diff --git a/python/packages/core/agent_framework/_workflows/_checkpoint_encoding.py b/python/packages/core/agent_framework/_workflows/_checkpoint_encoding.py
index 3a7d361b61..f6fd376692 100644
--- a/python/packages/core/agent_framework/_workflows/_checkpoint_encoding.py
+++ b/python/packages/core/agent_framework/_workflows/_checkpoint_encoding.py
@@ -403,7 +403,7 @@ def _base64_to_unpickle(encoded: str, *, allowed_types: frozenset[str] | None =
             format is incompatible, or a disallowed type is encountered.
     """
     try:
-        pickled = base64.b64decode(encoded.encode("ascii"))
+        pickled = base64.b64decode(encoded.encode("ascii"), validate=True)
         if allowed_types is not None:
             return _RestrictedUnpickler(pickled, allowed_types).load()
         return pickle.loads(pickled)  # nosec  # ruff:ignore[suspicious-pickle-usage]
diff --git a/python/packages/core/tests/workflow/test_checkpoint_decode.py b/python/packages/core/tests/workflow/test_checkpoint_decode.py
index a8115d17c2..9038410482 100644
--- a/python/packages/core/tests/workflow/test_checkpoint_decode.py
+++ b/python/packages/core/tests/workflow/test_checkpoint_decode.py
@@ -8,6 +8,7 @@
 
 from agent_framework import WorkflowCheckpointException
 from agent_framework._workflows._checkpoint_encoding import (
+    _PICKLE_MARKER,  # type: ignore
     _TYPE_MARKER,  # type: ignore
     decode_checkpoint_value,
     encode_checkpoint_value,
@@ -188,6 +189,16 @@ def test_decode_raises_on_type_mismatch() -> None:
         decode_checkpoint_value(encoded)
 
 
+def test_decode_raises_on_malformed_base64() -> None:
+    """Test that decoding rejects non-Base64 characters in a pickle payload."""
+    encoded = encode_checkpoint_value((1, 2, 3))
+    assert isinstance(encoded, dict)
+    encoded[_PICKLE_MARKER] = cast(str, encoded[_PICKLE_MARKER]) + "!!!!"
+
+    with pytest.raises(WorkflowCheckpointException, match="Failed to decode pickled checkpoint data"):
+        decode_checkpoint_value(encoded)
+
+
 class NotADataclass:  # noqa: B903
     """A regular class that is not a dataclass."""