diff --git a/third_party/blink/renderer/core/xml/parser/xml_document_parser.cc b/third_party/blink/renderer/core/xml/parser/xml_document_parser.cc
index aac93c5b62a19..ee5995d82a737 100644
--- a/third_party/blink/renderer/core/xml/parser/xml_document_parser.cc
+++ b/third_party/blink/renderer/core/xml/parser/xml_document_parser.cc
@@ -752,7 +752,7 @@ static void ErrorFunc(void*, const char*, ...) {
   // FIXME: It would be nice to display error messages somewhere.
 }
 
-static void EnsureLibXMLInitialized() {
+void EnsureLibXMLInitialized() {
   static bool did_init = false;
   if (did_init)
     return;
diff --git a/third_party/blink/renderer/core/xml/parser/xml_document_parser.h b/third_party/blink/renderer/core/xml/parser/xml_document_parser.h
index 8bac86e1cfb04..fad605cd859f0 100644
--- a/third_party/blink/renderer/core/xml/parser/xml_document_parser.h
+++ b/third_party/blink/renderer/core/xml/parser/xml_document_parser.h
@@ -238,6 +238,11 @@ class XMLDocumentParser final : public ScriptableDocumentParser,
   SegmentedString pending_src_;
 };
 
+// Registers Blink's libxml IO callbacks. Must be called before any libxml
+// parse that may load external entities/DTDs, otherwise libxml's default
+// xmlFdOpen()/open() handler is used (XXE / local file read).
+void EnsureLibXMLInitialized();
+
 xmlDocPtr XmlDocPtrForString(Document*,
                              const String& source,
                              const String& url);
diff --git a/third_party/blink/renderer/core/xml/xsl_style_sheet_libxslt.cc b/third_party/blink/renderer/core/xml/xsl_style_sheet_libxslt.cc
index 4bc346b1186f6..ef55f4d6f42c2 100644
--- a/third_party/blink/renderer/core/xml/xsl_style_sheet_libxslt.cc
+++ b/third_party/blink/renderer/core/xml/xsl_style_sheet_libxslt.cc
@@ -29,6 +29,7 @@
 #include "third_party/blink/renderer/core/execution_context/execution_context.h"
 #include "third_party/blink/renderer/core/frame/local_frame.h"
 #include "third_party/blink/renderer/core/loader/resource/xsl_style_sheet_resource.h"
+#include "third_party/blink/renderer/core/xml/parser/xml_document_parser.h"
 #include "third_party/blink/renderer/core/xml/parser/xml_document_parser_scope.h"
 #include "third_party/blink/renderer/core/xml/parser/xml_parser_input.h"
 #include "third_party/blink/renderer/core/xml/xsl_style_sheet.h"
@@ -120,6 +121,12 @@ void XSLStyleSheet::ClearDocuments() {
 }
 
 bool XSLStyleSheet::ParseString(const String& source) {
+  // Ensure Blink's libxml IO callbacks (MatchFunc/OpenFunc) are registered
+  // before parsing. Without this, if ParseString is the first libxml call in
+  // the process, libxml's default xmlFdOpen()/open() handler resolves external
+  // entities directly from the filesystem (XXE).
+  EnsureLibXMLInitialized();
+
   // Parse in a single chunk into an xmlDocPtr
   if (!stylesheet_doc_taken_)
     xmlFreeDoc(stylesheet_doc_);