An analysis of an in-the-wild iOS Safari WebContent to GPU Process exploit

By Ian Beer

A graph representation of the sandbox escape NSExpression payload

In April this year Google's Threat Analysis Group, in collaboration with Amnesty International, discovered an in-the-wild iPhone zero-day exploit chain being used in targeted attacks delivered via malicious link. The chain was reported to Apple under a 7-day disclosure deadline and Apple released iOS 16.4.1 on April 7, 2023 fixing CVE-2023-28206 and CVE-2023-28205.

Over the last few years Apple has been hardening the Safari WebContent (or "renderer") process sandbox attack surface on iOS, recently removing the ability for the WebContent process to access GPU-related hardware directly. Access to graphics-related drivers is now brokered via a GPU process which runs in a separate sandbox.

Analysis of this in-the-wild exploit chain reveals the first known case of attackers exploiting the Safari IPC layer to "hop" from WebContent to the GPU process, adding an extra link to the exploit chain (CVE-2023-32409).

On the surface this is a positive sign: clear evidence that the renderer sandbox was hardened sufficiently that (in this isolated case at least) the attackers needed to bundle an additional, separate exploit. Project Zero has long advocated for attack-surface reduction as an effective tool for improving security and this would seem like a clear win for that approach.

On the other hand, upon deeper inspection, things aren't quite so rosy. Retroactively sandboxing code which was never designed with compartmentalization in mind is rarely simple to do effectively. In this case the exploit targeted a very basic buffer overflow vulnerability in unused IPC support code for a disabled feature - effectively new attack surface which exists only because of the introduced sandbox. A simple fuzzer targeting the IPC layer would likely have found this vulnerability in seconds.

Nevertheless, it remains the case that attackers will still need to exploit this extra link in the chain each time to reach the GPU driver kernel attack surface. A large part of this writeup is dedicated to analysis of the NSExpression-based framework the attackers developed to ease this and vastly reduce their marginal costs.

Setting the stage

After gaining native code execution exploiting a JavaScriptCore Garbage Collection vulnerability the attackers perform a find-and-replace on a large ArrayBuffer in JavaScript containing a Mach-O binary to link a number of platform- and version-dependent symbol addresses and structure offsets using hardcoded values:

// find and rebase symbols for current target and ASLR slide:

dt: {

ce: false,

["16.3.0"]: {

_e: 0x1ddc50ed1,

de: 0x1dd2d05b8,

ue: 0x19afa9760,

he: 1392,

me: 48,

fe: 136,

pe: 0x1dd448e70,

ge: 305,

Ce: 0x1dd2da340,

Pe: 0x1dd2da348,

ye: 0x1dd2d45f0,

be: 0x1da613438,

...

["16.3.1"]: {

_e: 0x1ddc50ed1,

de: 0x1dd2d05b8,

ue: 0x19afa9760,

he: 1392,

me: 48,

fe: 136,

pe: 0x1dd448e70,

ge: 305,

Ce: 0x1dd2da340,

Pe: 0x1dd2da348,

ye: 0x1dd2d45f0,

be: 0x1da613438,

// mach-o Uint32Array:

xxxx = new Uint32Array([0x77a9d075,0x88442ab6,0x9442ab8,0x89442ab8,0x89442aab,0x89442fa2,

// deobfuscate xxx

...

// find-and-replace symbols:

xxxx.on(new m("0x2222222222222222"), p.Le);

xxxx.on(new m("0x3333333333333333"), Gs);

xxxx.on(new m("0x9999999999999999"), Bs);

xxxx.on(new m("0x8888888888888888"), Rs);

xxxx.on(new m("0xaaaaaaaaaaaaaaaa"), Is);

xxxx.on(new m("0xc1c1c1c1c1c1c1c1"), vt);

xxxx.on(new m("0xdddddddddddddddd"), p.Xt);

xxxx.on(new m("0xd1d1d1d1d1d1d1d1"), p.Jt);

xxxx.on(new m("0xd2d2d2d2d2d2d2d2"), p.Ht);

The initial Mach-O which this loads has a fairly small __TEXT (code) segment and is itself in fact a Mach-O loader, which loads another binary from a segment called __embd. It's this inner Mach-O which this analysis will cover.

Part I - Mysterious Messages

Looking through the strings in the binary there's a collection of familiar IOKit userclient matching strings referencing graphics drivers:

"AppleM2ScalerCSCDriver",0

"IOSurfaceRoot",0

"AGXAccelerator",0

But following the cross references to "AGXAccelerator" (which opens userclients for the GPU) this string never gets passed to IOServiceOpen. Instead, all references to it end up here (the binary is stripped so all function names are my own):

kern_return_t

get_a_user_client(char *matching_string,

u32 type,

void* s_out) {

kern_return_t ret;

struct uc_reply_msg;

mach_port_name_t reply_port;

struct msg_1 msg;

reply_port = 0;

mach_port_allocate(mach_task_self_,

MACH_PORT_RIGHT_RECEIVE,

&reply_port);

memset(&msg, 0, sizeof(msg));

msg.hdr.msgh_bits = 0x1413;

msg.hdr.msgh_remote_port = a_global_port;

msg.hdr.msgh_local_port = reply_port;

msg.hdr.msgh_id = 5;

msg.hdr.msgh_size = 200;

msg.field_a = 0;

msg.type = type;

__strcpy_chk(msg.matching_str, matching_string, 128LL);

ret = mach_msg_send(&msg.hdr);

...

// return a port read from the reply message via s_out

Whilst it's not unusual for a userclient matching string to end up inside a mach message (plenty of exploits will include or generate their own MIG serialization code for interacting with IOKit) this isn't a MIG message.

Trying to track down the origin of the port right to which this message was sent was non-trivial; there was clearly more going on. My guess was that this must be communicating with something else, likely some other part of the exploit. The question was: what other part?

Down the rabbit hole

At this point I started going through all the cross-references to the imported symbols which could send or receive mach messages, hoping to find the other end of this IPC. This just raised more questions than it answered.

In particular, there were a lot of cross-references to a function sending a variable-sized mach message with a msgh_id of 0xDBA1DBA.

There is exactly one hit on Google for that constant:

Ignoring Google's helpful advice that maybe I wanted to search for "cake recipes" instead of this hex constant and following the single result leads to this snippet on opensource.apple.com in ConnectionCocoa.mm:

namespace IPC {

static const size_t inlineMessageMaxSize = 4096;

// Arbitrary message IDs that do not collide with Mach notification messages (used my initials).

constexpr mach_msg_id_t inlineBodyMessageID = 0xdba0dba;

constexpr mach_msg_id_t outOfLineBodyMessageID = 0xdba1dba;

This is a constant used in Safari IPC messages!

Whilst Safari has had a separate networking process for a long time it's only recently started to isolate GPU and graphics-related functionality into a GPU process. Knowing this, it's fairly clear what must be going on here: since the renderer process can presumably no longer open the AGXAccelerator userclients, the exploit is somehow going to have to get the GPU process to do that. This is likely the first case of an in-the-wild iOS exploit targeting Safari's IPC layer.

The path less trodden

Googling for info on Safari IPC doesn't yield many results (apart from some very early Project Zero vulnerability reports) and looking through the WebKit source reveals heavy use of generated code and C++ operator overloading, neither of which are conducive to quickly getting a feel for the binary-level structure of the IPC messages.

But the high-level structure is easy enough to figure out. As we can see from the code snippet above, IPC messages containing the msgh_id value 0xdba1dba send their serialized message body as an out-of-line descriptor. That serialized body always starts with a common header defined in the IPC namespace as:

void Encoder::encodeHeader()

{

*this << defaultMessageFlags;

*this << m_messageName;

*this << m_destinationID;

}

The flags and name fields are both 16-bit values and destinationID is 64 bits. The serialization uses natural alignment so there's 4 bytes of padding between the name and destinationID:

It's easy enough to enumerate all the functions in the exploit which serialize these Safari IPC messages. None of them hardcode the messageName values; instead there's a layer of indirection indicating that the messageName values aren't stable across builds. The exploit uses the device's uname string, product and OS version to choose the correct hardcoded table of messageName values.

The IPC::description function in the iOS shared cache maps messageName values to IPC names:

const char * IPC::description(unsigned int messageName)

{

if ( messageName > 0xC78 )

return "<invalid message name>";

else

return off_1D61ED988[messageName];

}

The size of the bounds check gives you an idea of the size of the IPC attack surface - that's over 3000 IPC messages between all pairs of communicating processes.

Using the table in the shared cache to map the message names to human-readable strings we can see the exploit uses the following 24 IPC messages:

0x39: GPUConnectionToWebProcess_CreateRemoteGPU

0x3a: GPUConnectionToWebProcess_CreateRenderingBackend

0x9B5: InitializeConnection

0x9B7: ProcessOutOfStreamMessage

0xBA2: RemoteAdapter_RequestDevice

0xBA5: RemoteBuffer_MapAsync

0x271: RemoteBuffer_Unmap

0xBA6: RemoteCDMFactoryProxy_CreateCDM

0x2A2: RemoteDevice_CreateBuffer

0x2C7: RemoteDisplayListRecorder_DrawNativeImage

0x2D4: RemoteDisplayListRecorder_FillRect

0x2DF: RemoteDisplayListRecorder_SetCTM

0x2F3: RemoteGPUProxy_WasCreated

0xBAD: RemoteGPU_RequestAdapter

0x402: RemoteMediaRecorderManager_CreateRecorder

0xA85: RemoteMediaRecorderManager_CreateRecorderReply

0x412: RemoteMediaResourceManager_RedirectReceived

0x469: RemoteRenderingBackendProxy_DidInitialize

0x46D: RemoteRenderingBackend_CacheNativeImage

0x46E: RemoteRenderingBackend_CreateImageBuffer

0x474: RemoteRenderingBackend_ReleaseResource

0x9B8: SetStreamDestinationID

0x9B9: SyncMessageReply

0x9BA: Terminate

This list of IPC names solidifies the theory that this exploit is targeting a GPU process vulnerability.

Finding a way

The destination port which these messages are being sent to comes from a global variable which looks like this in the raw Mach-O when loaded into IDA:

__data:000000003E4841C0 dst_port DCQ 0x4444444444444444

I mentioned earlier that the outer JS which loaded the exploit binary first performed a find-and-replace using patterns like this. Here's the snippet computing this particular value:

let Ls = o(p.Ee);

let Ds = o(Ls.add(p.qe));

let Ws = o(Ds.add(p.$e));

let vs = o(Ws.add(p.Ze));

jBHk.on(new m("0x4444444444444444"), vs);

Replacing all the constants we can see it's following a pointer chain from a hardcoded offset inside the shared cache:

let Ls = o(0x1dd453458);

let Ds = o(Ls.add(256));

let Ws = o(Ds.add(24);

let vs = o(Ws.add(280));

At the initial symbol address (0x1dd453458) we find the WebContent process's singleton process object which maintains its state:

WebKit:__common:00000001DD453458 WebKit::WebProcess::singleton(void)::process

Following the offsets we can see they follow this pointer chain to be able to find the mach port right representing the WebProcess's connection to the GPU process:

process->m_gpuProcessConnection->m_connection->m_sendPort

The exploit also reads the m_receivePort field allowing it to set up bidirectional communication with the GPU process and fully imitate the WebContent process.

Defining features

Webkit defines its IPC messages using a simple custom DSL in files ending with the suffix .messages.in. These definitions look like this:

messages -> RemoteRenderPipeline NotRefCounted Stream {

void GetBindGroupLayout(uint32_t index, WebKit::WebGPUIdentifier identifier);

void SetLabel(String label)

}

These are parsed by this python script to generate the necessary boilerplate code to handle serializing and deserializing the messages. Types which wish to cross the serialization boundary define ::encode and ::decode methods:

void encode(IPC::Encoder&) const;

static WARN_UNUSED_RETURN bool decode(IPC::Decoder&, T&);

There are a number of macros defining these coders for the built-in types.

A pattern appears

Renaming the methods in the exploit which send IPC messages and reversing some more of their arguments a clear pattern emerges:

image_buffer_base_id = rand();

for (i = 0; i < 34; i++) {

IPC_RemoteRenderingBackend_CreateImageBuffer(

image_buffer_base_id + i);

}

semaphore_signal(semaphore_b);

remote_device_buffer_id_base = rand();