GHSA-6g55-p6wh-862q

Summary

PostCSS's PreviousMap parses the /*# sourceMappingURL=PATH */ comment from any CSS string passed to process() and dereferences PATH against the local filesystem with no scheme, allowlist, or traversal check. An attacker who controls the CSS input can cause the host process to read any file readable by Node and leak the first ~10 bytes of its content through the resulting JSON.parse SyntaxError message. The bug also yields a precise file-existence oracle and a controllable-read primitive that may be combined with large-file targets for DoS. The behaviour is triggered with PostCSS's default options — no from, no map, no plugins required — and is therefore reachable from any pipeline that runs untrusted CSS through PostCSS (CMS themes, user-uploaded styles, browser-extension/userstyle processors, build pipelines for third-party packages, blog comment renderers, etc.).

Details

The dangerous chain lives in lib/previous-map.js and is wired into every Input construction at lib/input.js:70-77.

Input constructor (lib/input.js:70-77):

if (pathAvailable && sourceMapAvailable) {
  let map = new PreviousMap(this.css, opts)
  if (map.text) {
    this.map = map
    let file = map.consumer().file
    if (!this.file && file) this.file = this.mapResolve(file)
  }
}

PreviousMap constructor (lib/previous-map.js:17-29):

constructor(css, opts) {
  if (opts.map === false) return
  this.loadAnnotation(css)
  this.inline = this.startWith(this.annotation, 'data:')

  let prev = opts.map ? opts.map.prev : undefined
  let text = this.loadMap(opts.from, prev)
  ...
}

Note opts.map === false is the only short-circuit. With default options (opts.map === undefined), the rest of the constructor — including the filesystem read — executes.

loadAnnotation (lib/previous-map.js:72-84) extracts the URL without sanitisation:

loadAnnotation(css) {
  let comments = css.match(/\/\*\s*# sourceMappingURL=/g)
  if...