move fast and fix things

Last year, we wrote about why a memory safety strategy that focuses on vulnerability prevention in new code quickly yields durable and compounding gains. This year we look at how this approach isn’t just fixing things, but helping us move faster.

The 2025 data continues to validate the approach, with memory safety vulnerabilities falling below 20% of total vulnerabilities for the first time.

Updated data for 2025. This data covers first-party and third-party (open source) code changes to the Android platform across C, C++, Java, Kotlin, and Rust. This post is published a couple of months before the end of 2025, but Android’s industry-standard 90-day patch window means that these results are very likely close to final. We can and will accelerate patching when necessary.

We adopted Rust for its security and are seeing a 1000x reduction in memory safety vulnerability density compared to Android’s C and C++ code. But the biggest surprise was Rust’s impact on software delivery. With Rust changes having a 4x lower rollback rate and spending 25% less time in code review, the safer path is now also the faster one.

In this post, we dig into the data behind this shift and also cover:

• How we’re expanding our reach: We’re pushing to make secure code the default across our entire software stack. We have updates on Rust adoption in first-party apps, the Linux kernel, and firmware.
• Our first rust memory safety vulnerability…almost: We’ll analyze a near-miss memory safety bug in unsafe Rust: how it happened, how it was mitigated, and steps we’re taking to prevent recurrence. It’s also a good chance to answer the question “if Rust can have memory safety issues, why bother at all?”

Developing an operating system requires the low-level control and predictability of systems programming languages like C, C++, and Rust. While Java and Kotlin are important for Android platform development, their role is complementary to the systems languages rather than interchangeable. We introduced Rust into Android as a direct alternative to C and C++, offering a similar level of control but without many of their risks. We focus this analysis on new and actively developed code because our data shows this to be an effective approach.

When we look at development in systems languages (excluding Java and Kotlin), two trends emerge: a steep rise in Rust usage and a slower but steady decline in new C++.

Net lines of code added: Rust vs. C++, first-party Android code.
This chart focuses on first-party (Google-developed) code (unlike the previous chart that included all first-party and third-party code in Android.) We only include systems languages, C/C++ (which is primarily C++), and Rust.

The chart shows that the volume of new Rust code now rivals that of C++, enabling reliable comparisons of software development process metrics. To measure this, we use the DORA¹ framework, a decade-long research program that has become the industry standard for evaluating software engineering team performance. DORA metrics focus on:

• Throughput: the velocity of delivering software changes.
• Stability: the quality of those changes.

Cross-language comparisons can be challenging. We use several techniques to ensure the comparisons are reliable.

• Similar sized changes: Rust and C++ have similar functionality density, though Rust is slightly denser. This difference favors C++, but the comparison is still valid. We use Gerrit’s change size definitions.
• Similar developer pools: We only consider first-party changes from Android platform developers. Most are software engineers at Google, and there is considerable overlap between pools with many contributing in both.
• Track trends over time: As Rust adoption increases, are metrics changing steadily, accelerating the pace, or reverting to the mean?

Throughput

Code review is a time-consuming and high-latency part of the development process. Reworking code is a primary source of these costly delays. Data shows that Rust code requires fewer revisions. This trend has been consistent since 2023. Rust changes of a similar size need about 20% fewer revisions than their C++ counterparts.

In addition, Rust changes currently spend about 25% less time in code review compared to C++. We speculate that the significant change in favor of Rust between 2023 and 2024 is due to increased Rust expertise on the Android team.

While less rework and faster code reviews offer modest productivity gains, the most significant improvements are in the stability and quality of the changes.

Stability

Stable and high-quality changes differentiate Rust. DORA uses rollback rate for evaluating change stability. Rust’s rollback rate is very low and continues to decrease, even as its adoption in Android surpasses C++.

For medium and large changes, the rollback rate of Rust changes in Android is ~4x lower than C++. This low rollback rate doesn’t just indicate stability; it actively improves overall development throughput. Rollbacks are highly disruptive to productivity, introducing organizational friction and mobilizing resources far beyond the developer who submitted the faulty change. Rollbacks necessitate rework and more code reviews, can also lead to build respins, postmortems, and blockage of other teams. Resulting postmortems often introduce new safeguards that add even more development overhead.

In a self-reported survey from 2022, Google software engineers reported that Rust is both easier to review and more likely to be correct. The hard data on rollback rates and review times validates those impressions.

Putting it all together

Historically, security improvements often came at a cost. More security meant more process, slower performance, or delayed features, forcing trade-offs between security and other product goals. The shift to Rust is different: we are significantly improving security and key development efficiency and product stability metrics.

With Rust support now mature for building Android system services and libraries, we are focused on bringing its security and productivity advantages elsewhere.

• Kernel: Android’s 6.12 Linux kernel is our first kernel with Rust support enabled and our first production Rust driver. More exciting projects are underway, such as our ongoing collaboration with Arm and Collabora on a Rust-based kernel-mode GPU driver.
• Firmware: The combination of high privilege, performance constraints, and limited applicability of many security measures makes firmware both high-risk, and challenging to secure. Moving firmware to Rust can yield a major improvement in security. We have been deploying Rust in firmware for years now, and even released tutorials, training, and code for the wider community. We’re particularly excited about our collaboration with Arm on Rusted Firmware-A.
• First-party applications: Rust is ensuring memory safety from the ground up in several security-critical Google applications, such as:
• Nearby Presence: The protocol for securely and privately discovering local devices over Bluetooth is implemented in Rust and is currently running in Google Play Services.
• MLS: The protocol for secure RCS messaging is implemented in Rust and will be included in the Google Messages app in a future release.
• Chromium: Parsers for PNG, JSON, and web fonts have been replaced with memory-safe implementations in Rust, making it easier for Chromium engineers to deal with data from the web while following the Rule of 2.

These examples highlight Rust’s role in reducing security risks, but memory-safe languages are only one part of a comprehensive memory safety strategy. We continue to employ a defense-in-depth approach, the value of which was clearly demonstrated in a recent near-miss.

We recently avoided shipping our very first Rust-based memory safety vulnerability: a linear buffer overflow in CrabbyAVIF. It was a near-miss. To ensure the patch received high priority and was tracked through release channels, we assigned it the identifier CVE-2025-48530. While it’s great that the vulnerability never made it into a public release, the near-miss offers valuable lessons. The following sections highlight key takeaways from our postmortem.

Scudo Hardened Allocator for the Win

A key finding is that Android’s Scudo hardened allocator deterministically rendered this vulnerability non-exploitable due to guard pages surrounding secondary allocations. While Scudo is Android’s default allocator, used on Google Pixel and many other devices, we continue to work with partners to make it mandatory. In the meantime, we will issue CVEs of sufficient severity for vulnerabilities that could be prevented by Scudo.

In addition to protecting against overflows, Scudo’s use of guard pages helped identify this issue by changing an overflow from silent memory corruption into a noisy crash. However, we did discover a gap in our crash reporting: it failed to clearly show that the crash was a result of an overflow, which slowed down triage and response. This has been fixed, and we now have a clear signal when overflows occur into Scudo guard pages.

Unsafe Review and Training

Operating system development requires unsafe code, typically C, C++, or unsafe Rust (for example, for FFI and interacting with hardware), so simply banning unsafe code is not workable. When developers must use unsafe, they should understand how to do so soundly and responsibly

To that end, we are adding a new deep dive on unsafe code to our Comprehensive Rust training. This new module, currently in development, aims to teach developers how to reason about unsafe Rust code, soundness and undefined behavior, as well as best practices like safety comments and encapsulating unsafe code in safe abstractions.

Better understanding of unsafe Rust will lead to even higher quality and more secure code across the open source software ecosystem and within Android. As we’ll discuss in the next section, our unsafe Rust is already really quite safe. It’s exciting to consider just how high the bar can go.

This near-miss inevitably raises the question: “If Rust can have memory safety vulnerabilities, then what’s the point?”

The point is that the density is drastically lower. So much lower that it represents a major shift in security posture. Based on our near-miss, we can make a conservative estimate. With roughly 5 million lines of Rust in the Android platform and one potential memory safety vulnerability found (and fixed pre-release), our estimated vulnerability density for Rust is 0.2 vuln per 1 million lines (MLOC).

Our historical data for C and C++ shows a density of closer to 1,000 memory safety vulnerabilities per MLOC. Our Rust code is currently tracking at a density orders of magnitude lower: a more than 1000x reduction.

Memory safety rightfully receives significant focus because the vulnerability class is uniquely powerful and (historically) highly prevalent. High vulnerability density undermines otherwise solid security design because these flaws can be chained to bypass defenses, including those specifically targeting memory safety exploits. Significantly lowering vulnerability density does not just reduce the number of bugs; it dramatically boosts the effectiveness of our entire security architecture.

The primary security concern regarding Rust generally centers on the approximately 4% of code written within unsafe{} blocks. This subset of Rust has fueled significant speculation, misconceptions, and even theories that unsafe Rust might be more buggy than C. Empirical evidence shows this to be quite wrong.

Our data indicates that even a more conservative assumption, that a line of unsafe Rust is as likely to have a bug as a line of C or C++, significantly overestimates the risk of unsafe Rust. We don’t know for sure why this is the case, but there are likely several contributing factors:

• unsafe{} doesn’t actually disable all or even most of Rust’s safety checks (a common misconception).
• The practice of encapsulation enables local reasoning about safety invariants.
• The additional scrutiny that unsafe{} blocks receive.

Historically, we had to accept a trade-off: mitigating the risks of memory safety defects required substantial investments in static analysis, runtime mitigations, sandboxing, and reactive patching. This approach attempted to move fast and then pick up the pieces afterwards. These layered protections were essential, but they came at a high cost to performance and developer productivity, while still providing insufficient assurance.

While C and C++ will persist, and both software and hardware safety mechanisms remain critical for layered defense, the transition to Rust is a different approach where the more secure path is also demonstrably more efficient. Instead of moving fast and then later fixing the mess, we can move faster while fixing things. And who knows, as our code gets increasingly safe, perhaps we can start to reclaim even more of that performance and productivity that we exchanged for security, all while also improving security.

Thank you to the following individuals for their contributions to this post:

• Ivan Lozano for compiling the detailed postmortem on CVE-2025-48530.
• Chris Ferris for validating the postmortem’s findings and improving Scudo’s crash handling as a result.
• Dmytro Hrybenko for leading the effort to develop training for unsafe Rust and for providing extensive feedback on this post.
• Alex Rebert and Lars Bergstrom for their valuable suggestions and extensive feedback on this post.
• Peter Slatala, Matthew Riley, and Marshall Pierce for providing information on some of the places where Rust is being used in Google’s apps.

Finally, a tremendous thank you to the Android Rust team, and the entire Android organization for your relentless commitment to engineering excellence and continuous improvement.

Notes