Introducing OSS Rebuild: Open Source, Rebuilt to Last

Posted by Matthew Suozzo, Google Open Source Security Team (GOSST)

Today we're excited to announce OSS Rebuild, a new project to strengthen trust in open source package ecosystems by reproducing upstream artifacts. As supply chain attacks continue to target widely-used dependencies, OSS Rebuild gives security teams powerful data to avoid compromise without burden on upstream maintainers.

The project comprises:

• Automation to derive declarative build definitions for existing PyPI (Python), npm (JS/TS), and Crates.io (Rust) packages.

• SLSA Provenance for thousands of packages across our supported ecosystems, meeting SLSA Build Level 3 requirements with no publisher intervention.

• Build observability and verification tools that security teams can integrate into their existing vulnerability management workflows.

• Infrastructure definitions to allow organizations to easily run their own instances of OSS Rebuild to rebuild, generate, sign, and distribute provenance.

Challenges

Open source software has become the foundation of our digital world. From critical infrastructure to everyday applications, OSS components now account for 77% of modern applications. With an estimated value exceeding $12 trillion, open source software has never been more integral to the global economy.

Yet this very ubiquity makes open source an attractive target: Recent high-profile supply chain attacks have demonstrated sophisticated methods for compromising widely-used packages. Each incident erodes trust in open ecosystems, creating hesitation among both contributors and consumers.

The security community has responded with initiatives like Security Scorecard, pypi's Trusted Publishers, and npm's native SLSA support. However, there is no panacea: Each effort targets a certain aspect of the problem, often making tradeoffs like shifting work onto publishers and maintainers.

Our Aim

Our aim with OSS Rebuild is to empower the security community to deeply understand and control their supply chains by making package consumption as transparent as using a source repository. Our rebuild platform unlocks this transparency by utilizing a declarative build process, build instrumentation, and network monitoring capabilities which, within the SLSA Build framework, produces fine-grained, durable, trustworthy security metadata.

Building on the hosted infrastructure model that we pioneered with OSS Fuzz for memory issue detection, OSS Rebuild similarly seeks to use hosted resources to address security challenges in open source, this time aimed at securing the software supply chain.

Our vision extends beyond any single ecosystem: We are committed to bringing supply chain transparency and security to all open source software development. Our initial support for the PyPI (Python), npm (JS/TS), and Crates.io (Rust) package registries—providing rebuild provenance for many of their most popular packages—is just the beginning of our journey.

How OSS Rebuild Works

Through automation and heuristics, we determine a prospective build definition for a target package and rebuild it. We semantically compare the result with the existing upstream artifact, normalizing each one to remove instabilities that cause bit-for-bit comparisons to fail (e.g. archive compression). Once we reproduce the package, we publish the build definition and outcome via SLSA Provenance. This attestation allows consumers to reliably verify a package's origin within the source history, understand and repeat its build process, and customize the build from a known-functional baseline (or maybe even use it to generate more detailed SBOMs).

With OSS Rebuild's existing automation for PyPI, npm, and Crates.io, most packages obtain protection effortlessly without user or maintainer intervention. Where automation isn't currently able to fully reproduce the package, we offer manual build specification so the whole community benefits from individual contributions.

And we are also excited at the potential for AI to help reproduce packages: Build and release processes are often described in natural language documentation which, while difficult to utilize with discrete logic, is increasingly useful to language models. Our initial experiments have demonstrated the approach's viability in automating exploration and testing, with limited human intervention, even in the most complex builds.

Our Capabilities

OSS Rebuild helps detect several classes of supply chain compromise:

• Unsubmitted Source Code - When published packages contain code not present in the public source repository, OSS Rebuild will not attest to the artifact.

• Real world attack: solana/webjs (2024)

• Build Environment Compromise - By creating standardized, minimal build environments with comprehensive monitoring, OSS Rebuild can detect suspicious build activity or avoid exposure to compromised components altogether.

• Real world attack: tj-actions/changed-files (2025)

• Stealthy Backdoors - Even sophisticated backdoors like xz often exhibit anomalous behavioral patterns during builds. OSS Rebuild's dynamic analysis capabilities can detect unusual execution paths or suspicious operations that are otherwise impractical to identify through manual review.

• Real world attack: xz-utils (2024)

For enterprises and security professionals, OSS Rebuild can...

• Enhance metadata without changing registries by enriching data for upstream packages. No need to maintain custom registries or migrate to a new package ecosystem.

• Augment SBOMs by adding detailed build observability information to existing Software Bills of Materials, creating a more complete security picture.

• Accelerate vulnerability response by providing a path to vendor, patch, and re-host upstream packages using our verifiable build definitions.

For publishers and maintainers of open source packages, OSS Rebuild can...

• Strengthen package trust by providing consumers with independent verification of the packages' build integrity, regardless of the sophistication of the original build.

• Retrofit historical packages' integrity with high-quality build attestations, regardless of whether build attestations were present or supported at the time of publication.

• Reduce CI security-sensitivity allowing publishers to focus on core development work. CI platforms tend to have complex authorization and execution models and by performing separate rebuilds, the CI environment no longer needs to be load-bearing for your packages' security.

Check it out!

The easiest (but not only!) way to access OSS Rebuild attestations is to use the provided Go-based command-line interface. It can be compiled and installed easily:

$ go install github.com/google/oss-rebuild/cmd/oss-rebuild@latest

You can fetch OSS Rebuild's SLSA Provenance:

$ oss-rebuild get cratesio syn 2.0.39

..or explore the rebuilt versions of a particular package:

$ oss-rebuild list pypi absl-py

..or even rebuild the package for yourself:

$ oss-rebuild get npm lodash 4.17.20 --output=dockerfile | \

   docker run $(docker buildx build -q -)

Join Us in Helping Secure Open Source

OSS Rebuild is not just about fixing problems; it's about empowering end-users to make open source ecosystems more secure and transparent through collective action. If you're a developer, enterprise, or security researcher interested in OSS security, we invite you to follow along and get involved!

• Check out the code, share your ideas, and voice your feedback at github.com/google/oss-rebuild.

• Explore the data and contribute to improving support for your critical ecosystems and packages.

• Learn more about SLSA Provenance at slsa.dev

Advancing Protection in Chrome on Android

Posted by David Adrian, Javier Castro & Peter Kotwicz, Chrome Security Team

Android recently announced Advanced Protection, which extends Google’s Advanced Protection Program to a device-level security setting for Android users that need heightened security—such as journalists, elected officials, and public figures. Advanced Protection gives you the ability to activate Google’s strongest security for mobile devices, providing greater peace of mind that you’re better protected against the most sophisticated threats.

Advanced Protection acts as a single control point for at-risk users on Android that enables important security settings across applications, including many of your favorite Google apps, including Chrome. In this post, we’d like to do a deep dive into the Chrome features that are integrated with Advanced Protection, and how enterprises and users outside of Advanced Protection can leverage them.

Android Advanced Protection integrates with Chrome on Android in three main ways:

• Enables the “Always Use Secure Connections” setting for both public and private sites, so that users are protected from attackers reading confidential data or injecting malicious content into insecure plaintext HTTP connections. Insecure HTTP represents less than 1% of page loads for Chrome on Android.
• Enables full Site Isolation on mobile devices with 4GB+ RAM, so that potentially malicious sites are never loaded in the same process as legitimate websites. Desktop Chrome clients already have full Site Isolation.
• Reduces attack surface by disabling Javascript optimizations, so that Chrome has a smaller attack surface and is harder to exploit.

Let’s take a look at all three, learn what they do, and how they can be controlled outside of Advanced Protection.

Always Use Secure Connections

“Always Use Secure Connections” (also known as HTTPS-First Mode in blog posts and HTTPS-Only Mode in the enterprise policy) is a Chrome setting that forces HTTPS wherever possible, and asks for explicit permission from you before connecting to a site insecurely. There may be attackers attempting to interpose on connections on any network, whether that network is a coffee shop, airport, or an Internet backbone. This setting protects users from these attackers reading confidential data and injecting malicious content into otherwise innocuous webpages. This is particularly useful for Advanced Protection users, since in 2023, plaintext HTTP was used as an exploitation vector during the Egyptian election.

Beyond Advanced Protection, we previously posted about how our goal is to eventually enable “Always Use Secure Connections” by default for all Chrome users. As we work towards this goal, in the last two years we have quietly been enabling it in more places beyond Advanced Protection, to help protect more users in risky situations, while limiting the number of warnings users might click through:

• We added a new variant of the setting that only warns on public sites, and doesn’t warn on local networks or single-label hostnames (e.g. 192.168.0.1, shortlink/, 10.0.0.1). These names often cannot be issued a publicly-trusted HTTPS certificate. This variant protects against most threats—accessing a public website insecurely—but still allows for users to access local sites, which may be on a more trusted network, without seeing a warning.
• We’ve automatically enabled “Always Use Secure Connections” for public sites in Incognito Mode for the last year, since Chrome 127 in June 2024.
• We automatically prevent downgrades from HTTPS to plaintext HTTP on sites that Chrome knows you typically access over HTTPS (a heuristic version of the HSTS header), since Chrome 133 in January 2025.

Always Use Secure Connections has two modes—warn on insecure public sites, and warn on any insecure site.

Any user can enable “Always Use Secure Connections” in the Chrome Privacy and Security settings, regardless of if they’re using Advanced Protection. Users can choose if they would like to warn on any insecure site, or only insecure public sites. Enterprises can opt their fleet into either mode, and set exceptions using the HTTPSOnlyMode and HTTPAllowlist policies, respectively. Website operators should protect their users' confidentiality, ensure their content is delivered exactly as they intended, and avoid warnings, by deploying HTTPS.

Full Site Isolation

Site Isolation is a security feature in Chrome that isolates each website into its own rendering OS process. This means that different websites, even if loaded in a single tab of the same browser window, are kept completely separate from each other in memory. This isolation prevents a malicious website from accessing data or code from another website, even if that malicious website manages to exploit a vulnerability in Chrome’s renderer—a second bug to escape the renderer sandbox is required to access other sites. Site isolation improves security, but requires extra memory to have one process per site. Chrome Desktop isolates all sites by default. However, Android is particularly sensitive to memory usage, so for mobile Android form factors, when Advanced Protection is off, Chrome will only isolate a site if a user logs into that site, or if the user submits a form on that site. On Android devices with 4GB+ RAM in Advanced Protection (and on all desktop clients), Chrome will isolate all sites. Full Site Isolation significantly reduces the risk of cross-site data leakage for Advanced Protection users.

JavaScript Optimizations and Security

Advanced Protection reduces the attack surface of Chrome by disabling the higher-level optimizing Javascript compilers inside V8. V8 is Chrome’s high-performance Javascript and WebAssembly engine. The optimizing compilers in V8 make certain websites run faster, however they historically also have been a source of known exploitation of Chrome. Of all the patched security bugs in V8 with known exploitation, disabling the optimizers would have mitigated ~50%. However, the optimizers are why Chrome scores the highest on industry-wide benchmarks such as Speedometer. Disabling the optimizers blocks a large class of exploits, at the cost of causing performance issues for some websites.

Javascript optimizers can be disabled outside of Advanced Protection Mode via the “Javascript optimization & security” Site Setting. The Site Setting also enables users to disable/enable Javascript optimizers on a per-site basis. Disabling these optimizing compilers is not limited to Advanced Protection. Since Chrome 133, we’ve exposed this as a Site Setting that allows users to enable or disable the higher-level optimizing compilers on a per-site basis, as well as change the default.

Settings -> Privacy and Security -> Javascript optimization and security

This setting can be controlled by the DefaultJavaScriptOptimizerSetting enterprise policy, alongside JavaScriptOptimizerAllowedForSites and JavaScriptOptimizerBlockedForSites for managing the allowlist and denylist. Enterprises can use this policy to block access to the optimizer, while still allowlisting¹ the SaaS vendors their employees use on a daily basis. It’s available on Android and desktop platforms

Chrome aims for the default configuration to be secure for all its users, and we’re continuing to raise the bar for V8 security in the default configuration by rolling out the V8 sandbox.

Protecting All Users

Billions of people use Chrome and Android, and not all of them have the same risk profile. Less sophisticated attacks by commodity malware can be very lucrative for attackers when done at scale, but so can sophisticated attacks on targeted users. This means that we cannot expect the security tradeoffs we make for the default configuration of Chrome to be suitable for everyone.

Advanced Protection, and the security settings associated with it, are a way for users with varying risk profiles to tailor Chrome to their security needs, either as an individual at-risk user. Enterprises with a fleet of managed Chrome installations can also enable the underlying settings now. Advanced Protection is available on Android 16 in Chrome 137+.

We additionally recommend at-risk users join the Advanced Protection Program with their Google accounts, which will require the account to use phishing-resistant multi-factor authentication methods and enable Advanced Protection on any of the user’s Android devices. We also recommend users enable automatic updates and always keep their Android phones and web browsers up to date.

Notes

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1. Allowlisting only works on platforms capable of full site isolation—any desktop platform and Android devices with 2GB+ RAM. This is because internally allowlisting is dependent on origin isolation. ↩