Obtain target execution exit code
wait(&status);
retCode = WEXITSTATUS(ret);
Check abnormal exit code and record inputs
that cause the abnormal
if ( retCode == 128+11 || retCode ==128+6) /* segmentation fault (11) or Abort (6) */
{
printf("retCode=%d ## Input: firstInt = %d, arraySize = %d, secondInt = %d\n",
retCode, firstInt, arraySize, secondInt);
fflush(stdout); /*make sure output is print out immediately ! */
}

Repeat from start in generating inputs
8
How to Record Fuzzing Result?



When abnormal happens, record down inputs that cause
the abnormal
Record the corresponding abnormal message printout by
target code
Unix OS I/O definition:


I/O redirection:





stdin (0), stdout (1), stderr (2)
$ Command < data.txt: let stdin get from file (instead of keyboard)
$ Command > output.txt: let stdout redirect to file
$ Command 2> error.txt: let stderr redirect to file
$ Command &> output.txt: let stdout and stderr redirect to file
For our example:

$./fuzzTest100 &> output.txt
9
Program Project 2
Introduction
10
Manual Read Sample.jpg File

To understand the jpeg file format and the
project’s ‘sample.format’ description, you need a
HEX Editor:

In Unix: use “$hexdump sample.jpg > hex.txt”



HexEdit for Win: http://www.physics.ohio-state.edu/~prewett/hexedit/


Each two-byte value is shown as ‘daff’ where the first byte is
‘ff’ and second byte is ‘da’ !
A bit confusing on the byte order
This program shows each byte value, so no confusion on
big-endian or little-endian stuff.
You can use windows accessories “calculator” to
translate between decimal and hexadecimal values

Use ‘programmer’ option in ‘view’ menu
11
One-Round Fuzzing Outline




In our fuzzer, we need to first read
sample.jpg into a char buffer array
Then, modify the buffer (randomly or
follows format rules)
Then, write the content of the buffer to
test.jpg file.
Then, invoke jpegconv on test.jpg to do
fuzz test
12
Read sample.jpg into Buffer
char imageBuf[10000]; /*enough to hold sample.jpg */
int fSize;
FILE *fin, *fout;
fin = fopen(“./sample.jpg”, "rb"); fout = fopen(“./test.jpg”, “wb");
fseek(fin, 0, SEEK_END); /* set file pointer to the file end */
fSize=ftell(fin); /*get input file size */
fseek(fin, 0, SEEK_SET); /* rewind the pointer to the start of file fin */
fread(imageBuf, 1, fSize, fin); /* read byte stream of the file */
fclose(fin);
/* then, modify imageBuf randomly, */
/* or follow jpeg format on the header section*/
13
Jpeg Header Format



Now the ‘sample.jpg’ is in the char array imageBuf[]
Check the ‘sample.format’ for the Jpeg format
For example:




imageBuf[0] = 0xff; imageBuf[1] = 0xd8;  SOI header
imageBuf[158]=0xff; imageBuf[159]=0xc0;  SOF header
imageBuf[609]=0xff; imageBuf[610]=0xda;  SOS header
Simple fuzzing: Mutation-based fuzzing


Only work on Jpeg Header section since all bugs are in here
You may only be able to find a few bugs in this way


Of course, trying millions of inputs may find all bugs if you are lucky
Advanced fuzzing: Protocol-aware fuzzing


Follow the guide in project description, modify format sections
step-by-step
Modify different section could trigger different bugs
14
Write fuzzed image to test.jpg
fwrite (imageBuf , 1, fSize, fout );
/* if you modified the image size, then use the new fSize */
fclose (fout);
/* then, invoke jpegconv on test.jpg for testing */

Note that the ‘test.jpg’ will only save the
newest fuzzed file!
15
Save Fuzzed Input That Causes Bug
int status, ret, retCode;
int crashNum = 0;
char fileName[20]; /* saved fuzzed image file name */
char comBuf[200]; /* save the command line string */
sprintf(comBuf, “./jpegconv -ppm -outfile foo.ppm test.jpg");
ret = system(buffer);
wait(&status);
retCode = WEXITSTATUS(ret);
if (retCode == 139){ /* Segmentation fault for a bug */
crashNum ++;
sprintf(fileName, “./crashed-%d.jpg”, crashNum);
fout = fopen(fileName, “wb");
fwrite (imageBuf, 1, fSize, fout );
fclose (fout);
}
16
Notes

Remember, do not save every fuzzed input into
image files!




There is no enough disk space for that on Eustis!
You will still have multiple fuzzed image saved for the same
bug.
You can find smart way to only save one copy for each bug.
When one or two bugs are repeatedly triggered


Try to modify image on other format sections
Mutate image file in different ways

Change to different values


random, negative, zero, upper-bound…
Change different number of bytes

Consecutive, randomly picked….
17
Unsolved Task

How to Match crashed-x.jpg to its bug ID?


Hint: Jpegconv uses stderr to print out “BUG
X TRIGGERED”
I will leave this task to you
18
Notes

Do not directly copy code in the slide!


The quotation mark has been changed by Word!
How many runs should I do?
 No. of fuzzed input files
 No. of saved fuzzed image files


No. of different bugs found


In order to not blow your disk space quota in
Eustis
Need your code to process stderr message
Your code needs to check if fopen() succeeds
or not!
19
Working Environment

You can do this project on Eustis, or any Linux
machine you set up


Make sure ‘jpegconv’ works on your computer (see
project description)
You can use any programming langrage in
Linux for the project



But your code must be able to run under Eustis for
project submission
Eustis support: Perl, Java, C, Python, Sbcl
Your report must explain how we can run your code
in Eustis!
20
Last Words

After this detailed explanation and coding,
the project should be not hard

My own mutation-based fuzzer only
contains less than 60 lines in C


Find two bugs in 1300 inputs
Protocol-aware fuzzer will be longer
21