CAP6135: Malware and Software Vulnerability Analysis Fuzzing Test Example Cliff Zou Spring 2014

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

Example Code $ fuzzTest-target 200 “what is this?” 2 Example code needs three inputs Int, string, Int int inputInteger; /* global variable */ void (*foo)(short); 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

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

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 */ }

Bug #3: divide by zero sscanf(divStr, "%d", &denominator); int denominator; float x; sscanf(divStr, "%d", &denominator); if (denominator == 0){ x = argLen / denominator; fprintf(stderr, "Bug #3: division by zero triggered\n"); foo = (void *)0xbffbffff; foo(argLen); /* trigger illegal instruction fault */ }else return 0;

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! */

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+4) /* segmentation fault (11) or illegal (4) */ { 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

List of Unix Signal Number You can find it at: http://man7.org/linux/man-pages/man7/signal.7.html The WEXITSTATUS() returns a value that is the signal number that caused the termination plus 128

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

Program Project 2 Introduction

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

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 follow some format rules) Then, write the content of the buffer to test.jpg file. Then, invoke jpegconv on test.jpg to do fuzz test

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*/

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

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 if you repeatedly do the above fuzzing! On the other hand, we cannot save all tested jpg files since most of them do not trigger bugs

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); }

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 images 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 n bytes, randomly picked n bytes, change value of n, ….

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

Notes Do not directly copy code in this slide to your code! 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!

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 in order for me to verify your code! Eustis support: Perl, Java, C, Python, Sbcl Your report must explain how I can run your code under Eustis!

Last Words After this detailed explanation and coding, the project should be not too 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