chore: import upstream snapshot with attribution

tensorflow/security/advisory/tfsa-2021-116.md

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+## TFSA-2021-116: Heap OOB in `RaggedGather`
+
+### CVE Number
+CVE-2021-37641
+
+### Impact
+If the arguments to `tf.raw_ops.RaggedGather` don't determine a valid ragged
+tensor code can trigger a read from outside of bounds of heap allocated buffers.
+
+```python
+import tensorflow as tf
+
+tf.raw_ops.RaggedGather(
+  params_nested_splits = [0,0,0],
+  params_dense_values = [1,1],
+  indices = [0,0,9,0,0],
+  OUTPUT_RAGGED_RANK=0)
+```
+
+In debug mode, the same code triggers a `CHECK` failure.
+
+The
+[implementation](https://github.com/tensorflow/tensorflow/blob/8d72537c6abf5a44103b57b9c2e22c14f5f49698/tensorflow/core/kernels/ragged_gather_op.cc#L70)
+directly reads the first dimension of a tensor shape before checking that said
+tensor has rank of at least 1 (i.e., it is not a scalar). Furthermore, the
+implementation does not check that the list given by `params_nested_splits` is
+not an empty list of tensors.
+
+### Patches
+We have patched the issue in GitHub commit
+[a2b743f6017d7b97af1fe49087ae15f0ac634373](https://github.com/tensorflow/tensorflow/commit/a2b743f6017d7b97af1fe49087ae15f0ac634373).
+
+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.