From d39f86845d948a4830a9809cf7b84f43e9193641 Mon Sep 17 00:00:00 2001
From: Victor Tan <victortan@chromium.org>
Date: Wed, 23 Sep 2026 07:49:09 -0700
Subject: [PATCH] Document Client Hints persistence and delegation in content/SECURITY.md

Clarify in content/SECURITY.md that Client Hints (Accept-CH and
Critical-CH) and their persistence are a content-negotiation and
privacy mechanism rather than a process-isolated security boundary.
Also document that Client Hints intentionally propagate main-frame
context and permissions policies to subframes, subresources, and
preloads, so edge cases in Accept-CH persistence or inheritance should
be treated as functional bugs rather than security vulnerabilities.

Bug: 563016452, 563015686, 563015337
Change-Id: I22a8ca6abf715b2d4787bafe3ff97f338287dc8f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8423367
Reviewed-by: Ari Chivukula <arichiv@chromium.org>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Commit-Queue: Victor Tan <victortan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1703424}
---
diff --git a/content/SECURITY.md b/content/SECURITY.md
index 7d52b65..f561ae9 100644
--- a/content/SECURITY.md
+++ b/content/SECURITY.md
@@ -165,6 +165,19 @@
   access to unpartitioned storage and third-party cookies via JavaScript, but
   network requests from the document (or workers) must reflect the revoked
   permission.
+* **Client Hints (`Accept-CH` / `Critical-CH`):** Client Hints and their
+  persistence (`ContentSettingsType::CLIENT_HINTS`) are a content-negotiation
+  and privacy mechanism, not a process-isolated security boundary. High-entropy
+  client hints expose device and user-agent properties that are already
+  accessible to active scripts via JavaScript APIs (e.g.,
+  `navigator.userAgentData.getHighEntropyValues()`), and any site can already
+  trigger `Accept-CH` persistence for a third-party origin via a top-level
+  redirect. Additionally, Client Hints intentionally propagate main-frame
+  context and policies (`main_frame_origin`, `is_outermost_main_frame`, and
+  `PermissionsPolicy`) to subframes, subresources, preloads, and network-service
+  interceptors. Issues where preloading, prefetching, or subresource fetches
+  persist or inherit `Accept-CH` state should be filed as functional bugs rather
+  than security vulnerabilities.
 
 
 ## Mitigating Factors