chore: import upstream snapshot with attribution

tensorflow/security/advisory/tfsa-2021-137.md

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+## TFSA-2021-137: Heap OOB in boosted trees
+
+### CVE Number
+CVE-2021-37664
+
+### Impact
+An attacker can read from outside of bounds of heap allocated data by sending
+specially crafted illegal arguments to
+`BoostedTreesSparseCalculateBestFeatureSplit`:
+
+```python
+import tensorflow as tf
+
+tf.raw_ops.BoostedTreesSparseCalculateBestFeatureSplit(
+  node_id_range=[0,10],
+  stats_summary_indices=[[1, 2, 3, 0x1000000]],
+  stats_summary_values=[1.0],
+  stats_summary_shape=[1,1,1,1],
+  l1=l2=[1.0],
+  tree_complexity=[0.5],
+  min_node_weight=[1.0],
+  logits_dimension=3,
+  split_type='inequality')
+```
+
+The
+[implementation](https://github.com/tensorflow/tensorflow/blob/84d053187cb80d975ef2b9684d4b61981bca0c41/tensorflow/core/kernels/boosted_trees/stats_ops.cc)
+needs to validate that each value in `stats_summary_indices` is in range.
+
+### Patches
+We have patched the issue in GitHub commit
+[e84c975313e8e8e38bb2ea118196369c45c51378](https://github.com/tensorflow/tensorflow/commit/e84c975313e8e8e38bb2ea118196369c45c51378).
+
+The fix will be included in TensorFlow 2.6.0. We will also cherrypick this
+commit on TensorFlow 2.5.1, TensorFlow 2.4.3, and TensorFlow 2.3.4, as these are
+also affected and still in supported range.
+
+### For more information
+Please consult [our security
+guide](https://github.com/tensorflow/tensorflow/blob/master/SECURITY.md) for
+more information regarding the security model and how to contact us with issues
+and questions.
+
+### Attribution
+This vulnerability has been reported by members of the Aivul Team from Qihoo
+360.