Chrome's multi-process architecture provides many benefits for speed, stability, and security. It allows web pages in unrelated tabs to run in parallel, and it allows users to continue using the browser and other tabs when a renderer process crashes. Because the renderer processes don't require direct access to disk, network, or devices, Chrome can also run them inside a restricted sandbox. This limits the damage that an attacker can cause if he exploits a vulnerability in the renderer, including making it difficult for the attacker to access the user's filesystem or devices, as well as privileged pages (e.g., settings or extensions) and pages in other profiles (e.g., Incognito mode).
However, there is still a large opportunity to use Chrome's sandbox for greater security benefits: isolating web sites from each other. Chrome currently makes an effort to place pages from different web sites in different renderer processes, but due to compatibility constraints, there are many cases in which pages from different sites share a process (e.g., cross-site iframes). In these cases, we rely on the renderer process to enforce the Same Origin Policy and keep web sites isolated from each other.
This page describes our "site isolation" efforts to improve Chrome to use renderer processes as a security boundary between web sites, even in the presence of WebKit vulnerabilities. Our goal is to add support for a "site-per-process" policy that ensures each renderer process contains pages from at most one web site. The browser process can then give each renderer process limited access to cookies and other resources, based on that process's site.
For the "site-per-process" security policy, we assume that an attacker can convince the user to visit a page that exploits a vulnerability in the renderer process, allowing the attacker to run arbitrary code within the sandbox. We consider attackers that want to steal information or abuse privileges granted to other web sites.
Here, we use a precise definition for a site that we use as a principal: a page's site includes the scheme and registered domain name, including the public suffix, but ignoring subdomains, port, or path. We use sites instead of origins to avoid breaking compatibility with existing web pages that might modify their document.domain to communicate across subdomains.
We consider the following threats in scope for the proposed policy:
We do not expect this policy to mitigate traditional cross-site attacks or attacks that occur within the page of a victim site, such as XSS, CSRF, XSSI, or clickjacking.
To support a site-per-process policy in a multi-process web browser, we need to identify the smallest unit that cannot be split across renderer processes. This is not actually a single page, but rather a group of pages from the same web site that have references to each other. Such pages have full script access to each other's content, and they must run on a single thread, not concurrently. These pages may span multiple frames or tabs, and they may come from multiple sub-domains of the same site.
The HTML spec refers to this group as a "unit of related similar-origin browsing contexts." In Chrome, we refer to this as a SiteInstance. All of the pages within a SiteInstance are allowed to script each other, and we must thus render them in the same process.
Note that a single tab might be navigated from one web site to another, and thus it may show different SiteInstances at different times. To support a site-per-process policy, a browser must be able to swap between renderer processes for these navigations.
In addition, top-level pages may contain iframes from different web sites. These iframes have their own security context and must be rendered in a process based on their own site, not the site of their parent frame.
As described on our Process Models page, there are currently several cases in which Chrome will place pages from different sites in the same renderer process. This keeps Chrome compatible with pages that make script calls across windows, at least until the project tasks described below are completed. Some examples of cross-site pages that may share a process:
As a result of this progress, we have adopted a stricter security policy for certain types of pages, including WebUI pages (like the Settings page), extensions, and hosted apps. These pages are never allowed to share a process with regular web pages, even when navigating in a single tab. This is generally acceptable from a compatibility perspective because no scripting is expected between normal pages and WebUI or app pages.
To support a site-per-process policy in Chrome, we need to complete the tasks outlined below. These will ensure that cross-site navigations and script interactions will not break, despite having all pages from different sites in different processes.
For users eager to try site isolation before it is enabled by default, we have added two experimental command-line flags.
First, the --enable-strict-site-isolation flag forces a process swap on all cross-site navigations, even if this might disrupt valid script calls on some web sites. This mode also blocks access to cross-site cookies, which may cause compatibility issues with many web sites. Cross-origin iframes remain in their parent process, but because of cross-site cookie blocking, such pages are unauthenticated. This mode is effectively usable on most web sites today.
Second, the --site-per-process flag starts to enforce the security policy described in this document. It does not block cross-site cookies (to preserve compatibility with existing pages). With this flag, Chrome 34 supports painting and simple input events for a single out-of-process iframe (such as this test page: http://csreis.github.io/tests/cross-site-iframe.html). We recommend starting Chrome with "about:blank" as a command-line argument, since the New Tab Page has multiple iframes that can cause problems. This mode should only be used for experimentation and not normal browsing, as crashes are highly likely.
The mailing list for technical discussions on Site Isolation is email@example.com.