GHSA-R28C-9Q8G-F849

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

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)
    ...
  }
}

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.

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

GHSA-R28C-9Q8G-F849 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (8.5.18); upgrading removes the vulnerable code path.

Kodem intelligence

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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.

1. What is GHSA-R28C-9Q8G-F849? GHSA-R28C-9Q8G-F849 is a high-severity path traversal vulnerability in postcss (npm), affecting versions <= 8.5.17. It is fixed in 8.5.18. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is GHSA-R28C-9Q8G-F849? GHSA-R28C-9Q8G-F849 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of postcss are affected by GHSA-R28C-9Q8G-F849? postcss (npm) versions <= 8.5.17 is affected.
4. Is there a fix for GHSA-R28C-9Q8G-F849? Yes. GHSA-R28C-9Q8G-F849 is fixed in 8.5.18. Upgrade to this version or later.
5. Is GHSA-R28C-9Q8G-F849 exploitable, and should I be worried? Whether GHSA-R28C-9Q8G-F849 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether GHSA-R28C-9Q8G-F849 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix GHSA-R28C-9Q8G-F849? Upgrade postcss to 8.5.18 or later.

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