diff --git a/src/mistune/plugins/footnotes.py b/src/mistune/plugins/footnotes.py
index 9c9b0c5..8783304 100644
--- a/src/mistune/plugins/footnotes.py
+++ b/src/mistune/plugins/footnotes.py
@@ -38,7 +38,7 @@ def parse_inline_footnote(inline: "InlineParser", m: Match[str], state: "InlineS
         if not indexes:
             indexes = {note_key: index for index, note_key in enumerate(notes)}
             state.env["footnote_indexes"] = indexes
-        if key not in notes:
+        if key not in indexes:
             notes.append(key)
             indexes[key] = len(notes) - 1
             state.env["footnotes"] = notes
diff --git a/tests/test_security_footnotes.py b/tests/test_security_footnotes.py
new file mode 100644
index 0000000..06e999c
--- /dev/null
+++ b/tests/test_security_footnotes.py
@@ -0,0 +1,36 @@
+import re
+from unittest import TestCase
+
+from mistune.core import InlineState
+from mistune.inline_parser import InlineParser
+from mistune.plugins.footnotes import INLINE_FOOTNOTE, parse_inline_footnote
+
+
+class _NoMembershipList(list[str]):
+    def __contains__(self, item: object) -> bool:
+        raise AssertionError("footnote membership must use footnote_indexes")
+
+
+class TestFootnoteSecurity(TestCase):
+    def test_inline_footnote_membership_uses_index_mapping(self):
+        notes = _NoMembershipList(["FIRST"])
+        indexes = {"FIRST": 0}
+        state = InlineState(
+            {
+                "ref_footnotes": {"FIRST": "First note", "SECOND": "Second note"},
+                "footnotes": notes,
+                "footnote_indexes": indexes,
+            }
+        )
+        match = re.match(INLINE_FOOTNOTE, "[^second]")
+        self.assertIsNotNone(match)
+        assert match is not None
+
+        parse_inline_footnote(InlineParser(), match, state)
+
+        self.assertEqual(notes, ["FIRST", "SECOND"])
+        self.assertEqual(indexes, {"FIRST": 0, "SECOND": 1})
+        self.assertEqual(
+            state.tokens,
+            [{"type": "footnote_ref", "raw": "SECOND", "attrs": {"index": 2}}],
+        )