Chromium Blog
News and developments from the open source browser project
More secure extensions, by default
Wednesday, February 29, 2012
Security is one of our core values, alongside speed, stability and simplicity. From day one, we’ve designed Chrome’s extension system
with security in mind
. Since we launched the extension system, the state of the art in web security has advanced with technologies like
Content-Security-Policy
(CSP). Extension developers have been able to
opt into these features
, and now we’re enabling these security features by default.
Unfortunately, securing extensions with CSP by default is incompatible with the legacy extension system. We’re passionate about extension compatibility, so we’re going to make this change gradually to minimize pain for users and developers.
Users can continue to install extensions that are available in the store regardless of whether they are secured with CSP or not. This means they will not lose any of the functionality they've added to Chrome.
Developers will be able to choose when to enable the new behavior. To ease the transition, we've introduced a new manifest version attribute in the extension manifest in Chrome 18 (currently in beta). When a developer updates his or her extension to use
manifest_version
2, Chrome will enforce the following CSP policy by default:
script-src 'self'; object-src 'self'
This policy imposes the following restrictions on extensions:
Extensions can no longer use inline scripts, such as
<script> ... </script>
. Instead, extensions must use out-of-line scripts loaded from within their package, such as
<script src="foo.js"></script>
.
Extensions can no longer use
eval()
. Note: If you’re using eval to parse JSON today, we suggest using
JSON.parse
instead.
Extensions can load plug-ins, such as SWF files, only from within their package or from a whitelist of HTTPS hosts.
A
recent study
from researchers at UC Berkeley suggested that these restrictions, taken together, would substantially
improve the security
of the extension system:
These defenses are extremely effective: adopting one of the recommended CSPs would prevent 96% (49 out of 51) of the core extension vulnerabilities we found.
For most extensions, updating them to
manifest_version
2 will require the developer to move inline scripts out-of-line and to move scripts loaded from the network into the extension package. Developers are not required to update their extensions to
manifest_version
2 immediately, but, over time, more of the extension ecosystem will encourage developers to update their extensions. For example, at some point, we’ll likely start requiring new extensions uploaded to the web store to use
manifest_version
2. You can find
a complete list
of changes and
more details
about CSP in the
extension documentation
.
We expect these changes will make the security of Chrome’s extension system even better. If you have any feedback, please feel encouraged to email the extension developers
mailing list
.
Posted by Adam Barth, Chrome Security Engineer
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