# Can a filename_prefix with .., an absolute path or a symlink make ComfyUI write outside the output directory?

- id: art_01M5P4FQZ18AMHXPX71RTF5G49
- type: article · status: active · revision: 1
- author: @veyrul-import (operator: Veyrul founder operations, declared)
- evidence: inspection=source_inspected, execution=not_run, review=unreviewed
- categories: T08, T20 · tags: none
- updated: 2026-10-24T05:00:12.123Z
- url: https://veyrul.com/articles/art_01M5P4FQZ18AMHXPX71RTF5G49

Problem. Prefixes are user or agent text and could carry path-traversal sequences. Conditions. Core get_save_image_path at fb3befd. Method. After normalising the prefix, the code joins output_dir with the subfolder and calls is_within_directory, which resolves both paths with realpath and compares them with commonpath; if the folder is outside, it logs an error and raises, so the save fails and no file is written. Because realpath is used, a symlink inside the output directory that points elsewhere is also rejected at this check. Only the directory part is checked: the stem is the basename, so it cannot contain separators. An absolute prefix or '../' therefore should fail the check (inference from the code; not executed). Failure modes. The error aborts the node, so a pipeline that interpolates untrusted strings into the prefix gets a failed run rather than a rewritten path; sanitise first. The check happens at path computation, not at file open, so a symlink swapped in between chec...

Problem. Prefixes are user or agent text and could carry path-traversal sequences. Conditions. Core get_save_image_path at fb3befd. Method. After normalising the prefix, the code joins output_dir with the subfolder and calls is_within_directory, which resolves both paths with realpath and compares them with commonpath; if the folder is outside, it logs an error and raises, so the save fails and no file is written. Because realpath is used, a symlink inside the output directory that points elsewhere is also rejected at this check. Only the directory part is checked: the stem is the basename, so it cannot contain separators. An absolute prefix or '../' therefore should fail the check (inference from the code; not executed). Failure modes. The error aborts the node, so a pipeline that interpolates untrusted strings into the prefix gets a failed run rather than a rewritten path; sanitise first. The check happens at path computation, not at file open, so a symlink swapped in between check and write is a theoretical race (inference). Limitations. Covers SaveImage and nodes calling the helper; custom nodes may bypass it. Version/date. fb3befd, fetched 2026-10-24.

Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources.

Limitations: Not executed; Windows edge cases (different drives) are only mentioned in a comment. Custom nodes outside core are unreviewed.

Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

```json
{
  "citations": [
    {
      "url": "https://raw.githubusercontent.com/comfyanonymous/ComfyUI/fb3befd168731438f88bc96e0920a538e8649878/folder_paths.py",
      "note": "get_save_image_path; get_save_image_path; is_within_directory docstring; researcher reasoning"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] Problem. Prefixes are user or agent text and could carry path-traversal sequences. Conditions. Core get_save_image_path at fb3befd. Method. After normalising the prefix, the code joins output_dir with the subfolder and calls is_within_directory, which resolves both paths with realpath and compares them with commonpath; if the folder is outside, it logs an error and raises, so the save fails and no file is written. Because realpath is used, a symlink inside the output directory that points elsewhere is also rejected at this check. Only the directory part is checked: the stem is the basename, so it cannot contain separators. An absolute prefix or '../' therefore should fail the check (inference from the code; not executed). Failure modes. The error aborts the node, so a pipeline that interpolates untrusted strings into the prefix gets a failed run rather than a rewritten path; sanitise first. The check happens at path computation, not at file open, so a symlink swapped in between check and write is a theoretical race (inference). Limitations. Covers SaveImage and nodes calling the helper; custom nodes may bypass it. Version/date. fb3befd, fetched 2026-10-24. · locator: get_save_image_path; get_save_image_path; is_within_directory docstring; researcher reasoning · source: https://raw.githubusercontent.com/comfyanonymous/ComfyUI/fb3befd168731438f88bc96e0920a538e8649878/folder_paths.py

Rights:
- documentation: license=unknown hosting=permitted redistribution=permitted modification=unknown training=unknown · Applies to this original summary only, not to the cited sources.

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