The Fuzzing NoteBook

Now lets Start the Real Fun^^

How to find Bugs ?#

1. Static Analysis: look at the code without running it and find bugs (mistakes).
2. Fuzzing: Give the program lots of unexpected inputs , If one input crashes the program, you may have found a bug (^^ cash ^^) .

imagine this:

Normal user: 
	input : hello

fuzzer:
	input : A
	input : AAAAAAAA
	input : \xff\x00\x01
	input : 10000 random bytes

So what’s fuzzing? What is the concept??#

• In basic terms: Fuzzing is automatically giving a program lots of weird, unexpected, or random inputs to see if it breaks.

The concept =>#

• The following concept is translated From Frequently asked questions (FAQ)

Documentation A program contains functions, functions contain the compiled machine code. The compiled machine code in a function can be in a single or many basic blocks. A basic block is the largest possible number of subsequent machine code instructions that has exactly one entry point (which can be be entered by multiple other basic blocks) and runs linearly without branching or jumping to other addresses (except at the end).

Example: The following A, B, C, D, E are the basic blocks:

function() {
  A:
    some
    code
  B:
    if (x) goto C; else goto D;
  C:
    some code
    goto E
  D:
    some code
    goto B
  E:
    return
}

An edge is then the unique relationship between two directly connected basic blocks (from the code example above), Every line between two blocks is an edge

Types of fuzzing :>#

1. Based on input Awareness:

   1. Dumb Fuzzer: know nothing , Just throws random data, and that makes it to miss the deep bugs
   2. Smart Fuzzer: Understands the input format, instead of random JPEG bytes, it creates almost valid JPEG files this makes it explore much more code

2. Based on input generation:

   1. Mutation-based: starts from valid files, and changes them slightly maybe bit by bit or byte. (like what AFL++ mainly does)
   2. Generation based: creates files from scratch using rules, instead of mutating a PDF, it builds a PDF according to its specification.

3. Based on knowledge of Target:

   1. White box: source code available , you can use every detail
   2. Black box: No source code , you can only observe the input > program > crash or not.
   3. Grey box: Most popular , no full source , but uses feedback like (code coverage) (AFL++ belongs here as well)

4. Based on Target: Different targets need different fuzzers. Examples:

   target	input
   File fuzzing	PDFs, PNGs
   Network fuzzing	Protocols
   Command-line fuzzing	linux tools
   Kernel fuzzing	syscalls or drivers
   Web fuzzing	HTTP requests
   Firmware fuzzing	target it via emulation or hardware interfaces
   Browser fuzzing	HTML/JS/CSS
   Stateful fuzzing	FTP, DHCP
   Windows fuzzing	winAFL

   Always understand your target before choosing a fuzzer.

Instrumenting The Target#

• Instumetation is adding extra code into the binary during compilation often at specific points like before (basic blocks, function calls) to collect information and it makes the compiled program looks like this

• This tracks which lines, basic blocks, or branches of code are executed by a given input.

Instrumenting Types#

1. PCGUARD:
   • Compiler: afl-clang-fast
   • LLVM SanitizerCoverage
   • Instrumentation during compilation
   • Faster compile time
2. LTO (Recommended)
   • Compiler: afl-clang-lto
   • Instrumentation during linking
   • Better optimization
   • Usually best performance & coverage
3. GCC Plugin
   • Compiler: afl-gcc-fast
   • Uses GCC plugin
   • Use only if Clang/LLVM isn’t available

Sanitizers#

• Sanitizers insert runtime checks to detect bugs.

• ASAN is the most commonly used during fuzzing.
• Sanitizers slow execution (ASAN ≈ 2–3× slower).
• Real fuzzing often runs:
  • One instance with ASAN
  • Other instances without sanitizers for speed.

Demo#

• Lets consider the following c program just to examine the basic usage of fuzzing and its idea

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

int isBigPrime(int n) {
  if (n <= 5)
    return 0;
  for (int i = 2; i * i <= n; i++)
    if (n % i == 0)
      return 0;
  return 1;
}

int main(void) {
  char s[35];
  scanf("%s", s);

  char cnt[300] = {0};

  for (int i = 0; s[i]; i++) {
    cnt[s[i]]++;
    if (s[i] < 'x' || s[i] > 'z') {
      puts("unacceptable");
      return 0;
    }
  }

  if (isBigPrime(cnt['x']) && isBigPrime(cnt['y']) && isBigPrime(cnt['z']))
    abort();

  puts("Nice string");

  return 0;
}

• Input Limits: The program only accepts strings consisting of characters x,y,z. If other characters are included, The program will output the unacceptable and exit normally.
• Counting statistics: Counting the number of times each appears in the input string using cnt arrays
• Trigger crash:
  • The isBigPrime function checks whether a number is greater than 5 prime (e.g. 7, 11, 13, 17 etc.)
  • The program executes abort only if the number x, y and z are all prime numbers greater than 5 at the same time
• Additional Bugs:
  • scanf does not limit input length, there is stack overflow

According to Selecting the best AFL++ compiler for instrumenting the target we will use afl-clang-lt as the compiler for instrumentation

afl-clang-lto ./test.c -o test

• we can provide initial simples for the inputs as well and put them in the inputs directory

mkdir inputs
nano inputs/text

aaaaabbbbddddddd
helloworld
xxxyyyzzzzzzxxxyyy
xxxzzzyyy

• maybe none of them will crash the program , but the thing is AFL++ can mutate these samples on its own and find an input that can crash the program Now Lets Fuzz

afl-fuzz -i inputs -out/ ./test

and after just few seconds we will notice that we got some crashes, (aborts and stack overflow)

• The inputs that caused the program to crash can be found in out/default/crashes
• And I use this script to check the input and crashes

#! /usr/bin/env bash

for f in out/default/crashes/id:*; do
  echo "=== $f ==="
  hexdump -C "$f" | head
  ./test <"$f"
done

Fuzzing-Module#

Fuzzing-Module is the official AFL++ Beginners practice. there are three small exercises

Exercise 1#

• The source code =>

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

using namespace std;

int main() {

  string str;

  cout << "enter input string: ";
  getline(cin, str);
  cout << str << endl << str[0] << endl;

  if (str[0] == 0 || str[str.length() - 1] == 0) {
    abort();
  } else {
    int count = 0;
    char prev_num = 'x';
    while (count != str.length() - 1) {
      char c = str[count];
      if (c >= 48 && c <= 57) {
        if (c == prev_num + 1) {
          abort();
        }
        prev_num = c;
      }
      count++;
    }
  }

  return 0;
}

• okay I will use the afl-clang-lto++ as the instrumentation

CC=afl-clang-lto CXX=afl-clang-lto++
cmake ..
make

• First lets analyze a few places that can cause a crash
  1. str[0] == \x00
  2. str[str.length() - 1] == \x00
  3. The next read num is one greater than the prev one example 12
  4. EOF
  • okay now As required by the exercise 1 we will generate 5 seeds using this script:

  #!/usr/bin/env bash
  
    mkdir seeds
    for i in {0..4}; do
      dd if=/dev/urandom of=seeds/seed_"$i" bs=64 count=10
    done
  

• now lets fuzz afl-fuzz -i seeds -o out -m 0 -d -- ./build/simple_crash

After a few seconds I found 3 crashes

• it feels amazing when i see those crashes XD

Note: crashes analyses

1. okay the first crash id:000000 this matches our second case
2. the second crash id:000001 matches our third case
3. and the third one id:000002 matches our first case

As you surely notice the fuzzer didn’t detect our last case which is EOF This is mainly caused by its result this specific input EOF will cause undefine Behaviour

Possible results:

• Crash
• No crash
• Garbage value
• Different behavior every run

Because it is not reproducible, AFL++ usually marks it as flaky and discards it.

• so we need to enhance its crash performance somehow to make it more certain like this maybe:

  if(str.length() == 0)
	  abort();

• or we can use the AddressSanitize ASAN and UndefinedBehaviorSanitizer UBSAN

Note: Compiling with sanitizer

cmake -S . -B build \
-DCMAKE_C_COMPILER=afl-clang-lto \
-DCMAKE_CXX_COMPILER=afl-clang-lto++ \
-DCMAKE_C_FLAGS="-O1 -g -fno-omit-frame-pointer -fsanitize=address,undefined" \
-DCMAKE_CXX_FLAGS="-O1 -g -fno-omit-frame-pointer -fsanitize=address,undefined"

Why these flags?

1. -O1 small optimization just to make debugging easier
2. -g also makes debugging with gdb much easier it tells u exactly at which line in the source the binary crashed
3. -fno-omit-frame-pointer Produces accurate stack traces.
4. fsanitize=address,undefined enables both ASan and UBSan.

---

Okay before fuzzing we need to set these vars

export ASAN_OPTIONS=abort_on_error=1:symbolize=0:detect_leaks=0
export UBSAN_OPTIONS=abort_on_error=1

WHY?

1. symbolize=1 it prints simple_crash.cpp:15 instead of just an address “much easier to debug”
2. print_stacktrace=1 prints the call stack where the bug occurred And for better Debugging Use this one

export ASAN_OPTIONS=abort_on_error=1:symbolize=1:detect_leaks=0
export UBSAN_OPTIONS=abort_on_error=1:print_stacktrace=1

Exercise 2#

• This one is easy , the only condition to abort is to enter ffl this will trigger it

} else if (input[i] == 'l') {
    if (crew.num == 0) {
        abort();
    }
    land();
}

• I used the same pervious seed ( I know its a bit missy But I like it XD)

• by observing the crashes they all about the ffl but with different paths in the code

Exercise 3#

• Hmmmm Too much code in here so we will use Sourcetrail as the guide says
• okay Next lets search for the abort points
  1. choose_color If we entered a pure number
  2. main_alt input negative number
  3. main_airspeed input negative number
  4. fuel_cap input negative number
  5. check_alt if the alt is less than 0
  6. check_fuel if the fuel is less than 0
  7. check_speed if the speed is less than 0

Note: Sourcetrail in such projects that have lots of files, headers, classes sourcetrail is a really big help for us it helps us to trace the functions and map it in a very easy way

• In this exercise we need to use a really important technique create a slice its basically that we want to narrow down the code that will be running as much as possible. so instead for example fuzzing a full drone system we will slice it to just fuzz the GPS module or something
• For the slice, you can comment out some of the code in the target to ignore features that you are not interested in fuzzing.

This exercise gives us a template for a slice specs_slice so we will fuzz only the specs class and test the choose_color function specifically

/*
 *
 *
 * This file isolates the Specs class and tests out the 
 * choose_color function specifically.
 * 
 * 
 * 
 */

#include "specs.h"

int main(int argc, char** argv) {
    // In order to call any functions in the Specs class, a Specs
    // object is necessary. This is using one of the constructors
    // found in the Specs class.
    Specs spec(505, 110, 50);
    // By looking at all the code in our project, this is all the 
    // necessary setup required. Most projects will have much more
    // that is needed to be done in order to properly setup objects.

    // This section should be in your code that you write after all the 
    // necessary setup is done. It allows AFL++ to start from here in 
    // your main() to save time and just throw new input at the target.
    #ifdef __AFL_HAVE_MANUAL_CONTROL
        __AFL_INIT();
    #endif

    spec.choose_color();
    //spec.min_alt();

    return 0;
}

• We can set the fuzz entry by defining __AFL_HAVE_MANUAL_CONTROL,
  • __AFL_INIT() tells AFL++ to start fuzzing only after your program’s initialization is complete, so it skips one-time setup and focuses on the target code

We will test chosse_color first , we know if we entered a pure number it will collapse so lets fuzz and see

wooh more crashes than i expected, lets analyze them.

1. okay the first crash input is like this \x39\x0c\x36 we know that cin stops at white space so it will take the first digit 9 and ignores the others , so the digit now aborts the program
2. the second and fourth one are simple they are pure digits so it will crash the program
3. the third one its \x09\x09 its \t\t so when it tries to saves an empty color so this loop right here will return true , and returning true causes a crash

   std::cin >> color;
   if (isNumber(color))
     abort();
   
   bool Specs::isNumber(std::string str) {
     for (int i = 0; i < str.length(); i++) {
       if (isdigit(str[i]) == 0)
         Return to False;
     }
     return true;
   }
   

Lets fuzz one more function min-airspeed

• first i used this slice

#include "specs.h"

int main(int argc, char** argv) {
    Specs spec(505, 110, 50);
    #ifdef __AFL_HAVE_MANUAL_CONTROL
        __AFL_INIT();
    #endif

    spec.min_airspeed();

    return 0;
}

Don’t forget to add this line in the CMakeLists.txt

add_executable(min-airspeed-slice min-airspeed-slice.cpp specs.cpp)

and then build again

• I fuzzed this function specifically to see how the exec speed gonna increase since this function has a loop and may ask for more than one input in different paths.

void Specs::min_airspeed() {
  bool out_of_bounds = true;
  std::cout << "enter aircraft minimum airspeed:";
  std::cin >> speed;
  do {
    out_of_bounds = false;
    if (speed < 0)
      abort();
    if (speed < 100) {
      std::cout << "too low. please re-enter:";
      std::cin >> speed;
      out_of_bounds = true;
    } else if (speed > 200) {
      std::cout << "too high. please re-enter:";
      std::cin >> speed;
      out_of_bounds = true;
    }
  while (out_of_bounds);
}

• As I expected the exec speed was too slow 60/sec but we got some crashes

• The fourth crash is a bit interesting the input 11111-11+13 crashes the program because std::cin >> automatically splits the input while reading integers It first reads 11111 and leaves -11+13 in the input buffer since 11111 is greater than 200 the program asks for another input the next std::cin >> speed reads -11 from the remaining buffer and stops before the + now speed is negative so the program reaches abort()

okay that’s it , I won’t fuzz the other function cause they are easier and it will be a waste of time to fuzz them.