Integrating Rust into Chromium's C++ Codebase for Security

This article explores how the Chromium project is integrating the Rust programming language into its massive C++ codebase to eliminate memory safety vulnerabilities. It covers the primary security motivations behind this shift, the technical mechanisms and interoperability tools used to bridge C++ and Rust, and the strategic approach engineers are taking to implement Rust in high-risk browser components without rewriting the entire browser from scratch.

The Security Motivation: Eliminating Memory Bugs

The primary driver for bringing Rust into Chromium—the open-source project behind Google Chrome, Microsoft Edge, and Brave—is memory safety. According to the Chromium security team, approximately 70% of all high-severity security bugs in the browser are memory safety issues. These are mistakes made in C++ regarding how memory is allocated, read, and freed, such as "use-after-free" errors and buffer overflows.

While C++ relies on the developer to manage memory correctly, Rust enforces memory safety at compile time through its ownership and borrowing system. By writing components in Rust, developers can mathematically guarantee the absence of these specific vulnerabilities before the code is ever shipped to users.

Simplifying the Rule of Two

Historically, Chromium’s security architecture has relied on the "Rule of 2." This rule states that code can pick no more than two of the following traits:

  1. It processes untrustworthy inputs.
  2. It is written in an unsafe language (like C or C++).
  3. It runs with high privileges (outside a sandbox).

Because C++ is an unsafe language, any C++ code that processes untrusted web data (like an image or a font) must be isolated in a restricted sandbox process. Sandbox processes are expensive in terms of system memory and performance. Because Rust is a memory-safe language, Chromium engineers can now process untrusted inputs using Rust without always needing a heavy process sandbox, thereby improving both security and browser performance.

Bridging C++ and Rust with Interoperability Tools

Chromium consists of tens of millions of lines of C++ code, making a complete rewrite in Rust impossible. Instead, Rust must coexist and communicate with the existing C++ code. This requires two-way interoperability, allowing C++ to call Rust functions and Rust to call C++ functions.

To achieve this safely, the Chromium team uses specialized Foreign Function Interface (FFI) tools, primarily a library called CXX.

  • How CXX works: Standard FFI requires writing unsafe Rust code, which defeats the purpose of using Rust for security. CXX allows developers to define a shared boundary between the two languages. It automatically generates the necessary bindings and statically checks that the data types on both the C++ side and the Rust side match safely.
  • Build System Integration: The Chromium team has added support for the official Rust compiler (rustc) directly into GN and Ninja, the build systems used to compile Chromium. This allows developers to add Rust libraries (crates) to the project just as easily as they add new C++ files.

Replacing High-Risk Third-Party Libraries

Rather than rewriting core browser architecture, the integration strategy focuses on the most vulnerable parts of the browser: third-party libraries that parse complex data formats. Web browsers must constantly decode fonts, images, audio, and video from unknown sources on the internet, making parsers a prime target for hackers.

Chromium is actively replacing older C and C++ parsing libraries with memory-safe Rust equivalents. Examples include:

  • Font Parsing: Moving away from C-based font processing toward Rust-based alternatives (like Skrifa) to prevent malicious web fonts from executing code on a user's machine.
  • QR Code Generation: Replacing C++ utilities with Rust crates to handle data parsing securely.
  • JSON and Image Parsers: Isolating and swapping out legacy decoders with audited, third-party Rust crates from the open-source ecosystem.

By strategically inserting Rust at the boundaries where the browser interacts with untrusted web content, the Chromium project is systematically reducing its attack surface while maintaining the stability of its foundational C++ codebase.