Lightweight DISA STIG Scanner

A simple and fast Python script to scan configurations for US Government Security Technical Implementation Guidance (STIG) compliance. The tool works in an offline mode using an extensible framework of YAML rulesets for each vulnerability of interest.

Contact information:
Email: njrusmc@gmail.com
Twitter: @nickrusso42518

• Supported platforms
• Usage
• Operation
• Testing
• FAQ

Supported platforms

Any platform that has a text-based configuration suited for matching by regex can be used. The examples in this repository are all based on Cisco network devices. The support structures are in place for other operating systems as well.

At the time of this writing, Cisco IOS and NXOS configurations are supported. Cisco IOS-XR and ASA will be supported in the near future.

Usage

usage: stig.py [-h] [-v {0,1,2}] [-f] config_file

A config_file is a relative path to the configuration file to scan, for example configs/l3pr.cfg. These files do not have to be in git but could be if they are being used as golden templates. This argument is required.

The -v or --verbosity argument determines the output style:

• 0: One line per rule showing the vuln ID, description, and result
• 1: Verbose output showing all rule info, including pass/fail objects
• 2: CSV format, one rule per line, including pass/fail objects

This argument is optional and when unspecified, 0 is assumed. See the samples/ folder for example outputs of each style.

The -f or --failonly argument enables the user to only print failed (out of compliance) rules. This reduces output and is good for on-demand testing or automated testing where the pass/NA results are not important. This a boolean option and does not take additional parameters. This argument is optional and when unspecified, false is assumed. All test results are printed by default (pass, fail, and NA).

Operation

Each individual rule or sub-rule goes in its own YAML file. Having many small files enables simpler searching, editing, adding, and deleting for the management of the rule set. Note that some rules as written in the STIG specifications may check multiple things. For example V18633 lists many banned tunneling protocols, but it is simpler to break these into separate sub-rule files as shown below. This way, if there are only a few missing protocols, the entire rule does not fail, and provides a more targeted notification for remediation.

# V18633a.yml
---
severity: 2
desc: Deny outdated tunneling protocol IPP 42
check:
  text: deny\s+42\s+any\s+any\s+log
  text_cnt: 1
  parent: ^ip\s+access-list\s+extended\s+ACL_EXTERNAL
  when: true
part_of_stig:
  - l3ps
  - l3pr

# V18633b.yml
---
severity: 2
desc: Deny outdated tunneling protocol IPP 93
check:
  text: deny\s+93\s+any\s+any\s+log
  text_cnt: 1
  parent: ^ip\s+access-list\s+extended\s+ACL_EXTERNAL
  when: true
part_of_stig:
  - l3ps
  - l3pr

Different operating systems will have different CLI syntax for the same features, so separate rulesets are needed per OS. In the rules/ directory, there is a subdirectory for each OS, such as ios, xr, asa, and nxos. Each configuration file must contain a !@#type:type_name directive at the top of the file to indicate what the OS is.

The components of a rule file are described below:

• severity: The category number of 1, 2, or 3. Documentation only.
• desc: Summarized explanation of the rule; be succinct.
• check: Nested dictionary containing the critical parts of the rule
  • text: The regex to search for. Do not quote the string.
  • text_cnt: The number of times to search for text. Often times this is set to 1, but could be greater if the regex is generic and looking for many things (e.g. multiple NTP or AAA servers). To test for a configuration item being totally absent, use 0 (e.g. ensure that ip directed-broadcast appears zero times under each interface).
  • parent: The regex of the parent under which the text regex should be searched. For example, searching for ACL entries under an ACL. Do not quote the string.
  • when: The sibling to text that tests for a regex to be present before looking for text. For example, only check for no ip proxy-arp under an interface if it has an IP address. Set this to true to always look for text. If when is false or the regex fails to match, the item is marked "N/A" versus "PASS" or "FAIL". Do not quote the string.
  • part_of_stig: List of strings that indicate when this rule should be evaluated. This string must match the directive at the top of each configuration file to be included. For example, if a rule is part of l3ps and l3pr, a configuration with either one of these directives will include this rule. The directive string is !@#stig:stig_name. See configs/ for examples.

Testing

A GNU Makefile is used for testing this codebase. There are currently two steps:

• lint: Runs YAML and Python linters, as well as a Python static code analyzer to check fo security flaws.
• run: Runs the STIG tool itself with a variety of input files at all available verbosities to test proper operation. The default input files should have no failures. If any failures do exist, this step fails. Failures can be STIG rule failures or catastrophic unhandled exceptions.

FAQ

Q: Does this tool have the logic to traverse complex dependencies?
A: No. It applies the text regex for each rule based on its position in the configuration, either globally or under a parent regex. For example, embedding blacklist items in an object-group and calling the object-group from an access-list will be counted by this tool unless the user defines the rules appropriately.

Q: Can I add my own rules or change the existing rules?
A: Yes. There is nothing specific about DISA STIGs for this tool, other than some naming conventions (e.g., vuln ID) and design intent. I have included several extra rules in the rules/ directory to illustrate this point. Users are encouraged to update the rules to fit their specific environment; this is not a static, click-button dogmatic tool.

Q: Can configurations be part of more than one STIG?
A: Yes. Use the !@#stig:stig_name directive at the top of the file as many times as necessary. Ensure the corresponding rules have this string in their part_of_stig YAML list.