https://bugs.gentoo.org/974278
https://codereview.qt-project.org/c/qt/qtsvg/+/732200
--- a/src/svg/qsvgstructure.cpp
+++ b/src/svg/qsvgstructure.cpp
@@ -415,7 +415,8 @@
     const auto markers = markersForNode(node);
     for (auto &i : markers) {
-        QSvgMarker *markNode = static_cast<QSvgMarker*>(node->document()->namedNode(i.markerId));
-        if (!markNode)
+        QSvgNode *referencedNode = node->document()->namedNode(i.markerId);
+        if (!referencedNode || referencedNode->type() != QSvgNode::Marker)
             continue;
+        QSvgMarker *markNode = static_cast<QSvgMarker *>(referencedNode);
 
         p->save();
@@ -722,6 +723,7 @@
     // Chrome seems to return the mask of the mask if a mask is set on the mask
     if (this->hasMask()) {
-        QSvgMask *maskNode = static_cast<QSvgMask*>(document()->namedNode(this->maskId()));
-        if (maskNode) {
+        QSvgNode *referencedNode = document()->namedNode(this->maskId());
+        if (referencedNode && referencedNode->type() == QSvgNode::Mask) {
+            QSvgMask *maskNode = static_cast<QSvgMask *>(referencedNode);
             QRectF boundsRect;
             return maskNode->createMask(p, states, localRect, &boundsRect);
--- a/tests/auto/qsvgrenderer/tst_qsvgrenderer.cpp
+++ b/tests/auto/qsvgrenderer/tst_qsvgrenderer.cpp
@@ -1868,4 +1868,15 @@
     QTest::newRow("excessive moveto in path") // id=406541912
             << R"(<svg><path stroke="#000" d="M- 7e8t9 ."/><marker id="c"/><use href=" c"/></svg>)"_ba;
+    // Bad-cast to QSvgMarker from QSvgLine -> Heap-buffer-overflow
+    QTest::newRow("line-as-marker") // id=496327371
+            << R"-(<svg><line x1="4" id="lledr" marker-end="url(#lledr)" stroke="#00f"/></svg>)-"_ba;
+    QTest::newRow("line-as-mask") // modeled after 496327371 to test similar problem, needs UBSAN
+            << R"-(<svg>
+                     <defs>
+                      <line x1="4" id="line"/>
+                      <mask id="mask" width="2" height="2" mask="url(#line)"/>
+                     </defs>
+                     <rect width="2" height="2" mask="url(#mask)"/>
+                   </svg>)-"_ba;
 }