🐛 Anchor ALLOWED_FONT_REGEX in a4a head validation#40530
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any update? |
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@madib06ops thanks for the submission. looking this over |
erwinmombay
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glevitzky
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powerivq
July 22, 2026 23:37
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Pull request overview
This PR tightens AMP A4A head sanitization by ensuring stylesheet <link> hrefs are validated as full-string matches against the font-provider allowlist, preventing malicious URLs that merely contain an allowlisted substring from being accepted and preloaded.
Changes:
- Anchors
ALLOWED_FONT_REGEXwith^(?: ... )$to alignRegExp.test()behavior with the validator’s full-match semantics. - Adds a regression test covering a foreign-origin URL that embeds an allowlisted provider substring via a fragment.
Reviewed changes
Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.
| File | Description |
|---|---|
| extensions/amp-a4a/0.1/head-validation.js | Anchors the font stylesheet allowlist regex to prevent substring-based bypasses during head sanitization. |
| extensions/amp-a4a/0.1/test/test-head-validation.js | Adds a regression test ensuring malicious “embedded substring” stylesheet URLs are removed and not preloaded. |
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@banaag could you help me take a look at this. @glevitzky heads up on this change. |
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handleLink gates ad-creative
<link rel=stylesheet>hrefs withALLOWED_FONT_REGEX.test(href), but the regex has no start/end anchors so test() succeeds on any href that merely contains an allowlisted provider, e.g.https://evil.example/x.css#https://fast.fonts.net/, which then survives head sanitization and gets preloaded from the host document. The validator that defines this allowlist (LINK_FONT_STYLESHEET in validator-main.protoascii) matches it with RE2::FullMatch against the whole value, so wrap the alternation in^(?:...)$to match that. Added a regression test for a foreign origin that embeds a provider substring.