pypdf has possible Infinite Loop when processing outlines/bookmarks

@@ -833,7 +833,10 @@ def outline(self) -> OutlineType:
         return self._get_outline()
 
     def _get_outline(
-        self, node: Optional[DictionaryObject] = None, outline: Optional[Any] = None
+        self,
+        node: Optional[DictionaryObject] = None,
+        outline: Optional[Any] = None,
+        visited: Optional[set[int]] = None,
     ) -> OutlineType:
         if outline is None:
             outline = []
@@ -855,15 +858,29 @@ def _get_outline(
             return outline
 
         # see if there are any more outline items
+        if visited is None:
+            visited = set()
         while True:
+            node_id = id(node)
+            if node_id in visited:
+                logger_warning(f"Detected cycle in outline structure for {node}", __name__)
+                break
+            visited.add(node_id)
+
             outline_obj = self._build_outline_item(node)
             if outline_obj:
                 outline.append(outline_obj)
 
             # check for sub-outline
             if "/First" in node:
                 sub_outline: list[Any] = []
-                self._get_outline(cast(DictionaryObject, node["/First"]), sub_outline)
+                # Pass a copy to allow multiple outer entries to reference the same inner one.
+                inner_visited = visited.copy()
+                self._get_outline(
+                    node=cast(DictionaryObject, node["/First"]),
+                    outline=sub_outline,
+                    visited=inner_visited,
+                )
                 if sub_outline:
                     outline.append(sub_outline)
 

@@ -12,7 +12,15 @@
 
 from pypdf import PdfReader, PdfWriter
 from pypdf.errors import PdfReadError
-from pypdf.generic import EmbeddedFile, NameObject, NullObject, TextStringObject, ViewerPreferences
+from pypdf.generic import (
+    ArrayObject,
+    DictionaryObject,
+    EmbeddedFile,
+    NameObject,
+    NullObject,
+    TextStringObject,
+    ViewerPreferences,
+)
 from tests import get_data_from_url
 
 TESTS_ROOT = Path(__file__).parent.resolve()
@@ -466,3 +474,84 @@ def test_flatten__cyclic_references():
 
     with pytest.raises(expected_exception=PdfReadError, match=r"^Detected cyclic page references\.$"):
         reader._flatten()
+
+
+@pytest.mark.enable_socket
+@pytest.mark.timeout(10)
+def test_get_outline__cyclic_references(caplog):
+    url = "https://github.com/user-attachments/files/24859044/circular_outline.pdf"
+    name = "circular_outline.pdf"
+    reader = PdfReader(BytesIO(get_data_from_url(url=url, name=name)))
+
+    assert reader.outline == [
+        {
+            "/%is_open%": True,
+            "/Page": reader.pages[0].indirect_reference,
+            "/Title": "Bookmark A",
+            "/Type": "/Fit"
+        },
+        {
+            "/%is_open%": True,
+            "/Page": reader.pages[0].indirect_reference,
+            "/Title": "Bookmark B",
+            "/Type": "/Fit"
+        }
+    ]
+    assert caplog.messages[0].startswith("Detected cycle in outline structure for {")
+
+
+@pytest.mark.enable_socket
+@pytest.mark.timeout(10)
+def test_get_outline__cyclic_references__nested_handling(caplog):
+    url = "https://github.com/user-attachments/files/24859044/circular_outline.pdf"
+    name = "circular_outline.pdf"
+    writer = PdfWriter(clone_from=BytesIO(get_data_from_url(url=url, name=name)))
+
+    nested_outline = DictionaryObject()
+    writer._add_object(nested_outline)
+    nested_outline.update({
+        NameObject("/Title"): TextStringObject("Nested entry"),
+        NameObject("/Parent"): writer.get_object(5),
+        NameObject("/Dest"): ArrayObject([writer.pages[0].indirect_reference, NameObject("/Fit")]),
+        NameObject("/Next"): writer.get_object(6),
+    })
+    writer.get_object(5)[NameObject("/First")] = nested_outline.indirect_reference
+    writer.get_object(6)[NameObject("/First")] = nested_outline.indirect_reference
+
+    assert writer.outline == [
+        {
+            "/%is_open%": True,
+            "/Page": writer.pages[0].indirect_reference,
+            "/Title": "Bookmark A",
+            "/Type": "/Fit"
+        },
+        [
+            {
+                "/%is_open%": True,
+                "/Page": writer.pages[0].indirect_reference,
+                "/Title": "Nested entry",
+                "/Type": "/Fit"
+            },
+            {
+                "/%is_open%": True,
+                "/Page": writer.pages[0].indirect_reference,
+                "/Title": "Bookmark B",
+                "/Type": "/Fit"
+            }
+        ],
+        {
+            "/%is_open%": True,
+            "/Page": writer.pages[0].indirect_reference,
+            "/Title": "Bookmark B",
+            "/Type": "/Fit"
+        },
+        [
+            {
+                "/%is_open%": True,
+                "/Page": writer.pages[0].indirect_reference,
+                "/Title": "Nested entry",
+                "/Type": "/Fit"
+            }
+        ]
+    ]
+    assert caplog.messages[0].startswith("Detected cycle in outline structure for {")