High Speed Bug Discovery with Fuzzing - with Speaker Notes

Unit testing is helpful at preventing regressions and guiding design, but it doesn't do a great job of helping you with exploratory testing. How can you find hidden defects in your code without a lot of manual analysis? Fuzzing is a simple but surprisingly effective technique which has been responsible for finding nearly all of the security vulnerabilities uncovered in Flash over the past five years. But it's not just limited to finding security defects! The technique was very successfully used to stabilize the Microsoft document importers for Open Office and check C++ compiler standards compliance. You'll leave this talk knowing when to use fuzzing to test your application, which tools you should use, how to implement a fuzzer from scratch, and when other techniques are a better choice.

1. High Speed Bug Discovery with Fuzzing Craig Stuntz ∈ Improving
2. Slides https://speakerdeck.com/craigstuntz
3. CodeMash
4. CodeMash
5. CodeMash
6. Sad Bunny https://www.flickr.com/photos/climber85268/446766956
7. Spoilers! Why should I care? (because it’s surprisingly effective at finding bugs in software) What is it? (a simple, property-based randomized testing technique) When should I use it? (integration testing complex systems with infinite input values) How do I get started? (I’ll suggest a bunch of tools) Should I write my own? (yes, and I have stories!) When should I not use it? Will start very non-technical and get more technical as we go on. Light bash scripting to rewriting binary assemblies
8. Why? already have other testing techniques, so…. Why is fuzzing interesting? Because it works!
9. 400 Crashes, 106 Distinct Security Bugs in Adobe Flash Player https://security.googleblog.com/2011/08/fuzzing-at-scale.html 1 month, 2000 cores
10. 325 C Compiler bugs in GCC, Clang, & Others https://www.flux.utah.edu/paper/yang-pldi11 25 GCC bugs classified as release-blocking Spent less than $1000
11. “…our most prolific bug-finding tool.” Robert Guo http://queue.acm.org/detail.cfm?ref=rss&id=3059007
12. DARPA Cyber Grand Challenge Mayhem, Xandra, Mechanical Phish, Galactica http://blogs.grammatech.com/the-cyber-grand-challenge
13. https://commons.wikimedia.org/wiki/File:Rabbit_american_fuzzy_lop_buck_white.jpg What is it?
14. Prevent Regressions Bug Discovery Help with Code Design Meets Specifications Testing goals and tools
15. Prevent Regressions Bug Discovery Help with Code Design Meets Specifications Testing goals and tools
16. Prevent Regressions Bug Discovery Help with Code Design Meets Specifications Testing goals and tools
17. Prevent Regressions Bug Discovery Help with Code Design Meets Specifications Testing goals and tools
18. Prevent Regressions Bug Discovery Help with Code Design Meets Specifications Testing goals and tools
19. Prevent Regressions Bug Discovery Help with Code Design Meets Specifications Integration testing Unit testing Formal verification Exploratory testing Testing goals and tools
20. Prevent Regressions Bug Discovery Help with Code Design Meets Specifications Fuzzing Integration testing Unit testing Formal verification Exploratory testing Testing goals and tools
21. How Many Cases Should We Test? One Only the Most Interesting Every Possible Case Unit Testing Fuzzing Formal Verification Formal verification example: a type signature
22. Property -Based Random Testing QuickCheck Chaos Monkey Fuzzing Scientist
24. Corpus (Collection of Examples)
25. System Under Test
26. Property
27. Magic!
28. Corpus A few handwritten examples Fuzzing databases Harvest from test suites, defect reports Harvest from public Internet
29. System Under Test A function Entire application Part of OS kernel Many testing techniques limited to testing at a particular scope
30. Properties Does it crash? Does it hang? Is the output “valid”? Does execution trip an address or memory sanitizer? Does the output match some other system?
31. Magic Mutation of corpus Coverage guidance Lots of test runs What you’ve heard so far sounds like taking a few sample inputs and doing a boring test on the output. That’s it?
32. Possible Inputs Random Inputs Interesting Inputs Random Inputs with Profile Guidance The idea is to find the interesting (desired properties don’t hold) inputs to the system under test faster than we could find them by either exhaustive testing or purely by chance.
33. Getting Started with afl - Compile system under test with instrumentation - Place corpus input(s) in a folder - Invoke afl - Wait for bugs https://fuzzing-project.org/tutorial3.html What does this look like in practice? There are more complicated options, but this is how you get started
34. Compile with Instrumentation $ ./configure CC="afl-gcc" \ CXX="afl-g++" \ --disable-shared; \ make
35. Place Corpus in Folder $ mkdir in $ cd in $ cat > foo.json { "a": "bc" } ^D $ cd .. You probably want more than one example in the corpus, but this gets us started
36. Invoke afl $ afl-fuzz -i in -o out \ my_json_parser @@ folder containing corpus “@@“ means “the current test case” system under test findings go here This is the simple case. There are many options!
37. This might run for weeks What will I find? Crashes, hangs Can do more, with effort!
38. This might run for weeks What will I find? Crashes, hangs Can do more, with effort!
39. some_project ├── in └─┬ out ├── queue ├── crashes └── hangs findings are just new input files.
40. afl In a Nutshell ⃗ Turn a few maybe uninteresting inputs into a lot of hopefully very interesting inputs, quickly
41. afl is a dumb fuzzer, meaning…
42. afl Fuzz Strategies Walking bit flips (try flipping each bit in input individually) Walking byte flips (try flipping each contiguous set of 8 bits) Simple arithmetic (increment or decrement bytes in the file by certain small values) Known integers (replace bytes with “problematic” 8, 16, and 32 bit integers like 0 and FF) Profile-guided stacked tweaks and test case splicing (magic!) https://lcamtuf.blogspot.com/2014/08/binary-fuzzing-strategies-what-works.html Deterministic, genetic
43. Walking Bit Flip Original 01010101 Flip bit 0 01010100 Flip bit 1 01010111 Flip bit 2 01010001 <etc.> Walking 2 Bit Flip Original 01010101 Flip bits 0,1 01010110 Flip bits 2,1 01010011 Flip bits 3,2 01001101 <etc.>
44. Trace Execution Paths A B D C E F
45. Trace Execution Paths A B D C E F
46. Trace Execution Paths A B D C E F ?
47. When Should I Use It? When is it useful? https://it.wikipedia.org/wiki/File:Coniglio_ariete.JPG
48. Unit Tests Fuzzing Useful For Preventing Regressions, Design Finding New Bugs Tests Functions Any Level Test Examples Hand-selected values Corpus + Mutation Execution Time Milliseconds Weeks Magic? No Yes
49. Instrumentation for profile guidance
50. $ ./configure CC="afl-gcc" \ CXX="afl-g++" \ --disable-shared; \ make You’ve seen this before; afl requires recompiling with instrumentation
51. $ pip install python-afl ——— afl.init()
52. Properties a.k.a. Specifications https://lorinhochstein.wordpress.com/2014/06/04/crossing-the-river-with-tla/
53. A simple specification: Programs shouldn’t crash.
54. Sanitizers and Canaries Security cookie (/GS)
55. STJSON A JSON Parser in Swift 3 compliant with RFC 7159 STJSON was written along with the article Parsing JSON is a Minefield. Basic usage: var p = STJSONParser(data: data) do { let o = p.parse() } catch let e { print(e) } Instantiation with options: var p = STJSON(data:data, maxParserDepth:1024, options:[.useUnicodeReplacementCharacter]) https://github.com/nst/STJSON https://github.com/CraigStuntz/Fizil/tree/master/StJson Is there a specification for the behavior of the program?
56. Can You Fuzz the Domain?
57. Dumb Fuzzer Mangling Byte Arrays many interesting programs take binary input sometimes this requires less domain knowledge (not always!) or at least less custom code What about programs or functions which can’t accept binary input?
58. public byte[] ResizePng( byte[] image) { Great candidate for dumb fuzzing
59. public boolean SomeFunction( SomeEnum firstArg, int secondArg) { ✗ not a good candidate for dumb fuzzing If I have a good property I can test exhaustively
60. Smart Fuzzing MongoDB Expression Grammar http://queue.acm.org/detail.cfm?ref=rss&id=3059007 Not an end to end test — testing the internals. Don’t test stuff which takes forever
61. public Assembly Compile( AbstractNode syntaxTree) { Good candidate for smart fuzzing?
62. Worth the Wait? Results Can Take Weeks
63. Isn’t it just for Security? https://www.flickr.com/photos/wocintechchat/25721078480/ Every security person knows about fuzzing. Almost no app devs do. Security people care more about bugs than app devs.
64. How do I get started?
65. How to Get Started with Fuzzing 1. Find a program to test 2. Find a fuzzer 3. Find a corpus 4. Choose a property 5. Let it run!
66. Fuzzers https://commons.wikimedia.org/wiki/File:Holland_Lop_with_Broken_Orange_Coloring.jpg
67. libfuzzer Fuzz testing for LLVM compilers http://llvm.org/docs/LibFuzzer.html
68. $ cargo install cargo-fuzz $ cargo fuzz init
69. $ gzip -c /bin/bash > sample.gz $ while true do radamsa sample.gz > fuzzed.gz gzip -dc fuzzed.gz > /dev/null test $? -gt 127 && break done ← Fuzz the corpus ← Execute S.O.T. ← Check a property ← Repeat a lot! https://github.com/aoh/radamsa Radamsa: Un*x philosophy of “do one thing well, chain with other stuff” Works on Windows with cygwin (and maybe
70. ClusterFuzz Submit a fuzzer, win a bounty https://security.googleblog.com/2016/08/guided-in-process-fuzzing-of-chrome.html
71. afl
72. OSS-Fuzz Submit your project https://github.com/google/oss-fuzz
73. burp, ZAP
74. Corpus https://en.wikipedia.org/wiki/File:Long_Room_Interior,_Trinity_College_Dublin,_Ireland_- _Diliff.jpg
75. “ We didn't call it fuzzing back in the 1950s, but it was our standard practice to test programs by inputting decks of punch cards taken from the trash. -Gerald M. Weinberg http://secretsofconsulting.blogspot.com/2017/02/fuzz-testing-and-fuzz-history.html
77. Fuzzing SQLite with afl Start with a single test case: create table t1(one smallint); insert into t1 values(1); select * from t1; Add a list of reserved words from documentation Then extract SQL statements from SQLite unit tests (550 files at around 220 bytes each) https://lcamtuf.blogspot.com/2015/04/finding-bugs-in-sqlite-easy-way.html
78. Properties
79. Don’t Crash or Hang
80. Sanitizers and Canaries https://docs.google.com/presentation/d/19OSgb1N9Ezef39Blb-5lkzycq7-tMtAvy825FofyrmY
81. Validators
82. End to End Input Output Same as Input? f(input) f-1(input)
83. Legacy Code https://commons.wikimedia.org/wiki/File:Blue-punch-card-front-horiz.png
84. One of The Things …Is Not Like the Others! Repeated testing with same input should yield same output
85. Should I write my own?
86. Heck Yeah! https://github.com/CraigStuntz/Fizil
87. Interesting Stuff I Learned While Writing a Fuzzer - F# bitwise operations - How to instrument .NET code - dnSpy is awesome - Same input -> Same code -> Different paths - Strong naming is painful - Unicode is also painful - MemoryMappedFile performance is straight-up awful
88. let jsonNetResult = try JsonConvert.DeserializeObject<obj>(str) |> ignore Success with | :? JsonReaderException as jre -> jre.Message |> Error | :? JsonSerializationException as jse -> jse.Message |> Error | :? System.FormatException as fe -> if fe.Message.StartsWith("Invalid hex character”) // hard coded in Json.NET then fe.Message |> Error else reraise() ⃪ T est ⬑ Special case error stuff Special test harness code specific to the system under test
89. use proc = new Process() proc.StartInfo.FileName <- executablePath inputMethod.BeforeStart proc testCase.Data proc.StartInfo.UseShellExecute <- false proc.StartInfo.RedirectStandardOutput <- true proc.StartInfo.RedirectStandardError <- true proc.StartInfo.EnvironmentVariables.Add(SharedMemory.environmentVariableName, sharedMemoryName) let output = new System.Text.StringBuilder() let err = new System.Text.StringBuilder() proc.OutputDataReceived.Add(fun args -> output.Append(args.Data) |> ignore) proc.ErrorDataReceived.Add (fun args -> err.Append(args.Data) |> ignore) proc.Start() |> ignore inputMethod.AfterStart proc testCase.Data proc.BeginOutputReadLine() proc.BeginErrorReadLine() proc.WaitForExit() let exitCode = proc.ExitCode let crashed = exitCode = WinApi.ClrUnhandledExceptionCode ⃪ Set up ⃪ Read results ⃪ Important bit Generic code in Fizil
90. /// An ordered list of functions to use when starting with a single piece of /// example data and producing new examples to try let private allStrategies = [ bitFlip 1 bitFlip 2 bitFlip 4 byteFlip 1 byteFlip 2 byteFlip 4 arith8 arith16 arith32 interest8 interest16 ]
91. let totalBits = bytes.Length * 8 let testCases = seq { for bit = 0 to totalBits - flipBits do let newBytes = Array.copy bytes let firstByte = bit / 8 let firstByteMask, secondByteMask = bitMasks(bit, flipBits) let newFirstByte = bytes.[firstByte] ^^^ firstByteMask newBytes.[firstByte] <- newFirstByte let secondByte = firstByte + 1 if secondByteMask <> 0uy && secondByte < bytes.Length then let newSecondByte = bytes.[secondByte] ^^^ secondByteMask newBytes.[secondByte] <- newSecondByte yield newBytes } Fuzz one byte → ^^^ means xor ↓
94. private static void F(string arg) { Console.WriteLine("f"); Console.Error.WriteLine("Error!"); Environment.Exit(1); }
95. private static void F(string arg) { instrument.Trace(29875); Console.WriteLine("f"); Console.Error.WriteLine("Error!"); Environment.Exit(1); } ← Random number
96. private static void F(string arg) { #if MANUAL_INSTRUMENTATION instrument.Trace(29875); #endif Console.WriteLine("f"); Console.Error.WriteLine("Error!"); Environment.Exit(1); }
97. let stringify (ob: obj) : string = JsonConvert.SerializeObject(ob)
98. let stringify (ob: obj) : string = JsonConvert.SerializeObject(ob) // Method: System.String\u0020Program::stringify(System.Object) .body stringify { arg_02_0 [generated] arg_07_0 [generated] nop() arg_02_0 = ldloc(ob) arg_07_0 = call(JsonConvert::SerializeObject, arg_02_0) ret(arg_07_0) }
99. let stringify (ob: obj) : string = JsonConvert.SerializeObject(ob) // Method: System.String\u0020Program::stringify(System.Object) .body stringify { arg_02_0 [generated] arg_07_0 [generated] nop() arg_02_0 = ldloc(ob) arg_07_0 = call(JsonConvert::SerializeObject, arg_02_0) ret(arg_07_0) } // Method: System.String\u0020Program::stringify(System.Object) .body stringify { arg_05_0 [generated] arg_0C_0 [generated] arg_11_0 [generated] arg_05_0 = ldc.i4(23831) call(Instrument::Trace, arg_05_0) nop() arg_0C_0 = ldloc(ob) arg_11_0 = call(JsonConvert::SerializeObject, arg_0C_0) ret(arg_11_0) }
101. let private insertTraceInstruction(ilProcessor: ILProcessor, before: Instruction, state) = let compileTimeRandom = state.Random.Next(0, UInt16.MaxValue |> Convert.ToInt32) let ldArg = ilProcessor.Create(OpCodes.Ldc_I4, compileTimeRandom) let callTrace = ilProcessor.Create(OpCodes.Call, state.Trace) ilProcessor.InsertBefore(before, ldArg) ilProcessor.InsertAfter (ldArg, callTrace) This margin is too narrow to contain a try/finally example, so see: https://goo.gl/W4y7JH Inserting the IL instructions I needed was fairly easy. Here is the important bit of the code which does it. How did I learn how to write this? I instrumented a small program “manually” by writing the instrumentation code myself, and then decompiled that program to figure out which IL instructions I needed. Inserting them with Mono.Cecil is just a few lines of code. try/finally is much, much harder. I won’t even try to walk you through it here. Look at the GitHub repo if you want to see how it’s done.
102. let private insertTraceInstruction(ilProcessor: ILProcessor, before: Instruction, state) = let compileTimeRandom = state.Random.Next(0, UInt16.MaxValue |> Convert.ToInt32) let ldArg = ilProcessor.Create(OpCodes.Ldc_I4, compileTimeRandom) let callTrace = ilProcessor.Create(OpCodes.Call, state.Trace) ilProcessor.InsertBefore(before, ldArg) ilProcessor.InsertAfter (ldArg, callTrace) This margin is too narrow to contain a try/finally example, so see: https://goo.gl/W4y7JH Inserting the IL instructions I needed was fairly easy. Here is the important bit of the code which does it. How did I learn how to write this? I instrumented a small program “manually” by writing the instrumentation code myself, and then decompiled that program to figure out which IL instructions I needed. Inserting them with Mono.Cecil is just a few lines of code. try/finally is much, much harder. I won’t even try to walk you through it here. Look at the GitHub repo if you want to see how it’s done.
103. let private insertTraceInstruction(ilProcessor: ILProcessor, before: Instruction, state) = let compileTimeRandom = state.Random.Next(0, UInt16.MaxValue |> Convert.ToInt32) let ldArg = ilProcessor.Create(OpCodes.Ldc_I4, compileTimeRandom) let callTrace = ilProcessor.Create(OpCodes.Call, state.Trace) ilProcessor.InsertBefore(before, ldArg) ilProcessor.InsertAfter (ldArg, callTrace) This margin is too narrow to contain a try/finally example, so see: https://goo.gl/W4y7JH Inserting the IL instructions I needed was fairly easy. Here is the important bit of the code which does it. How did I learn how to write this? I instrumented a small program “manually” by writing the instrumentation code myself, and then decompiled that program to figure out which IL instructions I needed. Inserting them with Mono.Cecil is just a few lines of code. try/finally is much, much harder. I won’t even try to walk you through it here. Look at the GitHub repo if you want to see how it’s done.
104. let private insertTraceInstruction(ilProcessor: ILProcessor, before: Instruction, state) = let compileTimeRandom = state.Random.Next(0, UInt16.MaxValue |> Convert.ToInt32) let ldArg = ilProcessor.Create(OpCodes.Ldc_I4, compileTimeRandom) let callTrace = ilProcessor.Create(OpCodes.Call, state.Trace) ilProcessor.InsertBefore(before, ldArg) ilProcessor.InsertAfter (ldArg, callTrace) This margin is too narrow to contain a try/finally example, so see: https://goo.gl/W4y7JH Inserting the IL instructions I needed was fairly easy. Here is the important bit of the code which does it. How did I learn how to write this? I instrumented a small program “manually” by writing the instrumentation code myself, and then decompiled that program to figure out which IL instructions I needed. Inserting them with Mono.Cecil is just a few lines of code. try/finally is much, much harder. I won’t even try to walk you through it here. Look at the GitHub repo if you want to see how it’s done.
106. http://www.json.org/ I need a way to determine if Json.NET is parsing the JSON correctly. So I thought I should write a JSON validator to check its behavior. Fortunately, there’s a standard! “Probably the boldest design decision I made was to not put a version number on JSON so there is no mechanism for revising it. We are stuck with JSON: whatever it is in its current form, that’s it.” -Crockford
107. https://tools.ietf.org/html/rfc4627 So, naturally, there’s also another JSON standard
108. http://www.ecma-international.org/ecma-262/5.1/#sec-15.12 And another JSON standard
109. http://www.ecma-international.org/publications/standards/Ecma-404.htm And another JSON standard
110. https://tools.ietf.org/html/rfc7158 And another JSON standard
111. https://tools.ietf.org/html/rfc7159 And another JSON standard. And no, they don’t all agree on everything, nor is there a single, “latest” version. Despite this multitude of standards, there are still edge cases intentionally delegated to the implementer — what we would call “undefined behavior” in C.
112. https://github.com/nst/STJSON I was going to write my own validator, but… Nicolas Seriot wrote a validator called STJSON which attempts to synthesize these as much as possible.
113. https://github.com/CraigStuntz/Fizil/blob/master/StJson/StJsonParser.fs Swift doesn’t readily compile to Windows, but if you squint hard enough it kind of looks like F#, so I ported the code and used it to validate Json.NET's behavior.
114. { "a" : "bc" }
117. Standard Accepts, Json.NET Rejects Value 88888888888888888888888888888888888888888888888888 88888888888888888888888888888888888888888888888888 88888888888888888888888888888888888888888888888888 88888888888888888888888888888888888888888888888888 88888888888888888888888888888888888888888888888888 Standard Says No limit Json.NET MaximumJavascriptIntegerCharacterLength = 380; Things JSON.NET fails on that the standard accepts
118. Standard Rejects, Json.NET Accepts Value [,,,] Standard Says A JSON value MUST be an object, array, number, or string, or one of the following three literal names: false null true Json.NET [null, null, null, null] Things JSON.NET succeeds on that the standard rejects
119. Strong naming was a consistent pain for me. I’m altering the binaries of assemblies, and part of the point of strong naming is to stop you from doing just that, so naturally if the assembly is strongly named it can’t be loaded when I’m finished.
120. let private removeStrongName (assemblyDefinition : AssemblyDefinition) = let name = assemblyDefinition.Name; name.HasPublicKey <- false; name.PublicKey <- Array.empty; assemblyDefinition.Modules |> Seq.iter ( fun moduleDefinition -> moduleDefinition.Attributes <- moduleDefinition.Attributes &&& ~~~ModuleAttributes.StrongNameSigned) let aptca = assemblyDefinition.CustomAttributes.FirstOrDefault( fun attr -> attr.AttributeType.FullName = typeof<System.Security.AllowPartiallyTrustedCallersAttribute>.FullName) assemblyDefinition.CustomAttributes.Remove aptca |> ignore assembly.MainModule.AssemblyReferences |> Seq.filter (fun reference -> Set.contains reference.Name assembliesToInstrument) |> Seq.iter (fun reference -> reference.PublicKeyToken <- null ) So I need to remove the strong name from any assembly I fuzz, but I also need to remove the PublicKeyToken from any other assembly which references it. Doing this in Mono.Cecil is not well-documented, and after quite a bit of time spent in GitHub issues and trial and error I figured out that it takes 5 distinct steps to do this.
121. “ “If marked BeforeFieldInit then the type’s initializer method is executed at, or sometime before, first access to any static field defined for that type.” -ECMA-335, Common Language Infrastructure (CLI), Partition I
122. Unicode Original JSON { "a": "bc" } ASCII Bytes 7B 20 22 61 22 20 3A 20 22 62 63 22 20 7D UTF-8 with Byte Order Mark EF BB BF 7B 20 22 61 22 20 3A 20 22 62 63 22 20 7D UTF-16 BE with BOM FE FF 00 7B 00 20 00 22 00 61 00 22 00 20 00 3A 00 20 00 22 00 62 00 63 00 22 00 20 00 7D
124. Resources MongoDB’s JavaScript Fuzzer http://queue.acm.org/detail.cfm?ref=rss&id=3059007 afl technical details http://lcamtuf.coredump.cx/afl/technical_details.txt afl Help Email List afl-users@googlegroups.com Fizil https://github.com/CraigStuntz/Fizil WTF, ACM?
125. Thank You! - Michał Zalewski, for afl documentation - Rehearsal audiences, employees of - Dynamit - Improving - Ineffable Solutions
126. Craig Stuntz craig.stuntz@improving.com www.craigstuntz.com @craigstuntz http://www.meetup.com/Papers-We-Love-Columbus/