Bandit

A Python AST-based static analyzer from OpenStack Security Group.

Overview

Bandit provides a framework for performing analysis against Python source code, utilizing the ast module from the Python standard library.

The ast module is used to convert source code into a parsed tree of Python syntax nodes. Bandit allows users to define custom tests that are performed against those nodes. At the completion of testing, a report is generated that lists security issues identified within the target source code.

Installation

Bandit is distributed as an installable package. To clone and install in a Python virtual environment:

$ git clone https://git.openstack.org/stackforge/bandit.git
$ cd bandit
$ virtualenv venv
$ source venv/bin/activate
$ python setup.py install

To test the new installation:

$ pip install tox
$ tox -epy27

Usage

Example usage across a code tree, showing one line of context for each issue:

$ find ~/openstack-repo/keystone -name '*.py' | xargs bandit -n 1

Example usage across the examples/ directory, showing three lines of context and only reporting on the high-severity issues:

$ bandit examples/*.py -n 3 -lll

Example usage across the examples/ directory, showing one line of context and running only tests in the ShellInjection profile:

$ bandit examples/*.py -n 1 -p ShellInjection

Usage:

$ bandit -h
usage: bandit [-h] [-a AGG_TYPE] [-n CONTEXT_LINES] [-c CONFIG_FILE]
              [-p PROFILE] [-l] [-o OUTPUT_FILE] [-d]
              file [file ...]

Bandit - a Python source code analyzer.

positional arguments:
  file                  source file/s to be tested

optional arguments:
  -h, --help            show this help message and exit
  -a AGG_TYPE, --aggregate AGG_TYPE
                        group results by (vuln)erability type or (file) it
                        occurs in
  -n CONTEXT_LINES, --number CONTEXT_LINES
                        number of context lines to print
  -c CONFIG_FILE, --configfile CONFIG_FILE
                        test config file (default: bandit.yaml)
  -p PROFILE, --profile PROFILE
                        test set profile in config to use (defaults to all
                        tests)
  -l, --level           results level filter
  -o OUTPUT_FILE, --output OUTPUT_FILE
                        write report to filename
  -d, --debug           turn on debug mode

Configuration

The default configuration file is bandit.yaml. This specifies a number of global options, and allows the creation of separate test profiles to include or exclude specific tests when Bandit is run.

Additional configuration files can be created and passed to Bandit as a command line argument.

Exclusions

In the event that a line of code triggers a Bandit issue, but that the line has been reviewed and the issue is a false positive or acceptable for some other reason, the line can be marked with a '# nosec' and any results associated with it will not be reported.

For example, although this line may cause Bandit to report a potential security issue, it will not be reported:

self.process = subprocess.Popen('/bin/echo', shell=True)  # nosec

Vulnerability Tests

Vulnerability tests or 'plugins' are defined in files in the plugins directory.

Tests are written in Python and are autodiscovered from the plugins directory. Each test can examine one or more type of Python statements. Tests are marked with the types of Python statements they examine (for example: function call, string, import, etc).

Tests are executed by the BanditNodeVisitor object as it visits each node in the AST.

Test results are maintained in the BanditResultStore and aggregated for output at the completion of a test run.

Writing Tests

To write a test:

• Identify a vulnerability to build a test for, and create a new file in examples/ that contains one or more cases of that vulnerability.
• Consider the vulnerability you're testing for, mark the function with one or more of the appropriate decorators:
  • @checks_functions
  • @checks_imports
  • @checks_strings
• Create a new Python source file to contain your test, you can reference existing tests for examples.
• The function that you create should take a parameter "context" which is an instance of the context class you can query for information about the current element being examined. You can also get the raw AST node for more advanced use cases. Please see the context.py file for more.
• Extend your Bandit configuration file as needed to support your new test.
• Execute Bandit against the test file you defined in examples/ and ensure that it detects the vulnerability. Consider variations on how this vulnerability might present itself and extend the example file and the test function accordingly.

References

Bandit wiki: https://wiki.openstack.org/wiki/Security/Projects/Bandit

Python AST module documentation: https://docs.python.org/2/library/ast.html

Green Tree Snakes - the missing Python AST docs: http://greentreesnakes.readthedocs.org/en/latest/

Documentation of the various types of AST nodes that Bandit currently covers or could be extended to cover: http://greentreesnakes.readthedocs.org/en/latest/nodes.html