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

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1. V8 ARCHITECTURE

Compilation Pipeline

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

Concept	Description
Tagged pointers	SMI (Small Integer): value << 1, HeapObject: ptr | 1
Pointer compression	V8 ≥ 8.0: objects addressed via 32-bit offset from cage base (4GB sandbox)
Maps (Hidden Classes)	Define object shape: property names, types, offsets
Elements kinds	Internal array type: PACKED_SMI_ELEMENTS, PACKED_DOUBLE_ELEMENTS, PACKED_ELEMENTS, etc.
Write barrier	GC bookkeeping when heap pointers are written
Garbage collection	Orinoco GC: minor (Scavenge) and major (Mark-Compact)

Object Representation (64-bit, pointer compression)

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

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2. COMMON V8 BUG CLASSES

Bug Class	Description	Example
JIT Type Confusion	TurboFan assumes wrong type after optimization	Speculative type guard eliminated, wrong operation applied
Incorrect Bounds Elimination	JIT removes array bounds check based on wrong range analysis	CheckBounds node eliminated → OOB access
Prototype Chain Confusion	Optimization assumes stable prototype, mutations invalidate	Prototype change after optimization → wrong property access
Turbofan Reduction Bug	Incorrect strength reduction or constant folding	Integer overflow in range analysis
Race Condition	SharedArrayBuffer + worker thread race	Type confusion via concurrent modification
Off-by-one in Builtin	Boundary error in built-in function implementation	String/Array bounds
Typer Bug	Incorrect type range computation in TurboFan	Typer says value is in [0, N] but can be N+1

Triggering JIT Optimization

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

// 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

// 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

// 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:

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:

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

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5. ARRAYBUFFER ARBITRARY R/W

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

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

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6. WASM RWX PAGE

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

// 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.

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7. V8 SANDBOX

Architecture (V8 ≥ 11.x)

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

Vector	Method
External pointer table	Corrupt entries in the external pointer table to reference arbitrary addresses
WASM code pointer	Overwrite WASM function entry to jump to controlled shellcode
JIT code corruption	Write to JIT code page via race condition or confused pointer
Mojo IPC (Chrome)	Exploit Chrome IPC to attack browser process from compromised renderer
Backing store seal bypass	Find type confusion to get unsandboxed pointer

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8. CHROME SANDBOX ESCAPE (OVERVIEW)

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

Stage	Target	Example
Renderer exploit	V8 / Blink DOM	Type confusion → shellcode
IPC/Mojo bug	Chrome IPC layer	Use-after-free in Mojo interface
Browser process exploit	Privileged browser process	Code execution outside sandbox

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

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9. TOOLS

# 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

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10. DECISION TREE

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)