cfn-nag doesn't fail the pipeline

[iWantToKeepAnon]

It isn't that cfn_nag doesn't return non-Zero; it's your ruby file. Instead of the last line being:

system(command)

It should be something like:

exit(system(command) ? 0 : 1)

[iWantToKeepAnon]

Actually, can't you dismiss with the gemfile stuff altogether and just execute

cfn_nag -r src/custom-rules/lib/rules/ templates/*.yaml

?

[iWantToKeepAnon]

Yes, this works w/o installing as a gem and it fails on errors:

cfn_nag --output-format=txt --rule-directory=custom-rules/lib/rules templates/*.yaml

That cures the output log having escape sequences; much simpler IMO and it works.