XSS on internal: privileged origin through reader mode

How it was found

Summary:

Brave iOS has two weaknesses described below. By combining them, XSS can be achieved on the privileged origin internal://local.

1. Exposure of uuidKey through REFERER header Reader mode in Brave has two HTML templates, Reader.html and ReaderViewLoading.html. The former template defines <meta name="referrer" content="never"> header for preventing referrer leakage, but the latter template does not. Therefore, by opening an external page through ReaderViewLoading.html, the uuidKey contained in the Reader mode page URL is leaked.

2. XSS in SessionRestoreHandler SessionRestoreHandler is used to restore a previously used tab, but it does not validate an URL to be restored. Therefore, if a javascript: URL is provided, the code is executed on the internal: domain.

Note that the first vulnerability is not reproduced on iOS 15 because WKWebView's referrer policy has been changed to hostname only. However, according to Apple's report in June 2021, more than 90% of users were using iOS 14.

Products affected:

• Brave iOS 1.32.3 and higher (include the latest Nightly) on iOS 14.x and below

Steps To Reproduce:

• Visit https://csrf.jp/brave/reader_uuid_leakage.php
• Open the page in Reader mode
• Long tap a hyperlink in the page and choose "Open in New Private Tab"
• Wait for several seconds and tap "Load original page"
• uuidKey in the reader mode URL is stolen through REFERER header
• Click an exploit URL in the page, then XSS is triggered on internal://local

Supporting Material/References:

• xss_on_internal_origin_through_reader_mode.mov: video of the attack against the vulnerabilities
• reader_uuid_leakage.php: server-side exploit code

Impact

• Attacker can elevate privileges to internal: origin