Reference binding to nullptr in unicode encoding

### Impact An attacker can cause undefined behavior via binding a reference to null pointer in `tf.raw_ops.UnicodeEncode`: ```python import tensorflow as tf from tensorflow.python.ops import gen_string_ops gen_string_ops.unicode_encode( input_values=[], input_splits=[], output_encoding='UTF-8', errors='ignore', replacement_char='a') ``` The [implementation](https://github.com/tensorflow/tensorflow/blob/460e000de3a83278fb00b61a16d161b1964f15f4/tensorflow/core/kernels/unicode_ops.cc#L533-L539) reads the first dimension of the `input_splits` tensor before validating that this tensor is not empty: ```cc const Tensor& input_splits = context->input(1); const auto input_splits_flat = input_splits.flat<SPLITS_TYPE>(); TensorShape output_shape({input_splits.dim_size(0) - 1}); ``` ### Patches We have patched the issue in GitHub commit [2e0ee46f1a47675152d3d865797a18358881d7a6](https://github.com/tensorflow/tensorflow/commit/2e0ee46f1a47675152d3d865797a18358881d7a6). 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.

CVSS 3.1

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H

Severity from

GitHub (reviewed advisory)

Weakness

CWE-824

Also known as

BIT-tensorflow-2021-37667, CVE-2021-37667, PYSEC-2021-289, PYSEC-2021-580, PYSEC-2021-778

• github.com/tensorflow/tensorflow/security/advisories/GHSA-w74j-v8xh-3w5h
• nvd.nist.gov/vuln/detail/CVE-2021-37667
• github.com/tensorflow/tensorflow/commit/2e0ee46f1a47675152d3d865797a18358881d7a6
• github.com/pypa/advisory-database/tree/main/vulns/tensorflow-cpu/PYSEC-2021-580.yaml
• github.com/pypa/advisory-database/tree/main/vulns/tensorflow-gpu/PYSEC-2021-778.yaml
• github.com/pypa/advisory-database/tree/main/vulns/tensorflow/PYSEC-2021-289.yaml
• github.com/tensorflow/tensorflow