Posted by Ivan Fratric, Google Project Zero
Around a year ago, we published the results of research about the resilience of modern browsers against DOM fuzzing, a well-known technique for finding browser bugs. Together with the bug statistics we also published Domato, our DOM fuzzing tool that was used to find those bugs.
Given that in the previous research, Apple Safari, or more specifically, WebKit (its DOM engine) did noticeably worse than other browsers, we decided to revisit it after a year using exactly the same methodology and exactly the same tools to see whether anything changed.
Test Setup
As in the original research, the fuzzing was initially done against WebKitGTK+ and then all the crashes were tested against Apple Safari running on a Mac. This makes the fuzzing setup easier as WebKitGTK+ uses the same DOM engine as Safari, but allows for fuzzing on a regular Linux machine. In this research, WebKitGTK+ version 2.20.2 was used which can be downloaded here.
To improve the fuzzing process, a couple of custom changes were made to WebKitGTK+:
- Made fixes to be able to build WebKitGTK+ with ASan (Address Sanitizer).
- Changed window.alert() implementation to immediately call the garbage collector instead of displaying a message window. This works well because window.alert() is not something we would normally call during fuzzing.
- Normally, when a DOM bug causes a crash, due to the multi-process nature of WebKit, only the web process would crash, but the main process would continue running. Code was added that monitors a web process and, if it crashes, the code would “crash” the main process with the same status.
- Created a custom target binary.
After the previous research was published, we got a lot of questions about the details of our fuzzing setup. This is why, this time, we are publishing the changes made to the WebKitGTK+ code as well as the detailed build instructions below. A patch file can be found here. Note that the patch was made with WebKitGTK+ 2.20.2 and might not work as is on other versions.
Once WebKitGTK+ code was prepared, it was built with ASan by running the following commands from the WebKitGTK+ directory:
export CC=/usr/bin/clang
export CXX=/usr/bin/clang++
export CFLAGS="-fsanitize=address"
export CXXFLAGS="-fsanitize=address"
export LDFLAGS="-fsanitize=address"
export ASAN_OPTIONS="detect_leaks=0"
mkdir build
cd build
cmake -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=. -DCMAKE_SKIP_RPATH=ON -DPORT=GTK -DLIB_INSTALL_DIR=./lib -DUSE_LIBHYPHEN=OFF -DENABLE_MINIBROWSER=ON -DUSE_SYSTEM_MALLOC=ON -DENABLE_GEOLOCATION=OFF -DENABLE_GTKDOC=OFF -DENABLE_INTROSPECTION=OFF -DENABLE_OPENGL=OFF -DENABLE_ACCELERATED_2D_CANVAS=OFF -DENABLE_CREDENTIAL_STORAGE=OFF -DENABLE_GAMEPAD_DEPRECATED=OFF -DENABLE_MEDIA_STREAM=OFF -DENABLE_WEB_RTC=OFF -DENABLE_PLUGIN_PROCESS_GTK2=OFF -DENABLE_SPELLCHECK=OFF -DENABLE_VIDEO=OFF -DENABLE_WEB_AUDIO=OFF -DUSE_LIBNOTIFY=OFF -DENABLE_SUBTLE_CRYPTO=OFF -DUSE_WOFF2=OFF -Wno-dev ..
make -j 4
mkdir -p libexec/webkit2gtk-4.0
cp bin/WebKit*Process libexec/webkit2gtk-4.0/
If you are doing this for the first time, the cmake/make step will likely complain about missing dependencies, which you will then have to install. You might note that a lot of features deemed not overly important for DOM fuzzing were disabled via -DENABLE flags. This was mainly to save us from having to install the corresponding dependencies but in some cases also to create a build that was more “portable”.
After the build completes, the fuzzing is as simple as creating a sample with Domato, running the target binary as
ASAN_OPTIONS=detect_leaks=0,exitcode=42 ASAN_SYMBOLIZER_PATH=/path/to/llvm-symbolizer LD_LIBRARY_PATH=./lib ./bin/webkitfuzz /path/to/sample <timeout>
and waiting for the exit code 42 (which, if you take a look at the command line above as well as the changes we made to the WebKitGTK+ code, indicates an ASan crash).
After collecting crashes, an ASan build of the most recent WebKit source code was created on the actual Mac hardware. This is as simple as running
./Tools/Scripts/set-webkit-configuration --release --asan
./Tools/Scripts/build-webkit
Each crash obtained on WebKitGTK+ was tested against the Mac build before reporting to Apple.
The Results
After running the fuzzer for 100.000.000 iterations (the same as a year ago) I ended up with 9 unique bugs that were reported to Apple. Last year, I estimated that the computational power to perform this number of iterations could be purchased for about $1000 and this probably hasn’t changed - an amount well within the payment range of a wide range of attackers with varying motivation.
The bugs are summarized in the table below. Please note that all of the bugs have been fixed at the time of release of this blog post.
Project Zero bug ID
|
CVE
|
Type
|
Affected Safari 11.1.2
|
Older than 6 months
|
Older than 1 year
|
CVE-2018-4197
|
UAF
|
YES
|
YES
|
NO
| |
CVE-2018-4318
|
UAF
|
NO
|
NO
|
NO
| |
CVE-2018-4317
|
UAF
|
NO
|
YES
|
NO
| |
CVE-2018-4314
|
UAF
|
YES
|
YES
|
NO
| |
CVE-2018-4306
|
UAF
|
YES
|
YES
|
NO
| |
CVE-2018-4312
|
UAF
|
NO
|
NO
|
NO
| |
CVE-2018-4315
|
UAF
|
YES
|
YES
|
NO
| |
CVE-2018-4323
|
UAF
|
YES
|
YES
|
NO
| |
CVE-2018-4328
|
OOB read
|
YES
|
YES
|
YES
|
UAF = use-after-free. OOB = out-of-bounds
As can be seen in the table, out of the 9 bugs found, 6 affected the release version of Apple Safari, directly affecting Safari users.
While 9 or 6 bugs (depending how you count) is significantly less than the 17 found a year ago, it is still a respectable number of bugs, especially if we take into an account that the fuzzer has been public for a long time now.
After the results were in, I looked into how long these bugs have been in the WebKit codebase. To check this, all the bugs were tested against a version of WebKitGTK+ that was more than 6 months old (WebKitGTK+ 2.19.6) as well as a version that was more than a year old (WebKitGTK+ 2.16.6).
The results are interesting—most of the bugs were sitting in the WebKit codebase for longer than 6 months, however, only 1 of them is older than 1 year. Here, it might be important to note that throughout the past year (between the previous and this blog post) I also did fuzzing runs using the same approach and reported 14 bugs. Unfortunately, it is impossible to know how many of those 14 bugs would have survived until now and how many would have been found in this fuzz run. It is also possible that some of the newly found bugs are actually older, but don’t trigger with the provided PoCs is in the older versions due to unrelated code changes in the DOM. I didn’t investigate this possibility.
However, even if we assume that all of the previously reported bugs would not have survived until now, the results still indicate that (a) the security vulnerabilities keep getting introduced in the WebKit codebase and (b) many of those bugs get incorporated into the release products before they are caught by internal security efforts.
While (a) is not unusual for any piece of software that changes as rapidly as a DOM engine, (b) might indicate the need to put more computational resources into fuzzing and/or review before release.
The Exploit
To prove that bugs like this can indeed lead to a browser compromise, I decided to write an exploit for one of them. The goal was not to write a very reliable or sophisticated exploit - highly advanced attackers would likely not choose to use the bugs found by public tools whose lifetime is expected to be relatively short. However, if someone with exploit writing skills was to use such a bug in, for example, a malware spreading campaign, they could potentially do a lot of damage even with an unreliable exploit.
Out of the 6 issues affecting the release version of Safari, I selected what I believed to be the easiest one to exploit—a use-after-free where, unlike in the other use-after-free issues found, the freed object is not on the isolated heap—a mitigation recently introduced in WebKit to make use-after-free exploitation harder.
Let us first start by examining the bug we’re going to exploit. The issue is a use-after-free in the SVGAnimateElementBase::resetAnimatedType() function. If you look at the code of the function, you are going to see that, first, the function gets a raw pointer to the SVGAnimatedTypeAnimator object on the line
SVGAnimatedTypeAnimator* animator = ensureAnimator();
and, towards the end of the function, the animator object is used to obtain a pointer to a SVGAnimatedType object (unless one already exists) on the line
m_animatedType = animator->constructFromString(baseValue);
The problem is that, in between these two lines, attacker-controlled JavaScript code could run. Specifically, this could happen during a call to computeCSSPropertyValue(). The JavaScript code could then cause SVGAnimateElementBase::resetAnimatedPropertyType() to be called, which would delete the animator object. Thus, the constructFromString() function would be called on the freed animator object - a typical use-after-free scenario, at least on the first glance. There is a bit more to this bug though, but we’ll get to that later.
The vulnerability has been fixed in the latest Safari by no longer triggering JavaScript callbacks through computeCSSPropertyValue(). Instead, the event handler is going to be processed at some later time. The patch can be seen here.
A simple proof of concept for the vulnerability is:
<body onload="setTimeout(go, 100)">
<svg id="svg">
<animate id="animate" attributeName="fill" />
</svg>
<div id="inputParent" onfocusin="handler()">
<input id="input">
</div>
<script>
function handler() {
animate.setAttribute('attributeName','fill');
}
function go() {
input.autofocus = true;
inputParent.after(inputParent);
svg.setCurrentTime(1);
}
</script>
</body>
Here, svg.setCurrentTime() results in resetAnimatedType() being called, which in turn, due to DOM mutations made previously, causes a JavaScript event handler to be called. In the event handler, the animator object is deleted by resetting the attributeName attribute of the animate element.
Since constructFromString() is a virtual method of the SVGAnimatedType class, the primitive the vulnerability gives us is a virtual method call on a freed object.
In the days before ASLR, such a vulnerability would be immediately exploitable by replacing the freed object with data we control and faking the virtual method table of the freed object, so that when the virtual method is called, execution is redirected to the attacker’s ROP chain. But due to ASLR we won’t know the addresses of any executable modules in the process.
A classic way to overcome this is to combine such a use-after-free bug with an infoleak bug that can leak an address of one of the executable modules. But, there is a problem: In our crop of bugs, there wasn’t a good infoleak we could use for this purpose. A less masochistic vulnerability researcher would simply continue to run the fuzzer until a good infoleak bug would pop up. However, instead of finding better bugs, I deliberately wanted to limit myself to just the bugs found in the same number of iterations as in the previous research. As a consequence, the majority of time spent working on this exploit was to turn the bug into an infoleak.
As stated before, the primitive we have is a virtual method call on the freed object. Without an ASLR bypass, the only thing we can do with it that would not cause an immediate crash is to replace the freed object with another object that also has a vtable, so that when a virtual method is called, it is called on the other object. Most of the time, this would mean calling a valid virtual method on a valid object and nothing interesting would happen. However, there are several scenarios where doing this could lead to interesting results:
- The virtual method could be something dangerous to call out-of-context. For example, if we can call a destructor of some object, its members could get freed while the object itself continues to live. With this, we could turn the original use-after-free issue into another use-after-free issue, but possibly one that gives us a better exploitation primitive.
- Since constructFromString() takes a single parameter of the type String, we could potentially cause a type confusion on the input parameter if the other virtual method expects a parameter of another type. Additionally, if the other virtual method takes more parameters than constructFromString(), these would be uninitialized which could also lead to exploitable behavior.
- As constructFromString() is expected to return a pointer of type SVGAnimatedType, if the other virtual method returns some other type, this will lead to the type confusion on the return value. Additionally, if the other virtual method does not return anything, then the return value remains uninitialized.
- If the vtables of the freed object and the object we replaced it with are of different size, calling a vtable pointer on the freed object could result in an out-of-bounds read on the vtable of the other object, resulting in calling a virtual function of some third class.
In this exploit we used option 3, but with a twist. To understand what the twist is, let’s examine the SVGAnimateElementBase class more closely: It implements (most of) the functionality of the SVG <animate> element. The SVG <animate> element is used to, as the name suggests, animate a property of another element. For example, having the following element in an SVG image
<animate attributeName="x" from="0" to="100" dur="10s" />
will cause the x coordinate of the target element (by default, the parent element) to grow from 0 to 100 over the duration of 10 seconds. We can use an <animate> element to animate various CSS or XML properties, which is controlled by the attributeName property of the <animate> element.
Here’s the interesting part: These properties can have different types. For example, we might use an <animate> element to animate the x coordinate of an element, which is of type SVGLengthValue (number + unit), or we might use it to animate the fill attribute, which is of type Color.
In an SVGAnimateElementBase class, the type of animated property is tracked via a member variable declared as
AnimatedPropertyType m_animatedPropertyType;
Where AnimatedPropertyType is the enumeration of possible types. Two other member variables of note are
std::unique_ptr<SVGAnimatedTypeAnimator> m_animator;
std::unique_ptr<SVGAnimatedType> m_animatedType;
The m_animator here is the use-after-free object, while m_animatedType is the object created from the (possibly freed) m_animator.
SVGAnimatedTypeAnimator (type of m_animator) is a superclass which has subclasses for all possible values of AnimatedPropertyType, such as SVGAnimatedBooleanAnimator, SVGAnimatedColorAnimator etc. SVGAnimatedType (type of m_animatedType) is a variant that contains a type and a union of possible values depending on the type.
The important thing to note is that normally, both the subclass of m_animator and the type of m_animatedType are supposed to match m_animatedPropertyType. For example, if m_animatedPropertyType is AnimatedBoolean, then the type of m_animatedType variant should be the same, and m_animator should be an instance of SVGAnimatedBooleanAnimator.
After all, why shouldn’t all these types match, since m_animator is created based on m_animatedPropertyType here and m_animatedType is created by m_animator here. Oh wait, that’s exactly where the vulnerability occurs!
So instead of replacing a freed animator with something completely different and causing a type confusion between SVGAnimatedType and another class, we can instead replace the freed animator with another animator subclass and confuse SVGAnimatedType with type = A to another SVGAnimatedType with type = B.
But one interesting thing about this bug is that it would still be a bug even if the animator object did not get freed. In that case, the bug turns into a type confusion: To trigger it, one would simply change the m_animatedPropertyType of the <animate> element to a different type in the JavaScript callback (we’ll examine how this happens in detail later). This led to some discussion in the office whether the bug should be called an use-after-free at all, or is this really a different type of bug where the use-after-free is merely a symptom.
Note that the animator object is always going to get freed as soon as the type of the <animate> element changes, which leads to an interesting scenario where to exploit a bug (however you choose to call it), instead of replacing the freed object with an object of another type, we could either replace it with the object of the same type or make sure it doesn’t get replaced at all. Due to how memory allocation in WebKit works, the latter is actually going to happen on its own most of the time anyway - objects allocated in a memory page will only start getting replaced once the whole page becomes full. Additionally, freeing an object in WebKit doesn’t corrupt it as would be the case in some other allocators, which allows us to still use it normally even after being freed.
Let’s now examine how this type confusion works and what effects it has:
- We start with an <animate> element for type A. m_animatedPropertyType, m_animator and m_animatedType all match type A.
- In the JavaScript callback, we change the type of <animate> element to B by changing its attributeName attribute. This causes SVGAnimateElementBase::resetAnimatedPropertyType() to be called. In it, m_animatedType and m_animator get deleted, while m_animatedPropertyType gets set to B according to the new attributeName here. Now, m_animatedType and m_animator are null, while m_animatedPropertyType is B.
- We return into resetAnimatedType(), where we still have a local variable animator which still points to (freed but still functional) animator for type A.
- m_animatedType gets created based on the freed animator here. Now, m_animatedType is of type A, m_animatedPropertyType is B and m_animator is null.
- resetAnimatedType() returns, and the animator local variable pointing to the freed animator of type A gets lost, never to be seen again.
- Eventually, resetAnimatedType() gets called again. Since m_animator is still null, but m_animatedPropertyType is B, it creates m_animator of type B here.
- Since m_animatedType is non-null, instead of creating it anew, we just initialize it by calling m_animatedType->setValueAsString() here. We now have m_animatedPropertyType for type B, m_animator for type B and m_animatedType for type A.
- At some point, the value of the animated property gets calculated. That happens in SVGAnimateElementBase::calculateAnimatedValue() on this line by calling m_animator->calculateAnimatedValue(..., m_animatedType). Here, there is a mismatch between the m_animator (type B) and m_animatedType (type A). However, because the mismatch wouldn’t normally occur, the animator won’t check the type of the argument (there might be some debug asserts but nothing in the release) and will attempt to write the calculated animated value of type B into the SVGAnimatedType with type A.
- After the animated value has been computed, it is read out as string and set to the corresponding CSS property. This happens here.
The actual type confusion only happens in step 10: there, we will write to the SVGAnimatedType of type A as if it actually was type B. The rest of the interactions with m_animatedType are not dangerous since they are simply getting and setting the value as string, an operation that is safe to do regardless of the actual type.
Note that, although the <animate> element supports animating XML properties as well as CSS properties, we can only do the above dance with CSS properties as the code for handling XML properties is different. The list of CSS properties we can work with can be found here.
So, how do we exploit this type confusion for an infoleak? The initial idea was to exploit with A = <some numeric type> and B = String. This way, when the type confusion on write occurs, a string pointer is written over a number and then we would be able to read it in step 11 above. But there is a problem with this (as well as with a large number of type combinations): The value read in step 11 must be a valid CSS property value in the context of the current animated property, otherwise it won’t be set correctly and we would not be able to read it out. For example, we were unable to find a string CSS property (from the list above) that would accept a value like 1.4e-45 or similar.
A more promising approach, due to limitations of step 11, would be to replace a numeric type with another numeric type. We had some success with A = FloatRect and B = SVGLengthListValues, which is a vector of SVGLengthValue values. Like above, this results in a vector pointer being written over FloatRect type. This sometimes leads to successfully disclosing a heap address. Why sometimes? Because the only CSS property with type SVGLengthListValues we can use is stroke-dasharray, and stroke-dasharray accepts only positive values. Thus, if lower 32-bits of the heap address we want to disclose look like a negative floating point number (i.e. the highest bit is set), then we would not be able to disclose that address. This problem can be overcome by spraying the heap with 2GB of data so that the lower 32-bits of heap addresses start becoming positive. But, since we need heap spraying anyway, there is another approach we can take.
The approach we actually ended up using is with A = SVGLengthListValues (stroke-dasharray CSS property) and B = float (stroke-miterlimit CSS property). What this type confusion does, is overwrites the lowest 32 bits of a SVGLengthValue vector with a floating point number.
Before we trigger this type confusion we need to spray the heap with approximately 4GB of data (doable on modern computers), which gives us a good probability that when we change an original heap address 0x000000XXXXXXXXXX to 0x000000XXYYYYYYYY, the resulting address is still going to be a valid heap address, especially if YYYYYYYY is high. This way, we can disclose not-quite-arbitrary data at 0x000000XX00000000 + arbitrary offset.
Why not-quite-arbitrary? Because there are still some limitations:
- As stroke-miterlimit must be positive, once again we can only disclose data from the heap interpretable as a 32-bit float.
- SVGLengthValue is a type which consists of a 32-bit float followed by an enumeration that describes the units used. When a SVGLengthValue is read out as string in step 11 above, if the unit value is valid, it will be appended to the number (e.g. ‘100px’). If we attempt to set a string like that to the stroke-miterlimit property it will fail. Thus, the next byte after the heap value we want to read must interpret as invalid unit (in which case the unit is not appended when reading out SVGLengthValue as string).
Note that both of these limitations can often be worked around by doing non-aligned reads.
Now that we have our more-or-less usable read, what do we read out? As the whole point is to defeat ASLR, we should read a pointer to an executable module. Often in exploitation, one would do that by reading out the vtable pointer of some object on the heap. However, on MacOS it appears that vtable pointers point to a separate memory region than the one containing executable code of the corresponding module. So instead of reading out a vtable pointer, we need to read a function pointer instead.
What we ended up doing is using VTTRegion objects in our heap spray. A VTTRegion object contains a Timer which contains a pointer to Function object which (in this case) contains a function pointer to VTTRegion::scrollTimerFired(). Thus, we can spray with VTTRegion objects (which takes about 10 seconds on a quite not-state-of-the-art Mac Mini) and then scan the resulting memory for a function pointer.
This gives us the ASLR bypass, but one other thing useful to have for the next phase is the address of the payload (ROP chain and shellcode). We disclose it by the following steps:
- Find a VTTRegion object in the heap spray.
- By setting the VTTRegion.height property during the heap spray to an index in the spray array, we can identify exactly which of the millions of VTTRegion objects we just read.
- Set the VTTRegion.id property of the VTTRegion object to the payload.
- Read out the VTTRegion.id pointer.
We are now ready for triggering the vulnerability a second time, this time for code exec. This time, it is the classic use-after-free exploitation scenario: we overwrite the freed SVGAnimatedTypeAnimator object with the data we control.
As Apple recently introduced gigacage (a separate large region of memory) for a lot of attacker-controlled datatypes (strings, arrays, etc.) this is no longer trivial. However, one thing still allocated on the main heap is Vector content. By finding a vector whose content we fully control, we can overcome the heap limitations.
What I ended up using is a temporary vector used when TypedArray.set() is called to copy values from one JavaScript typed array into another typed array. This vector is temporary, meaning it will be deleted immediately after use, but again, due to how memory allocation works in webkit it is not too horrible. Like other stability improvements, the task of finding a more permanent controllable allocation is left to the exercise of the reader. :-)
This time, in the JavaScript event handler, we can replace the freed SVGAnimatedTypeAnimator with a vector whose first 8 bytes are set to point to the ROP chain + shellcode payload.
The ROP chain is pretty straightforward, but one thing that is perhaps more interesting is the stack pivot gadget (or, in this case, gadgets) used. In the scenario we have, the virtual function on the freed object is called as
call qword ptr [rax+10h]
where rax points to our payload. Additionally, rsi points to the freed object (that we now also control). The first thing we want to do for ROP is control the stack, but I was unable to find any “classic” gadgets that accomplish this such as
mov rsp, rax; ret;
push rax; pop rsp; ret;
xchg rax, rsp; ret;
What I ended up doing is breaking the stack pivot into two gadgets:
push rax; mov rax, [rsi], call [rax + offset];
This first gadget pushes the payload address on the stack and is very common because, after all, that’s exactly how the original virtual function was called (apart from push rax that can be an epilogue of some other instruction). The second gadget can then be
pop whatever; pop rsp; ret;
where the first pop pops the return address from the stack and the second pop finally gets the controlled value into rsp. This gadget is less common, but still appears to be way more common than the stack pivot mentioned previously, at least in our binary.
The final ROP chain is (remember to start reading from offset 0x10):
[address of pop; pop; pop; ret]
0
[address of push rax; mov rax, [rsi], call [rax+0x28]];
0
[address of pop; ret]
[address of pop rbp; pop rsp; ret;]
[address of pop rdi; ret]
0
[address of pop rsi; ret]
shellcode length
[address of pop rdx; ret]
PROT_EXEC + PROT_READ + PROT_WRITE
[address of pop rcx; ret]
MAP_ANON + MAP_PRIVATE
[address of pop r8; pop rbp; ret]
-1
0
[address of pop r9; ret]
0
[address of mmap]
[address of push rax; pop rdi; ret]
[address of push rsp; pop rbp; ret]
[address of push rbp; pop rax; ret]
[address of add rax, 0x50; pop rbp; ret]
0
[address of push rax; pop rsi; pop rbp; ret]
0
[address of pop rdx; ret]
shellcode length
[address of memcpy]
[address of jmp rax;]
0
shellcode
The ROP chain calls
mmap(0, shellcode_length, PROT_EXEC | PROT_READ | PROT_WRITE, MAP_ANON + MAP_PRIVATE, -1, 0)
Then calculates the shellcode address and copies it to the address returned by mmap(), after which the shellcode is called.
In our case, the shellcode is just a sequence of ‘int 3’ instructions and when reaching it, Safari
will crash. If a debugger is attached, we can see that the shellcode was successfully reached as it will detect a breakpoint:
Process 5833 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BREAKPOINT (code=EXC_I386_BPT, subcode=0x0)
frame #0: 0x00000001b1b83001
-> 0x1b1b83001: int3
0x1b1b83002: int3
0x1b1b83003: int3
0x1b1b83004: int3
Target 0: (com.apple.WebKit.WebContent) stopped.
In the real-world scenario the shellcode could either be a second-stage exploit to break out of the Safari sandbox or, alternately, a payload that would turn the issue into an universal XSS, stealing cross-domain data.
The exploit was successfully tested on Mac OS 10.13.6 (build version 17G65). If you are still using this version, you might want to update. The full exploit can be seen here.
The impact of recent iOS mitigations
An interesting aspect of this exploit is that, on Safari for Mac OS it could be written in a very “old-school” way (infoleak + ROP) due to lack of control flow mitigations on the platform.
On the latest mobile hardware and in iOS 12, which was published after the exploit was already written, Apple introduced control flow mitigations by using Pointer Authentication Codes (PAC). While there are no plans to write another version of the exploit at this time, it is interesting to discuss how the exploit could be modified not to be affected by the recent mitigations.
The exploit, as presented here, consists of two parts: infoleak and getting code execution. PAC would not affect the infoleak part in any way, however it would prevent jumping to the ROP chain in the second part of the exploit, because we could not forge a correct signature for the vtable pointer.
Instead of jumping to the ROP code, the next stage of the exploit would likely need to be getting an arbitrary read-write primitive. This could potentially be accomplished by exploiting a similar type confusion that was used for the infoleak, but with a different object combination. I did notice that there are some type combinations that could result in a write (especially if the attacker already has an infoleak), but I didn’t investigate those in detail.
In the Webkit process, after the attacker has an arbitrary read-write primitive, they could find a way to overwrite JIT code (or, failing that, other data that would cause fully or partially controlled JIT code to be emitted) and achieve code execution that way.
So while the exploit could still be written, admittedly it would be somewhat more difficult to write.
On publishing the advisories
Before concluding this blog post, we want to draw some attention to how the patches for the issues listed in the blog post were announced and to the corresponding timeline. The issues were reported to Apple between June 15 and July 2nd, 2018. On September 17th 2018, Apple published security advisories for iOS 12, tvOS 12 and Safari 12 which fixed all of the issues. However, although the bugs were fixed at that time, the corresponding advisories did not initially mention them. The issues described in the blog post were only added to the advisories one week later, on September 24, 2018, when the security advisories for macOS Mojave 10.14 were also published.
To demonstrate the discrepancy between originally published advisories and the updated advisories, compare the archived version of Safari 12 advisories from September 18 here and the current version of the same advisories here (note that you might need to refresh the page if you still have the old version in your browser’s cache).
The original advisories most likely didn’t include all the issues because Apple wanted to wait for the issues to also be fixed on MacOS before adding them. However, this practice is misleading because customers interested in the Apple security advisories would most likely read them only once, when they are first released and the impression they would to get is that the product updates fix far less vulnerabilities and less severe vulnerabilities than is actually the case.
Furthermore, the practice of not publishing fixes for mobile or desktop operating systems at the same time can put the desktop customers at unnecessary risk, because attackers could reverse-engineer the patches from the mobile updates and develop exploits against desktop products, while the desktop customers would have no way to update and protect themselves.
Conclusion
While there were clearly improvements in WebKit DOM when tested with Domato, the now public fuzzer was still able to find a large number of interesting bugs in a non-overly-prohibitive number of iterations. And if a public tool was able to find that many bugs, it is expected that private ones might be even more successful.
And while it is easy to brush away such bugs as something we haven’t seen actual attackers use, that doesn’t mean it’s not happening or that it couldn’t happen, as the provided exploit demonstrates. The exploit doesn’t include a sandbox escape so it can’t be considered a full chain, however reports from other security researchers indicate that this other aspect of browser security, too, cracks under fuzzing (Note from Apple Security: this sandbox escape relies on attacking the WindowServer, access to which has been removed from the sandbox in Safari 12 on macOS Mojave 10.14). Additionally, a DOM exploit could be used to steal cross-domain data such as cookies even without a sandbox escape.
The fuzzing results might indicate that WebKit is getting fuzzed, but perhaps not with sufficient computing power to find all fuzzable, newly introduced bugs before they make it into the release version of the browser. We are hoping that this research will lead to improved user security by providing an incentive for Apple to allocate more resources into this area of browser security.
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