PostCSS: Path Traversal in Previous Source Map Auto-Loading (sourceMappingURL) leads to Arbitrary .map File Disclosure

Vulnerability Details

File: lib/previous-map.js Line: 87-98 (loadFile), 129-144 (loadMap)

Root Cause

PostCSS auto-detects a /*# sourceMappingURL=... */ comment inside the CSS text it is asked to parse and, unless the caller explicitly passes map: false, attempts to load that path from disk as a "previous source map." This happens on every postcss.parse() / postcss().process() call by default (opt-out, not opt-in).

loadMap() builds the candidate path via join(dirname(opts.from), annotation), where annotation is the raw, attacker-controlled string from the CSS comment. path.join() normalizes but does not sandbox .. segments, so a ../../../ prefix walks the resolved path outside the intended directory. If opts.from is not set at all, the annotation is used completely unmodified — an absolute path in the CSS comment is read verbatim.

8.5.12 already fixed a strictly worse variant of this (any file, any extension, could be read) by requiring the resolved path to end in .map (loadFile()). That fix did not address the traversal itself, only the target extension. Since the join(dirname(file), map) logic has existed unchanged since PostCSS 8.0.0 (Feb 2020), any file ending in .map remains readable through this path in the current release (8.5.16).

Once loaded, MapGenerator.isMap() treats the mere presence of a loaded "previous map" as an implicit request to generate result.map, even when the caller never set the map option. If the loaded map has a sourcesContent field (common for maps emitted by bundlers/transpilers), that content is merged into result.map and returned to the caller — disclosing the traversed-to file's content to whoever supplied the CSS.

Attack Scenario

1. A service accepts user-submitted CSS and runs it through PostCSS to lint/format/transform it, e.g. postcss().process(userCss, { from: '/app/uploads/user123/input.css', to: '/app/uploads/user123/output.css' }) — idiomatic usage; map option untouched.
2. Attacker submits CSS containing /*# sourceMappingURL=../../../../some/other/app/dist/bundle.js.map */ (or an absolute path if from is unset).
3. PostCSS reads that .map file and folds its sourcesContent into result.map.
4. The service does what most build pipelines do with a truthy result.map — writes it next to the CSS output or returns it via API (source maps are meant to be consumed by browser devtools, so this is commonly public/served).
5. Attacker retrieves the emitted map and reads out the traversed file's content.

Impact

Disclosure of the contents of arbitrary .map files reachable via path traversal (or absolute path when from is unset) from the process's filesystem. Affects any application processing CSS it does not fully trust without explicitly passing map: false. No authentication or user interaction beyond submitting CSS text is required.

Vulnerable Code

loadFile(path, cssFile, trusted) {
  if (!trusted && !this.unsafeMap) {
    if (!/\.map$/i.test(path)) {
      return undefined
    }
  }
  this.root = dirname(path)
  if (existsSync(path)) {
    this.mapFile = path
    return readFileSync(path, 'utf-8').toString().trim()
  }
}

loadMap(file, prev) {
  ...
  } else if (this.annotation) {
    let map = this.annotation
    if (file) map = join(dirname(file), map)
    let unknown = this.loadFile(map, file, false)
    ...
  }
}

Recommended Fix

Constrain the resolved path to remain inside the CSS file's own directory instead of relying solely on a filename-extension check:

loadFile(path, cssFile, trusted) {
  if (!trusted && !this.unsafeMap) {
    if (!/\.map$/i.test(path)) {
      return undefined
    }
    if (!cssFile) return undefined
    let root = resolve(dirname(cssFile))
    let resolvedPath = resolve(root, path)
    if (resolvedPath !== root && !resolvedPath.startsWith(root + sep)) {
      return undefined
    }
  }
  this.root = dirname(path)
  if (existsSync(path)) {
    this.mapFile = path
    return readFileSync(path, 'utf-8').toString().trim()
  }
}

I've implemented, tested (full existing test suite — 660/660 passing, plus new PoC-based regression checks for both the traversal and legitimate same-directory cases), and can share this fix on request or via a private fork if invited.

Verification

Dynamically confirmed on v8.5.16 (current npm release / repo HEAD) via a standalone Node.js harness against lib/postcss.js: a "secret" .map file placed two directories outside a simulated project directory was read via a crafted sourceMappingURL comment in otherwise-innocuous CSS, with its sourcesContent appearing verbatim in result.map.toString() — with no map option set by the caller. A second harness confirmed the simpler no-from case reads an absolute path directly. A third harness confirmed map: false is the only current workaround. The attached fix branch closes both vectors while keeping all 660 existing unit tests green.