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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.

Usage

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

find ~/openstack-repo/keystone -name '*.py' | xargs ./main.py -C 1

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

./main.py examples/*.py -C 3 -lll

Usage:

$ ./main.py -h
usage: main.py [-h] [-C CONTEXT_LINES] [-t TEST_CONFIG] [-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
  -C CONTEXT_LINES, --context CONTEXT_LINES
                        number of context lines to print
  -t TEST_CONFIG, --testconfig TEST_CONFIG
                        test config file (default: bandit.ini)
  -l, --level           results level filter
  -o OUTPUT_FILE, --output OUTPUT_FILE
                        write report to filename
  -d, --debug           turn on debug mode

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 are currently defined in files in the plugins/ directory.

Tests are associated with AST node types based on Bandit configuration file (bandit.ini), and loaded at run time.

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, and identify the AST node type that might best be used to detect that vulnerability.
Check that the Bandit framework already has a node visitor defined for that node type. Look at bandit/node_visitor.py and check that a visit_[typename] function is defined.
Add a new function to the relevant plugins/test_*.py file, that conducts the relevant test and returns an appropriate result. Review existing tests for examples of what this looks like.
Modify bandit.ini to include the new function in tests targeting the node type in question.
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.

The BanditNodeVisitor object provides a 'context' object that the test function can refer to as part of the testing being performed.

References

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/

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

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