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web_accessible_resource secret token accessible to webpages #550

konarkmodi opened this issue Apr 30, 2019 · 1 comment


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commented Apr 30, 2019


  • I verified that this is not a filter issue
  • This is not a support issue or a question
  • I performed a cursory search of the issue tracker to avoid opening a duplicate issue
    • Your issue may already be reported.
  • I tried to reproduce the issue when...
    • uBlock Origin is the only extension
    • uBlock Origin with default lists/settings
    • using a new, unmodified browser profile
  • I am running the latest version of uBlock Origin
  • I checked the documentation to understand that the issue I report is not a normal behavior


As per the documentation here:, even the files listed under web_accessible_resources are protected from being accessed by webpages using secret_token.

However, in Chromium based browser, we have found that under special circumstances webpages can steal that token:

  • Revealing user has uBlockOrigin installed.
  • Accessing files under web_accessible_resources.
  • Modifying the content of files added to the page via web_accessible_resources and potentially circumvent the functionality
  • The secret token seems to be generated on browser restart, hence it can be used as a session identifier to track the users across domains.

A specific URL where the issue occurs

Given the bug tracker is open, intentionally keeping from giving more details / PoC.
Is there a way to report security / privacy issues?

  • uBlock Origin version: 1.18.16
  • Browser Name and version: Version 74.0.3729.108 (Official Build) (64-bit)
  • Operating System and version: macOS 10.14.4

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commented Apr 30, 2019

Is there a way to report security / privacy issues?

I sent you an email.

gorhill added a commit to gorhill/uBlock that referenced this issue Apr 30, 2019

Web accessible secrets can be used for at most one second
Related issue:
- uBlockOrigin/uBlock-issues#550

Related Chromium issue (I can't access it):

Findings so far: affects browsers based on Chromium 74.
I could not reproduce the issue with either Chromium 73 or
Google Chrome 75.

This commit is a mitigation: to prevent sites from using
uBO's internal WAR secret for tracking purpose. A secret
can be used for at most one second, after which a new secret
is generated.

The original issue related to the implementation of
secret-gated web accessible resources is:
- #2823

gorhill added a commit to gorhill/uBlock that referenced this issue May 1, 2019

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