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Document Permissions Policy as a security mitigation #275

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Document Permissions Policy as a security mitigation
mlmrx Sep 1, 2026
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Address Permissions Policy review feedback
mlmrx Sep 10, 2026
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Expand Up @@ -1804,6 +1804,19 @@ respective private browsing modes are safely exposed to [=agents=] and that thes

<h3 id="mitigations">Mitigations</h3>

<h4 id="mitigation-disable-permissions-policy">Disabling WebMCP with Permissions Policy</h4>

**What:** Disable access to WebMCP APIs in documents where WebMCP is not an intended capability.

**Threats addressed:** Unintended tool registration or invocation by scripts executing in those
documents, including scripts introduced by injection vulnerabilities or compromised dependencies.

**How:** Site authors can deliver a <code>Permissions-Policy: tools=()</code> response header. The
empty allowlist makes the {{tools}} [=policy-controlled feature=] unavailable to the document and its
descendants; see [[#permissions-policy]]. This defense applies to every descendant frame, including
both same-origin and cross-origin ones. The [=user agent=] enforces this before script runs on the
page, protecting against malicious scripts or dependencies from using WebMCP APIs.

<h4 id="mitigation-restrict-input-lengths">Restricting maximum input lengths</h4>

**What:** Restrict the maximum amount of characters
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