This is part 3 of a 6-part series detailing a set of vulnerabilities found by Project Zero being exploited in the wild. To read the other parts of the series, see the introduction post.

Posted by Sergei Glazunov, Project Zero

Introduction

As we continue the series on the watering hole attack discovered in early 2020, in this post we’ll look at the rest of the exploits used by the actor against Chrome. A timeline chart depicting the extracted exploits and affected browser versions is provided below. Different color shades represent different exploit versions.

All vulnerabilities used by the attacker are in V8, Chrome’s JavaScript engine; and more specifically, they are JIT compiler bugs. While classic C++ memory safety issues are still exploited in real-world attacks against web browsers, vulnerabilities in JIT offer many advantages to attackers. First, they usually provide more powerful primitives that can be easily turned into a reliable exploit without the need of a separate issue to, for example, break ASLR. Secondly, the majority of them are almost interchangeable, which significantly accelerates exploit development. Finally, bugs from this class allow the attacker to take advantage of a browser feature called web workers. Web developers use workers to execute additional tasks in a separate JavaScript environment. The fact that every worker runs in its own thread and has its own V8 heap makes exploitation significantly more predictable and stable.

The bugs themselves aren’t novel. In fact, three out of four issues have been independently discovered by external security researchers and reported to Chrome, and two of the reports even provided a full renderer exploit. While writing this post, we were more interested in learning about exploitation techniques and getting insight into a high-tier attacker’s exploit development process.

1. CVE-2017-5070

The vulnerability

This is an issue in Crankshaft, the JIT engine Chrome used before TurboFan. The alias analyzer, which is used by several optimization passes to determine whether two nodes may refer to the same object, produces incorrect results when one of the two nodes is a constant. Consider the following code, which has been extracted from one of the exploits:

The first line of the trigger function makes Crankshaft perform a map check on global_array (a map in V8 describes the “shape” of an object and includes the element representation information). The next line may trigger the double -> tagged element representation transition for local_array. Since the compiler incorrectly assumes that local_array and global_array can’t point to the same object, it doesn’t invalidate the recorded map state of global_array and, consequently, eliminates the “redundant” map check in the last line of the function.

The vulnerability grants an attacker a two-way type confusion between a JS object pointer and an unboxed double, which is a powerful primitive and is sufficient for a reliable exploit.

The issue was reported to Chrome by security researcher Qixun Zhao (@S0rryMybad) in May 2017 and fixed in the initial release of Chrome 59. The researcher also provided a renderer exploit. The fix made made the alias analyser use the constant comparison only when both arguments are constants:

Exploit 1

The earliest exploit we’ve discovered targets Chrome 37-58. This is the widest version range we’ve seen, which covers the period of almost three years. Unlike the rest of the exploits, this one contains a separate constant table for every supported browser build.

The author of the exploit takes a known approach to exploiting type confusions in JavaScript engines, which involves gaining the arbitrary read/write capability as an intermediate step. The exploit employs the issue to implement the addrof and fakeobj primitives. It “constructs” a fake ArrayBuffer object inside a JavaScript string, and uses the above primitives to obtain a reference to the fake object. Because strings in JS are immutable, the backing store pointer field of the fake ArrayBuffer can’t be modified. Instead, it’s set in advance to point to an extra ArrayBuffer, which is actually used for arbitrary memory access. Finally, the exploit follows a pointer chain to locate and overwrite the code of a JIT compiled function, which is stored in a RWX memory region.

The exploit is quite an impressive piece of engineering. For example, it includes a small framework for crafting fake JS objects, which supports assigning fields to real JS objects, fake sub-objects, tagged integers, etc. Since the bug can only be triggered once per JIT-compiled function, every time addrof or fakeobj is called, the exploit dynamically generates a new set of required objects and functions using eval.

The author also made significant efforts to increase the reliability of the exploit: there is a sanity check at every minor step; addrof stores all leaked pointers, and the exploit ensures they are still valid before accessing the fake object; fakeobj creates a giant string to store the crafted object contents so it gets allocated in the large object space, where objects aren’t moved by the garbage collector. And, of course, the exploit runs inside a web worker.

However, despite the efforts, the amount of auxiliary code and complexity of the design make accidental crashes quite probable. Also, the constructed fake buffer object is only well-formed enough to be accepted as an argument to the typed array constructor, but it’s unlikely to survive a GC cycle. Reliability issues are the likely reason for the existence of the second exploit.

Exploit 2

The second exploit for the same vulnerability aims at Chrome 47-58, i.e. a subrange of the previous exploit’s supported version range, and the exploit server always gives preference to the second exploit. The version detection is less strict, and there are just three distinct constant tables: for Chrome 47-49, 50-53 and 54-58.

The general approach is similar, however, the new exploit seems to have been rewritten from scratch with simplicity and conciseness in mind as it’s only half the size of the previous one. addrof is implemented in a way that allows leaking pointers to three objects at a time and only used once, so the dynamic generation of trigger functions is no longer needed. The exploit employs mutable on-heap typed arrays instead of JS strings to store the contents of fake objects; therefore, an extra level of indirection in the form of an additional ArrayBuffer is not required. Another notable change is using a RegExp object for code execution. The possible benefit here is that, unlike a JS function, which needs to be called many times to get JIT-compiled, a regular expression gets translated into native code already in the constructor.

While it’s possible that the exploits were written after the issue had become public, they greatly differ from the public exploit in both the design and implementation details. The attacker has thoroughly investigated the issue, for example, their trigger function is much more straightforward than in the public proof-of-concept.

2. CVE-2020-6418

The vulnerability

This is a side effect modelling issue in TurboFan. The function InferReceiverMapsUnsafe assumes that a JSCreate node can only modify the map of its value output. However, in reality, the node can trigger a property access on the new_target parameter, which is observable to user JavaScript if new_target is a proxy object. Therefore, the attacker can unexpectedly change, for example, the element representation of a JS array and trigger a type confusion similar to the one discussed above:

A call reducer (i.e., an optimizer) for Array.prototype.pop invokes InferReceiverMapsUnsafe, which marks the inference result as reliable meaning that it doesn’t require a runtime check. When the proxy object is passed to the vulnerable function, it triggers the tagged -> double element transition. Then pop takes a double element and interprets it as a tagged pointer value.

Note that the attacker can’t call the array function directly because for the expression array.pop() the compiler would insert an extra map check for the property read, which would be scheduled after the proxy handler had modified the array.

This is the only Chrome vulnerability that was still exploited as a 0-day at the time we discovered the exploit server. The issue was reported to Chrome under the 7-day deadline. The one-line patch modified the vulnerable function to mark the result of the map inference as unreliable whenever it encounters a JSCreate node:

The reader can refer to the blog post published by Exodus Intel for more details on the issue and their version of the exploit.

Exploit 1

This time there’s no embedded list of supported browser versions; the appropriate constants for Chrome 60-63 are determined on the server side.

The exploit takes a rather exotic approach: it only implements a function for the confusion in the double -> tagged direction, i.e. the fakeobj primitive, and takes advantage of a side effect in pop to leak a pointer to the internal hole object. The function pop overwrites the “popped” value with the hole, but due to the same confusion it writes a pointer instead of the special bit pattern for double arrays.

The exploit uses the leaked pointer and fakeobj to implement a data leak primitive that can “survive'' garbage collection. First, it acquires references to two other internal objects, the class_start_position and class_end_position private symbols, owing to the fact that the offset between them and the hole is fixed. Private symbols are special identifiers used by V8 to store hidden properties inside regular JS objects. In particular, the two symbols refer to the start and end substring indices in the script source that represent the body of a class. When JSFunction::ToString is invoked on the class constructor and builds the substring, it performs no bounds checks on the “trustworthy” indices; therefore, the attacker can modify them to leak arbitrary chunks of data in the V8 heap.

The obtained data is scanned for values required to craft a fake typed array: maps, fixed arrays, backing store pointers, etc. This approach allows the attacker to construct a perfectly valid fake object. Since the object is located in a memory region outside the V8 heap, the exploit also has to create a fake MemoryChunk header and marking bitmap to force the garbage collector to skip the crafted objects and, thus, avoid crashes.

Finally, the exploit overwrites the code of a JIT-compiled function with a payload and executes it.

The author has implemented extensive sanity checking. For example, the data leak primitive is reused to verify that the garbage collector hasn’t moved critical objects. In case of a failure, the worker with the exploit gets terminated before it can cause a crash. Quite impressively, even when we manually put GC invocations into critical sections of the exploit, it was still able to exit gracefully most of the time.

The exploit employs an interesting technique to detect whether the trigger function has been JIT-compiled:

During compilation, TurboFan inlines the builtin function Number. This change is reflected in the JS call stack. Therefore, the attacker can scan a stack trace from inside a function that Number invokes to determine the compilation state.

The exploit was broken in Chrome 64 by the change that encapsulated both class body indices in a single internal object. Although the change only affected a minor detail of the exploit and had an obvious workaround, which is discussed below, the actor decided to abandon this 0-day and switch to an exploit for CVE-2019-5782. This observation suggests that the attacker was already aware of the third vulnerability around the time Chrome 64 came out, i.e. it was also used as a 0-day.

Exploit 2

After CVE-2019-5782 became unexploitable, the actor returned to this vulnerability. However, in the meantime, another commit landed in Chrome that stopped TurboFan from trying to optimize builtins invoked via Function.prototype.call or similar functions. Therefore, the trigger function had to be updated:

By making the result of Reflect.construct an argument to the pop call, the attacker can move the corresponding JSCreate node after the map check induced by the property load.

The new exploit also has a modified data leak primitive. First, the attacker no longer relies on the side effect in pop to get an address on the heap and reuses the type confusion to implement the addrof function. Because the exploit doesn’t have a reference to the hole, it obtains the address of the builtin asyncIterator symbol instead, which is accessible to user scripts and also stored next to the desired class_positions private symbol.

The exploit can’t modify the class body indices directly as they’re not regular properties of the object referenced by class_positions. However, it can replace the entire object, so it generates an extra class with a much longer constructor string and uses it as a donor.

This version targets Chrome 68-72. It was broken by the commit that enabled the W^X protection for JIT regions. Again, given that there are still similar RWX mappings in the renderer related to WebAssembly, the exploit could have been easily fixed. The attacker, nevertheless, decided to focus on an exploit for CVE-2019-13764 instead.

Exploit 3 & 4

The actor returned once again to this vulnerability after CVE-2019-13764 got fixed. The new exploit bypasses the W^X protection by replacing a JIT-compiled JS function with a WebAssembly function as the overwrite target for code execution. That’s the only significant change made by the author.

Exploit 3 is the only one we’ve discovered on the Windows server, and Exploit 4 is essentially the same exploit adapted for Android. Interestingly, it only appeared on the Android server after the fix for the vulnerability came out. A significant amount of number and string literals got updated, and the pop call in the trigger function was replaced with a shift call. The actor likely attempted to avoid signature-based detection with those changes.

The exploits were used against Chrome 78-79 on Windows and 78-80 on Android until the vulnerability finally got patched.

The public exploit presented by Exodus Intel takes a completely different approach and abuses the fact that double and tagged pointer elements differ in size. When the same bug is applied against the function Array.prototype.push, the backing store offset for the new element is calculated incorrectly and, therefore, arbitrary data gets written past the end of the array. In this case the attacker doesn’t have to craft fake objects to achieve arbitrary read/write, which greatly simplifies the exploit. However, on 64-bit systems, this approach can only be used starting from Chrome 80, i.e. the version that introduced the pointer compression feature. While Chrome still runs in the 32-bit mode on Android in order to reduce memory overhead, user agent checks found in the exploits indicate that the actor also targeted (possibly 64-bit) webview processes.

3. CVE-2019-5782

The vulnerability

CVE-2019-5782 is an issue in TurboFan’s typer module. During compilation, the typer infers the possible type of every node in a function graph using a set of rules imposed by the language. Subsequent optimization passes rely on this information and can, for example, eliminate a security-critical check when the predicted type suggests the check would be redundant. A mismatch between the inferred type and actual value can, therefore, lead to security issues.

Note that in this context, the notion of type is quite different from, for example, C++ types. A TurboFan type can be represented by a range of numbers or even a specific value. For more information on typer bugs please refer to the previous post.

In this case an incorrect type is produced for the expression arguments.length, i.e. the number of arguments passed to a given function. The compiler assigns it the integer range [0; 65534], which is valid for a regular call; however, the same limit is not enforced for Function.prototype.apply. The mismatch was abused by the attacker to eliminate a bounds check and access data past the end of the array:

Qixun Zhao used the same vulnerability in Tianfu Cup and reported it to Chrome in November 2018. The public report includes a renderer exploit. The fix, which landed in Chrome 72, simply relaxed the range of the length property.

The exploit

The discovered exploit targets Chrome 63-67. The exploit flow is a bit unconventional as it doesn’t rely on typed arrays to gain arbitrary read/write. The attacker makes use of the fact that V8 allocates objects in the new space linearly to precompute inter-object offsets. The vulnerability is only triggered once to corrupt the length property of a tagged pointer array. The corrupted array can then be used repeatedly to overwrite the elements field of an unboxed double array with an arbitrary JS object, which gives the attacker raw access to the contents of that object. It’s worth noting that this approach doesn’t even require performing manual pointer arithmetic. As usual, the exploit finishes by overwriting the code of a JS function with the payload.

Interestingly, this is the only exploit that doesn’t take advantage of running inside a web worker even though the vulnerability is fully compatible. Also, the amount of error checking is significantly smaller than in the previous exploits. The author probably assumed that the exploitation primitive provided by the issue was so reliable that all additional safety measures became unnecessary. Nevertheless, during our testing, we did occasionally encounter crashes when one of the allocations that the exploit makes managed to trigger garbage collection. That said, such crashes were indeed quite rare.

As the reader may have noticed, the exploit had stopped working long before the issue was fixed. The reason is that one of the hardening patches against speculative side-channel attacks in V8 broke the bounds check elimination technique used by the exploit. The protection was soon turned off for desktop platforms and replaced with site isolation; hence, the public exploit, which employs the same technique, was successfully used against Chrome 70 on Windows during the competition.

The public and private exploits have little in common apart from the bug itself and BCE technique, which has been commonly known since at least 2017. The public exploit turns out-of-bounds access into a type confusion and then follows the older approach, which involves crafting a fake array buffer object, to achieve code execution.

4. CVE-2019-13764

This more complex typer issue occurs when TurboFan doesn’t reflect the possible NaN value in the type of an induction variable. The bug can be triggered by the following code:

This vulnerability and exploit for Chrome 73-79 have been discussed in detail in the previous blog post. There’s also an earlier version of the exploit targeting Chrome 69-72; the only difference is that the newer version switched from a JS JIT function to a WASM function as the overwrite target.

The comparison with the exploit for the previous typer issue (CVE-2019-5782) is more interesting, though. The developer put much greater emphasis on stability of the new exploit even though the two vulnerabilities are identical in this regard. The web worker wrapper is back, and the exploit doesn’t corrupt tagged element arrays to avoid GC crashes. Also, it no longer relies completely on precomputed offsets between objects in the new space. For example, to leak a pointer to a JS object the attacker puts it between marker values and then scans the memory for the matching pattern. Finally, the number of sanity checks is increased again.

It’s also worth noting that the new typer bug exploitation technique worked against Chrome on Android despite the side-channel attack mitigation and could have “revived” the exploit for CVE-2019-5782.

Conclusion

The timeline data and incremental changes between different exploit versions suggest that at least three out of the four vulnerabilities (CVE-2020-6418, CVE-2019-5782 and CVE-2019-13764) have been used as 0-days.

It is no secret that exploit reliability is a priority for high-tier attackers, but our findings  demonstrate the amount of resources the attackers are willing to spend on making their exploits extra reliable, especially the evidence that the actor has switched from an already high-quality 0-day to a slightly better vulnerability twice.

The area of JIT engine security has received great attention from the wider security community over the last few years. In 2015, when Chrome 37 came out, the exploit for CVE-2017-5070 would be considered quite ahead of its time. In contrast, if we don’t take into account the stability aspect, the exploit for the latest typer issue is not very different from exploits that enthusiasts made for JavaScript challenges at CTF competitions in 2019. This attention also likely affects the average lifetime of a JIT vulnerability and, therefore, may force attackers to move to different bug classes in the future.

This is part 3 of a 6-part series detailing a set of vulnerabilities found by Project Zero being exploited in the wild. To continue reading, see In The Wild Part 4: Android Exploits.