Fuzzing for logic and state issues Gödel's Gourd Fuzzing for logic and state issues

Introductions Michael Eddington CTO and Principal Consultant @ Déjà vu Security 12+ years in security consulting Senior developer/architect in prior life Author of Peach, an open source fuzzer Device, Kernel, User, Web, Network

DARPA Cyber Fast track Thanks Mudge!

Defining the Problem Fuzzers are good at findings implementation issues …that crash the target …that are generically detectable (sqli, xss) Not good at finding design, logic, and state issues …that do not crash the target …that are not generically detectable

Examples Buffer Overflows Memory Corruption Resource Usage Easy Hard Buffer Overflows Memory Corruption Resource Usage SQL Injection Missing authentication State corruption Incorrect logic

Authentication Examples Out of 100 admin pages, 5 are missing authentication Microsoft SSPI skip a step auth bypass OpenBSD IPSEC incorrect if/then logic

Authentication – Detect Web – Missing Auth Trigger Request page w/o logging in MS SSPI/OBSD IPSEC Skip a step Status Code (200/403) What pages require auth Result (Pass) Did we complete all steps

Logic Example Windows 95 SMB Flaw Logic error in password checking code Length of loop determined by client input Modified SMB client, ~32 attemps always wins We never throw an exception or crash Typical generic fuzzer will never find this

Logic – Win95 SMB bool CheckPw( int userdata_len, char* userdata, int sysdata_len, char* sysdata ) { for(int i=0; i<userdata_len; i++) if(userdata[i] != sysdata[i]) return false; return true; }

Logic – Detect Win 95 SMB Trigger Result Does password match Try all chars Remove NULL Result Does password match

State Example Device (phone/tablet/laptop) with theft system Agent “heartbeats” to server Server can trigger “stolen” mode in laptop Laptop will trigger if unable to “heartbeat” Timer/counter runs down

State – Detect System Server Trigger Can we perform state flow? Cause exception Flow locked Unable to heartbeat Can we perform state flow? Check result of each step

How to detect? Goal – Modify existing fuzzer to detect these issues We already produce triggers How do we add detection?

How to detect? What do we need to detect these issues? Provide system constraints If not authenticated result is 402 If steps 1, 2, and 3 not performed step 4 is error Result is never 500 Verify we are still working Perform state flow w/o mutations

Proposed Solution Gödel's Gourd Re-use Peach fuzzing engine Mutation engine Fault detection/reporting Constraint language Control iterations (non mutation iterations) Mutate state model (skip, order, etc.)

Control Iterations Goal: Verify target is working correctly No mutations Constraints pass State model is followed Matches recorded control iteration

How it works R – Record iteration 1 – Fuzzing iteration C – Control iteration 2 – Fuzzing Iteration 3 – Fuzzing iteration … Remember all states/actions from record iteration Verify on control iterations Control iterations every N fuzzing iterations

Outcome If control does not match record – throw fault Identify conditions that stop normal operation

Constraints Verify logic via simple constraint expressions Apply constraints to state model State Action Does not modify fuzzer state

Language Options Existing Traditional Languages Pro Cons Well known JavaScript Python Ruby etc. Pro Well known Available via .NET scripting interface Cons Allows modification of fuzzer state.

Other Options Domain Specific Language (DSL) Use existing Create our own Pros Meet all requirements Cons Must implement Not well known

DSL Selection Object Constraint Language (OCL) Specification language, no side effects Developed as part of new UML standards Familiar syntax Relatively easy to implement

Object Constraint Language (OCL) Expression types Invariant (inv) Always true Pre (pre) Evaluated before [ something ] Post (post) Evaluated after [ something ] Can access state from Pre. (@pre)

OCL Examples “Car owner must be at least 18 years old” context Car inv: self.owner.age >= 18 “If passwords match result is true” context Login post: result = true implies pass1 = pass2

OCL Context Groups sets of constraints Constraints for a context are run together Association based on context

Normal Fuzzing Iteration Enter State Model State 1 Action 1.1 Send Data Action 1.2 Receive Data State N …

Fuzzing Iteration With Constraints Enter State Model State 1 Action 1.1 Send Data Action 1.2 Receive Data State N … Inv(pre) Pre EVENT Inv(post) Post

Applying (Authentication) Web Authentication # Verify authentication occurred post: (reply = 200 && url.indexOf(‘/admin’) > -1) implies auth.reply = 200

Applying (Authentication) Windows SSPI # Verify all steps completed post: reply = true implies ( auth.step1.reply = true && auth.step2.reply = true && auth.step3.reply = true)

Applying (Logic) Windows 95 Bug post: reply = true implies userpw = ‘password’

Applying (State) Antitheft System Perform control iteration

Implementation

Technologies Used Microsoft .NET Framework – C# Peach Fuzzer 3 Cross platform using Mono OS X Linux

Implementation Diagram

OCL Implementation Irony .NET Language Toolkit LINQ Expressions Many differences from traditional Grammar is code Easy AST hookups LINQ Expressions From IronPython work Last mile is already done

LINQ Expressions Exposes language constructs for use in AST classes. Does all the heavy lifting. return Expression.Condition( (Expression)ifNode.Evaluate(thread), (Expression)thenNode.Evaluate(thread), (Expression)elseNode.Evaluate(thread));

Gödel Usage All the things that do the stuff

Peach Pit vs. Gödel Gourd Data Model State Model Agents Test Data Model OCL Definitions State Model OCL Associations Agents Test

Gödel: Define Constraints <Ocl> <![CDATA[ context StatusCodeOk post: context.test.publishers[self.publisher].Result = 'OK' ]]> </Ocl>

Gödel: Associate Constraints <Action type="call" method="Logout"> <Ocl context="StatusCodeOk" /> </Action> Constraints will now run with this Action.

Gödel: Control Iterations <Test name=“Default” controlIteration=“1”> <Agent … /> <StateModel … /> <Publisher … /> <Logger … /> </Test> Define how often control iterations occur.

Usage Feasibility Time and Cost

Adding Gödel Process: Not recreating full application logic Existing Peach PIT Add OCL Constraints Test and Verify Definition Not recreating full application logic Just our “view of the world”

Time per Protocol Based on current experience of limited protocol set Decent in 1 – 2 days Complete in 1 week or less

Performance What performance impact does Gödel incur? Constraint evaluation Control iterations No performance optimizations…yet

Performance of Constraints

Performance Control Iterations Depends on how often, worst case half speed Never longer than mutation iterations

Performance Conclusions Performance impact dependent on speed of fuzzing Ability to scale fuzzing lowers impact For fast fuzzers, acceptable impact For slower fuzzers, adjust control iterations to occur less often

Conclusions Pentesting/Quick fuzzing General definition building Reasonable for “basics” (verify state’s work, critical logic flows) General definition building Reasonable to implement decent coverage 1-2 days “good enough”

Wrapping it up…

Lessons Learned Constraints applied only to control iterations Writing good constraints that apply to all mutation cases is challenging A few constraints can go along ways Performance overhead needs to be lowered when many constraints used. Optimize access to most used variables/objects

Looking towards next rev… Can we “learn” basic constraints? Performance optimizations Shorted “name” of common objects context.test.publishers[self.publisher].Result self.Result

Thanks for all the fish! Michael Eddington mike@dejavusecurity.com http://dejavusecurity.com http://peachfuzzer.com