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doc: clarify path.isAbsolute is not enough for mitigating path traversals #57073

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aduh95 merged 4 commits into from
Mar 2, 2025
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doc: clarify path.isAbsolute is not enough for mitigating path traversals #57073

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doc: clarify path.isAbsolute doesn’t resolve paths
ericfortis Feb 15, 2025
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doc: clarify path.isAbsolute doesn’t resolve paths
ericfortis Feb 16, 2025
f79fbba
doc: clarify path.isAbsolute doesn’t resolve paths
ericfortis Mar 2, 2025
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doc: clarify path.isAbsolute doesn’t resolve paths
ericfortis Mar 2, 2025
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22 changes: 17 additions & 5 deletions doc/api/path.md
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Original file line number Diff line number Diff line change
Expand Up @@ -317,17 +317,29 @@ added: v0.11.2
* `path` {string}
* Returns: {boolean}

The `path.isAbsolute()` method determines if `path` is an absolute path.
The `path.isAbsolute()` method determines if the literal (non-resolved) `path` is an
absolute path. Therefore, it’s likely that you want to run it through `path.normalize()`
if your plan is to use it as a subpath in order to mitigate path traversals. For example:

```js
// Normalizing the subpath mitigates traversing above the mount directory.
const mySubpath = path.normalize('/foo/../..');
if (path.isAbsolute(mySubpath)) {
const myPath = path.join(MOUNT_DIR, mySubpath)
}
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@aduh95 aduh95 Feb 16, 2025 •
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I don't really understand why you would call path.isAbsolute at all.

Suggested change
The `path.isAbsolute()` method determines if the literal (non-resolved) `path` is an
absolute path. Therefore, it’s likely that you want to run it through `path.normalize()`
if your plan is to use it as a subpath in order to mitigate path traversals. For example:
```js
// Normalizing the subpath mitigates traversing above the mount directory.
const mySubpath = path.normalize('/foo/../..');
if (path.isAbsolute(mySubpath)) {
const myPath = path.join(MOUNT_DIR, mySubpath)
}
The `path.isAbsolute()` method determines if `path` is an absolute path. Being an
absolute path doesn't not guarentee it would be safe to treat as a subpath:
```js
const MOUNT_DIR = '/home/example/';
const userProvidedPath = '/foo/../../../etc/passwd';
console.log(path.isAbsolute(userProvidedPath)); // true
console.log(path.join(MOUNT_DIR, userProvidedPath)); // /etc/passwd
// Instead, call `path.normalize`:
console.log(path.join(MOUNT_DIR, path.normalize(`/${userProvidedPath}`))); // /home/example/etc/passwd

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@ericfortis ericfortis Feb 16, 2025

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Flow control, e.g.

const mySubpath = path.normalize('/foo/../..')
if (!path.isAbsolute(mySubpath))
  throw 'FORBIDDEN'
const myPath = path.join(MOUNT_DIR, mySubpath)

I agree it‘s not needed.

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@ericfortis ericfortis Feb 16, 2025 •
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Perhaps, instead of documenting the behavior, a better approach could be adding path.isAbsoluteSubpath(). That way, when autocompleting, users can tell there's something not obvious about path.isAbsolute().

Something like,

function isAbsoluteSubpath(path) {
  const dpath = decodeURIComponent(path)
  if (!dpath.startsWith('/'))
    return false
  let nOutside = 0
  for (const part of dpath.split('/')) {
    if (part === '')
      continue
    if (part === '..')
      nOutside++
    else
      nOutside--
    if (nOutside > 0)
      return false
  }
  return true
}

function is(path, expected) {
  console.assert(expected === isAbsoluteSubpath(path), path)
}

;[
  '.',
  './',
  '..',
  '/..',
  '../',
  '/home//../../..',
  '/home/user/../../..'
].forEach(p => {
  is(p, false)
  is(encodeURIComponent(p), false)
})

;[
  '/',
  '/home/user/docs',
  '/home/user/./docs',
  '/home/user//docs'
].forEach(p => {
  is(p, true)
  is(encodeURIComponent(p), true)
})

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@aduh95 aduh95 Mar 1, 2025

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If the current behavior is confusing, let's update the doc to make them less confusing. Having clearer docs doesn't mean we can't add another API, that'd be a false dichotomy to think we can do one and not the other.

Would you like to take my suggestion so this PR can move forward?

```


If the given `path` is a zero-length string, `false` will be returned.

For example, on POSIX:

```js
path.isAbsolute('/foo/bar'); // true
path.isAbsolute('/baz/..'); // true
path.isAbsolute('qux/'); // false
path.isAbsolute('.'); // false
path.isAbsolute('/foo/bar'); // true
path.isAbsolute('/baz/..'); // true
path.isAbsolute('/baz/../..'); // true (doesn’t resolve)
path.isAbsolute('qux/'); // false
path.isAbsolute('.'); // false
```

On Windows:
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