fix: Prevent path traversal in nested stack TemplateURL resolution - #4640
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kddejong merged 1 commit intoAug 18, 2026
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Add path confinement checks to _nested_stack_get_atts to prevent reading arbitrary files outside the current working directory when resolving relative TemplateURL paths for nested CloudFormation stacks. Changes: - Block absolute paths specified in TemplateURL - Ensure resolved paths stay within os.getcwd() - Add unit tests for path traversal scenarios This is a defense-in-depth measure. When a path escapes the allowed boundary, cfn-lint gracefully falls back to the default Outputs.* pattern rather than attempting to read the file.
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## main #4640 +/- ##
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+ Coverage 94.58% 94.59% +0.01%
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Files 432 432
Lines 15417 15422 +5
Branches 2986 2988 +2
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+ Hits 14582 14589 +7
+ Misses 456 454 -2
Partials 379 379
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Summary
Add path confinement checks to prevent reading arbitrary files outside the current working directory when resolving relative
TemplateURLpaths for nested CloudFormation stacks.Changes
TemplateURL(e.g.,/etc/passwd)os.getcwd()after normalizationBehavior
When a
TemplateURLpath would resolve outside the current working directory, cfn-lint gracefully falls back to the defaultOutputs.*pattern rather than attempting to read the file. This maintains compatibility while adding defense-in-depth.Allowed:
./nested/child.yaml../sibling/child.yaml(if result is still under cwd)Blocked:
../../../../etc/passwd(escapes cwd)/etc/passwd(absolute path)User Impact
Users running cfn-lint with nested stacks in sibling directories should run from the common parent directory to ensure all templates are within the cwd boundary.