Getting RCE in Chrome with incomplete object initialization in the Maglev compiler
In this post I’ll exploit CVE-2023-4069, a type confusion vulnerability that I reported in July 2023. The vulnerability—which allows remote code execution (RCE) in the renderer sandbox of Chrome by a single visit to a malicious site—is found in v8, the Javascript engine of Chrome. It was filed as bug 1465326 and subsequently fixed in version 115.0.5790.170/.171.
Vulnerabilities like this are often the starting point for a “one-click” exploit, which compromises the victim’s device when they visit a malicious website. What’s more, renderer RCE in Chrome allows an attacker to compromise and execute arbitrary code in the Chrome renderer process. That being said, the renderer process has limited privilege and such a vulnerability needs to be chained with a second “sandbox escape” vulnerability (either another vulnerability in the Chrome browser process or one in the operating system) to compromise Chrome itself or the device.
While many of the most powerful and sophisticated “one-click” attacks are highly targeted, and average users may be more at risk from less sophisticated attacks such as phishing, users should still keep Chrome up-to-date and enable automatic updates, as vulnerabilities in v8 can often be exploited relatively quickly.
The current vulnerability, CVE-2023-4069, exists in the Maglev compiler, a new mid-tier JIT compiler in Chrome that optimizes Javascript functions based on previous knowledge of the input types. This kind of optimization is called speculative optimization and care must be taken to make sure that these assumptions on the inputs are still valid when the optimized code is used. The complexity of the JIT engine has led to many security issues in the past and has been a popular target for attackers.
Maglev compiler
The Maglev compiler is a mid-tier JIT compiler used by v8. Compared to the top-tier JIT compiler, TurboFan, Maglev generates less optimized code but with a faster compilation speed. Having multiple JIT compilers is common in Javascript engines, the idea being that with multiple tier compilers, you’ll find a more optimal tradeoff between compilation time and runtime optimization.
Generally speaking, when a function is first run, slow bytecode is generated, as the function is run more often, it may get compiled into more optimized code, first from a lowest-tier JIT compiler. If the function gets used more often, then its optimization tier gets moved up, resulting in better runtime performance—but at the expense of a longer compilation time. The idea here is that for code that runs often, the runtime cost will likely outweigh the compile time cost. You can consult An Introduction to Speculative Optimization in v8 by Benedikt Meurer for more details of how the compilation process works.
The Maglev compiler is enabled by default starting from version 114 of Chrome. Similar to TurboFan, it goes through the bytecode of a Javascript function, taking into account the feedback that was collected from previous runs, and transforms the bytecode into more optimized code. However, unlike TurboFan, which first transforms bytecodes into a “Sea of Nodes”, Maglev uses an intermediate representation and first transforms bytecodes into SSA (Static Single-Assignment) nodes, which are declared in the file maglev-ir.h. At the time of writing, the compilation process of Maglev consists mainly of two phases of optimizations: the first phase involves building a graph from the SSA nodes, while the second phase consists of optimizing the representations of Phi values.
Object construction in v8
The bug in this post really has more to do with object constructions than with Maglev, so now I’ll go through more details and some concepts of how v8 handles Javascript constructions. A Javascript function can be used as a constructor and called with the new keyword. When it is called with new, the new.target variable exists in the function scope that specifies the function being called with new. In the following case, new.target is the same as the function itself.
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