fix: validateOutputPath symlink bypass — resolve real path before safe-dir check#745
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Gonzih wants to merge 1 commit intogarrytan:mainfrom
Open
fix: validateOutputPath symlink bypass — resolve real path before safe-dir check#745Gonzih wants to merge 1 commit intogarrytan:mainfrom
Gonzih wants to merge 1 commit intogarrytan:mainfrom
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…cking safe dirs path.resolve() normalizes .. components but does NOT follow symlinks. A symlink inside /tmp or cwd pointing outside those dirs would pass the isPathWithin() check while the actual write goes to the symlink target. e.g.: ln -s /etc /tmp/evil; screenshot /tmp/evil/cron.d/payload → path.resolve gives /tmp/evil/cron.d/payload (passes check) → fs.writeFile writes to /etc/cron.d/payload (security fail) Fix: after the lexical check, call fs.realpathSync on the nearest existing ancestor directory and re-validate the real path. Falls back safely up the directory tree if the parent doesn't exist yet. Closes garrytan#665
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The Bug
`validateOutputPath()` calls `path.resolve(filePath)` and then checks if the result is within the safe directories. `path.resolve` normalizes `..` components lexically but does not follow symlinks.
Attack:
```bash
ln -s /etc /tmp/evil
then: screenshot /tmp/evil/cron.d/payload
```
`path.resolve('/tmp/evil/cron.d/payload')` returns `/tmp/evil/cron.d/payload` which passes `isPathWithin(_, '/tmp')`, but the actual `fs.writeFile` follows the symlink and writes to `/etc/cron.d/payload`.
Issue #665. Also related to #667.
Fix
After the lexical check, resolve the real path of the nearest existing ancestor directory using `fs.realpathSync` and re-validate against safe directories. Handles the case where the output file doesn't exist yet (walks up the tree to find the nearest existing parent).
```
/tmp/evil/ → realpathSync → /etc/
/tmp/evil/cron.d/ → /etc/cron.d/ (not in SAFE_DIRECTORIES) → BLOCKED
```
sent from mStack