Changed the loading behavior of resource:// URI since Nightly 57

[Chung-Sheng Fu]

Hi everyone,


== Background ==

Firefox and add-ons use the resource:// scheme to load resources
internally, but some of the information is available to sites the browser
connects to as well.

This means a web page can run internal scripts and inspect internal
resources of Firefox Browser, including the default preferences, which
could be a serious security and privacy issue.

== Threats ==

For example, a script on Browserleaks <https://www.browserleaks.com/firefox>
highlights what Firefox reveals when queried by a simple script running on
the site (you can find the code in https://browserleaks.com/firefox#more).

The file firefox.js passes preference names and values to the pref()
function.

Example:
http://searchfox.org/mozilla-central/rev/48ea452803907f2575d81021e8678634e8067fc2/browser/app/profile/firefox.js#575

Web sites can easily collect Firefox default preferences by overriding this
pref() function and using the script
“resource:///defaults/preferences/firefox.js”.

Furthermore, some default values of preferences differ between build
configurations, such as platform and locale, which means web sites could
identify individual users using this information.

== Solution ==

In order to fix these issues, we changed the behavior of loading
resource:// URIs in bug 863246
<https://bugzilla.mozilla.org/show_bug.cgi?id=863246>, which has been
landed in NIghtly 57. Now, web content is not able to access resource://
URIs by default, unless the resource:// URI is declared
contentaccessible=yes in the manifests.

For Mozilla developers who need to load resource:// URIs in the web
content, here are some tips.

1.

Simple resource files: add them to CONTENT_ACCESSIBLE_FILES in moz.build
<http://searchfox.org/mozilla-central/rev/51b3d67a5ec1758bd2fe7d7b6e75ad6b6b5da223/layout/style/moz.build#305>
and they will be located in resource://content-accessible/. Currently we
have these files moved:
1.

resouce://gre/res/ImageDocument.css =>
resource://content-accessible/ImageDocument.css
2. resource://gre/res/TopLevelImageDocument.css =>
resource://content-accessible/TopLevelImageDocument.css
3. resource://gre/res/TopLevelVideoDocument.css =>
resource://content-accessible/TopLevelVideoDocument.css
4. resource://gre-resources/viewsource.css =>
resource://content-accessible/viewsource.css
2.

Folders:
1.

Move your folder to resource://content-accessible/. If not
applicable,
2.

Add the contentaccessble=yes flag in jar.mn where you define the URI
mapping, e.g., about:newtab
<http://searchfox.org/mozilla-central/rev/51b3d67a5ec1758bd2fe7d7b6e75ad6b6b5da223/browser/extensions/onboarding/jar.mn#8>
and jsonview
<http://searchfox.org/mozilla-central/rev/51b3d67a5ec1758bd2fe7d7b6e75ad6b6b5da223/devtools/shared/jar.mn#7-8>
.


== Follow-up ==

If there is anything which was impacted by this change and not caught by
us, or you are not sure how to deal with resource:// URIs in your case,
please file a bug and set it as depending on bug 863246
<https://bugzilla.mozilla.org/show_bug.cgi?id=863246>. We will try the
best to resolve the compatibility issue.


Best regards,

Chung-Sheng Fu

[Ethan Tseng]

It means there is no need to use the "No Resource URI Leak" add-on [1]
anymore.


[1] No Resource URI Leak:
Deny resource:// access to Web content: We fill the hole to defend against
fingerprinting.
Very important to Firefox privacy. A direct workaround for bugzil.la/863246
https://addons.mozilla.org/en-US/firefox/addon/no-resource-uri-leak/

--
Ethan Tseng
Engineering Manager, Mozilla

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