diff --git a/Source/WebKit/ChangeLog b/Source/WebKit/ChangeLog
index 5399ee2..aae67e2 100644
--- a/Source/WebKit/ChangeLog
+++ b/Source/WebKit/ChangeLog
@@ -1,3 +1,23 @@
+2020-04-02  David Kilzer  <ddkilzer@apple.com>
+
+        WebPageProxy attachment methods should return early if attachment element is not enabled
+        <https://webkit.org/b/209451>
+        <rdar://problem/60253260>
+
+        Reviewed by Chris Dumez.
+
+        * UIProcess/WebPageProxy.cpp:
+        (WebKit::WebPageProxy::registerAttachmentIdentifierFromData):
+        (WebKit::WebPageProxy::registerAttachmentIdentifierFromFilePath):
+        (WebKit::WebPageProxy::registerAttachmentIdentifier):
+        (WebKit::WebPageProxy::registerAttachmentsFromSerializedData):
+        (WebKit::WebPageProxy::cloneAttachmentData):
+        (WebKit::WebPageProxy::serializedAttachmentDataForIdentifiers):
+        (WebKit::WebPageProxy::didInsertAttachmentWithIdentifier):
+        - Add a message check for the attachment element being enabled.
+        (WebKit::WebPageProxy::didRemoveAttachmentWithIdentifier): Ditto.
+        - Also add a missing message check to validate `identifier`.
+
 2020-04-02  Chris Dumez  <cdumez@apple.com>
 
         [iOS] Replace UIKit background task with a RunningBoard FinishTaskInterruptable assertion
diff --git a/Source/WebKit/UIProcess/WebPageProxy.cpp b/Source/WebKit/UIProcess/WebPageProxy.cpp
index 283ac48..02bc15d 100644
--- a/Source/WebKit/UIProcess/WebPageProxy.cpp
+++ b/Source/WebKit/UIProcess/WebPageProxy.cpp
@@ -9429,6 +9429,7 @@
 
 void WebPageProxy::registerAttachmentIdentifierFromData(const String& identifier, const String& contentType, const String& preferredFileName, const IPC::SharedBufferDataReference& data)
 {
+    MESSAGE_CHECK(m_process, m_preferences->attachmentElementEnabled());
     MESSAGE_CHECK(m_process, IdentifierToAttachmentMap::isValidKey(identifier));
 
     if (attachmentForIdentifier(identifier))
@@ -9443,6 +9444,7 @@
 
 void WebPageProxy::registerAttachmentIdentifierFromFilePath(const String& identifier, const String& contentType, const String& filePath)
 {
+    MESSAGE_CHECK(m_process, m_preferences->attachmentElementEnabled());
     MESSAGE_CHECK(m_process, IdentifierToAttachmentMap::isValidKey(identifier));
 
     if (attachmentForIdentifier(identifier))
@@ -9458,6 +9460,7 @@
 
 void WebPageProxy::registerAttachmentIdentifier(const String& identifier)
 {
+    MESSAGE_CHECK(m_process, m_preferences->attachmentElementEnabled());
     MESSAGE_CHECK(m_process, IdentifierToAttachmentMap::isValidKey(identifier));
 
     if (!attachmentForIdentifier(identifier))
@@ -9466,6 +9469,8 @@
 
 void WebPageProxy::registerAttachmentsFromSerializedData(Vector<WebCore::SerializedAttachmentData>&& data)
 {
+    MESSAGE_CHECK(m_process, m_preferences->attachmentElementEnabled());
+
     for (auto& serializedData : data) {
         auto identifier = WTFMove(serializedData.identifier);
         if (!attachmentForIdentifier(identifier))
@@ -9475,6 +9480,7 @@
 
 void WebPageProxy::cloneAttachmentData(const String& fromIdentifier, const String& toIdentifier)
 {
+    MESSAGE_CHECK(m_process, m_preferences->attachmentElementEnabled());
     MESSAGE_CHECK(m_process, IdentifierToAttachmentMap::isValidKey(fromIdentifier));
     MESSAGE_CHECK(m_process, IdentifierToAttachmentMap::isValidKey(toIdentifier));
 
@@ -9504,6 +9510,9 @@
 void WebPageProxy::serializedAttachmentDataForIdentifiers(const Vector<String>& identifiers, CompletionHandler<void(Vector<WebCore::SerializedAttachmentData>&&)>&& completionHandler)
 {
     Vector<WebCore::SerializedAttachmentData> serializedData;
+
+    MESSAGE_CHECK_COMPLETION(m_process, m_preferences->attachmentElementEnabled(), completionHandler(WTFMove(serializedData)));
+
     for (const auto& identifier : identifiers) {
         auto attachment = attachmentForIdentifier(identifier);
         if (!attachment)
@@ -9550,6 +9559,7 @@
 
 void WebPageProxy::didInsertAttachmentWithIdentifier(const String& identifier, const String& source, bool hasEnclosingImage)
 {
+    MESSAGE_CHECK(m_process, m_preferences->attachmentElementEnabled());
     MESSAGE_CHECK(m_process, IdentifierToAttachmentMap::isValidKey(identifier));
 
     auto attachment = ensureAttachment(identifier);
@@ -9563,6 +9573,9 @@
 
 void WebPageProxy::didRemoveAttachmentWithIdentifier(const String& identifier)
 {
+    MESSAGE_CHECK(m_process, m_preferences->attachmentElementEnabled());
+    MESSAGE_CHECK(m_process, IdentifierToAttachmentMap::isValidKey(identifier));
+
     if (auto attachment = attachmentForIdentifier(identifier))
         didRemoveAttachment(*attachment);
 }