CVE-2025-68467

Dark Reader is an accessibility browser extension that makes web pages colors dark. The dynamic dark mode feature of the extension works by analyzing the colors of web pages found in CSS style sheet files. In order to analyze cross-origin style sheets (stored on websites different from the original web page), Dark Reader requests such files via a background worker, ensuring the request is performed with no credentials and that the content type of the response is a CSS file. Prior to Dark Reader 4.9.117, this style content was assigned to an HTML Style Element in order to parse and loop through style declarations, and also stored in page's Session Storage for performance gains. This could allow a website author to request a style sheet from a locally running web server, for example by having a link pointing to http[:]//localhost[:]8080/style[.]css. The brute force of the host name, port and file name would be unlikely due to performance impact, that would cause the browser tab to hang shortly, but it could be possible to request a style sheet if the full URL was known in advance. As per December 18, 2025 there is no known exploit of the issue. The problem has been fixed in version 4.9.117 on December 3, 2025. The style sheets are now parsed using modern Constructed Style Sheets API and the contents of cross-origin style sheets is no longer stored in page's Session Storage. Version 4.9.118 (December 8, 2025) restricts cross-origin requests to localhost aliases, IP addresses, hosts with ports and non-HTTPS resources. The absolute majority of users have received an update 4.1.117 or 4.9.118 automatically within a week. However users must ensure their automatic updates are not blocked and they are using the latest version of the extension by going to chrome://extensions or about:addons pages in browser settings. Users utilizing manual builds must upgrade to version 4.9.118 and above. Developers using darkreader NPM package for their own websites are likely not affected, but must ensure the function passed to setFetchMethod() for performing cross-origin requests works within the intended scope. Developers using custom forks of earlier versions of Dark Reader to build other extensions or integrating into their apps or browsers must ensure they perform cross-origin requests safely and the responses are not accessible outside of the app or extension.

Dark Reader is an accessibility browser extension that makes web pages colors dark. The dynamic dark mode feature of the extension works by analyzing the colors of web pages found in CSS style sheet files. In order to analyze cross-origin style sheets (stored on websites different from the original web page), Dark Reader requests such files via a background worker, ensuring the request is performed with no credentials and that the content type of the response is a CSS file. Prior to Dark Reader 4.9.117, this style content was assigned to an HTML Style Element in order to parse and loop through style declarations, and also stored in page's Session Storage for performance gains. This could allow a website author to request a style sheet from a locally running web server, for example by having a link pointing to http[:]//localhost[:]8080/style[.]css. The brute force of the host name, port and file name would be unlikely due to performance impact, that would cause the browser tab to hang shortly, but it could be possible to request a style sheet if the full URL was known in advance. As per December 18, 2025 there is no known exploit of the issue. The problem has been fixed in version 4.9.117 on December 3, 2025. The style sheets are now parsed using modern Constructed Style Sheets API and the contents of cross-origin style sheets is no longer stored in page's Session Storage. Version 4.9.118 (December 8, 2025) restricts cross-origin requests to localhost aliases, IP addresses, hosts with ports and non-HTTPS resources. The absolute majority of users have received an update 4.1.117 or 4.9.118 automatically within a week. However users must ensure their automatic updates are not blocked and they are using the latest version of the extension by going to chrome://extensions or about:addons pages in browser settings. Users utilizing manual builds must upgrade to version 4.9.118 and above. Developers using darkreader NPM package for their own websites are likely not affected, but must ensure the function passed to setFetchMethod() for performing cross-origin requests works within the intended scope. Developers using custom forks of earlier versions of Dark Reader to build other extensions or integrating into their apps or browsers must ensure they perform cross-origin requests safely and the responses are not accessible outside of the app or extension.

Description

Dark Reader versions prior to 4.9.117 included a behavior where a website could request a style sheet from a locally running web server, for example http://localhost:8080/style.css, If an address was available and returned a text/css content type.

Patches

The problem was fixed in version 4.9.117, released on December 3, 2025. Most users received the update automatically. Users running manual builds must upgrade to version 4.9.117 or later.

The installed extension version number can be verified in Dark Reader's menu (More > All settings > About), browser settings, chrome://extensions or about:addons pages.

Users are encouraged not to disable automatic extension updates and use the latest browser version, as browser releases typically include multiple security fixes of varying severity.

NPM package

The issue does not affect developers using the darkreader NPM package for website integration. Developers using the setFetchMethod() API must ensure the cross-origin requests are restricted to the intended scope.

Custom forks

Developers using custom forks of earlier versions of Dark Reader to build other extensions, or integrating it into their apps or browsers, should review their implementation to ensure cross-origin requests are handled securely.

Acknowledgements

Security research performed by Brian Carpenter - Deep Fork Cyber.

Dark Reader is an accessibility browser extension that makes web pages colors dark. The dynamic dark mode feature of the extension works by analyzing the colors of web pages found in CSS style sheet files. In order to analyze cross-origin style sheets (stored on websites different from the original web page), Dark Reader requests such files via a background worker, ensuring the request is performed with no credentials and that the content type of the response is a CSS file. Prior to Dark Reader 4.9.117, this style content was assigned to an HTML Style Element in order to parse and loop through style declarations, and also stored in page's Session Storage for performance gains. This could allow a website author to request a style sheet from a locally running web server, for example by having a link pointing to http[:]//localhost[:]8080/style[.]css. The brute force of the host name, port and file name would be unlikely due to performance impact, that would cause the browser tab to hang shortly, but it could be possible to request a style sheet if the full URL was known in advance. As per December 18, 2025 there is no known exploit of the issue. The problem has been fixed in version 4.9.117 on December 3, 2025. The style sheets are now parsed using modern Constructed Style Sheets API and the contents of cross-origin style sheets is no longer stored in page's Session Storage. Version 4.9.118 (December 8, 2025) restricts cross-origin requests to localhost aliases, IP addresses, hosts with ports and non-HTTPS resources. The absolute majority of users have received an update 4.1.117 or 4.9.118 automatically within a week. However users must ensure their automatic updates are not blocked and they are using the latest version of the extension by going to chrome://extensions or about:addons pages in browser settings. Users utilizing manual builds must upgrade to version 4.9.118 and above. Developers using darkreader NPM package for their own websites are likely not affected, but must ensure the function passed to setFetchMethod() for performing cross-origin requests works within the intended scope. Developers using custom forks of earlier versions of Dark Reader to build other extensions or integrating into their apps or browsers must ensure they perform cross-origin requests safely and the responses are not accessible outside of the app or extension.

Dark Reader is an accessibility browser extension that makes web pages colors dark. The dynamic dark mode feature of the extension works by analyzing the colors of web pages found in CSS style sheet files. In order to analyze cross-origin style sheets (stored on websites different from the original web page), Dark Reader requests such files via a background worker, ensuring the request is performed with no credentials and that the content type of the response is a CSS file. Prior to Dark Reader 4.9.117, this style content was assigned to an HTML Style Element in order to parse and loop through style declarations, and also stored in page's Session Storage for performance gains. This could allow a website author to request a style sheet from a locally running web server, for example by having a link pointing to http[:]//localhost[:]8080/style[.]css. The brute force of the host name, port and file name would be unlikely due to performance impact, that would cause the browser tab to hang shortly, but it could be possible to request a style sheet if the full URL was known in advance. As per December 18, 2025 there is no known exploit of the issue. The problem has been fixed in version 4.9.117 on December 3, 2025. The style sheets are now parsed using modern Constructed Style Sheets API and the contents of cross-origin style sheets is no longer stored in page's Session Storage. Version 4.9.118 (December 8, 2025) restricts cross-origin requests to localhost aliases, IP addresses, hosts with ports and non-HTTPS resources. The absolute majority of users have received an update 4.1.117 or 4.9.118 automatically within a week. However users must ensure their automatic updates are not blocked and they are using the latest version of the extension by going to chrome://extensions or about:addons pages in browser settings. Users utilizing manual builds must upgrade to version 4.9.118 and above. Developers using darkreader NPM package for their own websites are likely not affected, but must ensure the function passed to setFetchMethod() for performing cross-origin requests works within the intended scope. Developers using custom forks of earlier versions of Dark Reader to build other extensions or integrating into their apps or browsers must ensure they perform cross-origin requests safely and the responses are not accessible outside of the app or extension.

Description

Dark Reader versions prior to 4.9.117 included a behavior where a website could request a style sheet from a locally running web server, for example http://localhost:8080/style.css, If an address was available and returned a text/css content type.

Patches

The problem was fixed in version 4.9.117, released on December 3, 2025. Most users received the update automatically. Users running manual builds must upgrade to version 4.9.117 or later.

The installed extension version number can be verified in Dark Reader's menu (More > All settings > About), browser settings, chrome://extensions or about:addons pages.

Users are encouraged not to disable automatic extension updates and use the latest browser version, as browser releases typically include multiple security fixes of varying severity.

NPM package

The issue does not affect developers using the darkreader NPM package for website integration. Developers using the setFetchMethod() API must ensure the cross-origin requests are restricted to the intended scope.

Custom forks

Developers using custom forks of earlier versions of Dark Reader to build other extensions, or integrating it into their apps or browsers, should review their implementation to ensure cross-origin requests are handled securely.

Acknowledgements

Security research performed by Brian Carpenter - Deep Fork Cyber.