CAP6135: Malware and Software Vulnerability Analysis

Fuzzing Test ExampleCliff 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

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Example Code

$ fuzzTest-target 200 “what is this?” 2 Example code needs three inputs

Int, string, Intint 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

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Bug # 1: Integer Overflowint 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

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Bug # 2: buffer Overflowchar 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 */ }

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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;

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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 linesprintf(buffer, "./fuzzTest-target %d \"%s\" %d\n", firstInt, charArray, secondInt);free(charArray); /* must free memory for repeat testing! */

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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

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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: stdin (0), stdout (1), stderr (2)

I/O redirection: $ 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

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Program Project 2 Introduction

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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”

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 HexEdit for Win: http://www.physics.ohio-state.edu/~prewett/hexedit/

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

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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

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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*/

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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

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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!

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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);}

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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….

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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

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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

In order to not blow your disk space quota in Eustis No. of different bugs found

Need your code to process stderr message Your code needs to check if fopen() succeeds or

not!

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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!

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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

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