CVE-2026-73646

Vulnerability Details

File: lib/previous-map.js Line: 87-98 (loadFile), 129-144 (loadMap)

Root Cause

PostCSS auto-detects a /*# sourceMappingURL=... */ comment inside the CSS text it is asked to parse and, unless the caller explicitly passes map: false, attempts to load that path from disk as a "previous source map." This happens on every postcss.parse() / postcss().process() call by default (opt-out, not opt-in).

loadMap() builds the candidate path via join(dirname(opts.from), annotation), where annotation is the raw, attacker-controlled string from the CSS comment. path.join() normalizes but does not sandbox .. segments, so a ../../../ prefix walks the resolved path outside the intended directory. If opts.from is not set at all, the annotation is used completely unmodified — an absolute path in the CSS comment is read verbatim.

8.5.12 already fixed a strictly worse variant of this (any file, any extension, could be read) by requiring the resolved path to end in .map (loadFile()). That fix did not address the traversal itself, only the target extension. Since the join(dirname(file), map) logic has existed unchanged since PostCSS 8.0.0 (Feb 2020), any file ending in .map remains readable through this path in the current release (8.5.16).

Once loaded, MapGenerator.isMap() treats the mere presence of a loaded "previous map" as an implicit request to generate result.map, even when the caller never set the map option. If the loaded map has a sourcesContent field (common for maps emitted by bundlers/transpilers), that content is merged into result.map and returned to the caller — disclosing the traversed-to file's content to whoever supplied the CSS.

Attack Scenario

1. A service accepts user-submitted CSS and runs it through PostCSS to lint/format/transform it, e.g. postcss().process(userCss, { from: '/app/uploads/user123/input.css', to: '/app/uploads/user123/output.css' }) — idiomatic usage; map option untouched.
2. Attacker submits CSS containing /*# sourceMappingURL=../../../../some/other/app/dist/bundle.js.map */ (or an absolute path if from is unset).
3. PostCSS reads that .map file and folds its sourcesContent into result.map.
4. The service does what most build pipelines do with a truthy result.map — writes it next to the CSS output or returns it via API (source maps are meant to be consumed by browser devtools, so this is commonly public/served).
5. Attacker retrieves the emitted map and reads out the traversed file's content.

Impact

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.

Vulnerable Code

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)
    ...
  }
}

Recommended Fix

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)) {
 …

PostCSS takes a CSS file and provides an API to analyze and modify its rules by transforming the rules into an Abstract Syntax Tree. Prior to 8.5.18, lib/previous-map.js loadMap() passes attacker-controlled sourceMappingURL values to join(dirname(opts.from), annotation), and loadFile() permits traversed or absolute .map paths, allowing untrusted CSS processed without map: false to disclose sourcesContent from arbitrary reachable .map files through result.map. This issue is fixed in version 8.5.18.

TuxCare rebuilt postcss to address 1 CVE. Fixed in 8.5.6-tuxcare.3.

TuxCare rebuilt postcss to address 1 CVE. Fixed in 8.3.6-tuxcare.5.

TuxCare rebuilt postcss to address 1 CVE. Fixed in 7.0.32-tuxcare.3.

TuxCare rebuilt postcss to address 1 CVE. Fixed in 7.0.17-tuxcare.3.

TuxCare rebuilt postcss to address 1 CVE. Fixed in 5.2.18-tuxcare.3.

TuxCare rebuilt postcss to address 1 CVE. Fixed in 8.4.14-tuxcare.5.

TuxCare rebuilt postcss to address 1 CVE. Fixed in 8.4.5-tuxcare.4.

TuxCare rebuilt postcss to address 1 CVE. Fixed in 8.4.31-tuxcare.4.

TuxCare rebuilt postcss to address 1 CVE. Fixed in 8.4.41-tuxcare.5.

TuxCare rebuilt postcss to address 1 CVE. Fixed in 8.2.15-tuxcare.4.

TuxCare rebuilt postcss to address 1 CVE. Fixed in 8.5.6-tuxcare.5.

TuxCare rebuilt postcss to address 1 CVE. Fixed in 8.2.13-tuxcare.3.

TuxCare rebuilt postcss to address 1 CVE. Fixed in 8.5.17-tuxcare.1.

TuxCare rebuilt postcss to address 6 CVEs. Fixed in 6.0.1-tuxcare.1.

TuxCare rebuilt postcss to address 2 CVEs. Fixed in 7.0.14-tuxcare.4.

TuxCare rebuilt postcss to address 2 CVEs. Fixed in 7.0.21-tuxcare.4.

TuxCare rebuilt postcss to address 2 CVEs. Fixed in 8.5.3-tuxcare.2.

TuxCare rebuilt postcss to address 2 CVEs. Fixed in 8.5.2-tuxcare.2.

TuxCare rebuilt postcss to address 4 CVEs. Fixed in 8.4.49-tuxcare.1.

TuxCare rebuilt postcss to address 2 CVEs. Fixed in 8.5.12-tuxcare.2.

TuxCare rebuilt postcss to address 2 CVEs. Fixed in 8.5.14-tuxcare.1.

TuxCare rebuilt postcss to address 3 CVEs. Fixed in 8.4.35-tuxcare.1.

TuxCare rebuilt postcss to address 3 CVEs. Fixed in 8.2.9-tuxcare.2.

TuxCare rebuilt postcss to address 1 CVE. Fixed in 8.4.17-tuxcare.2.

TuxCare rebuilt postcss to address 1 CVE. Fixed in 8.4.33-tuxcare.2.

TuxCare rebuilt postcss to address 2 CVEs. Fixed in 8.5.5-tuxcare.3.

These are all security issues fixed in the velociraptor-0.76.7.0.git145.b60ef1146-1.1 package on the GA media of openSUSE Tumbleweed.

Root has patched CVE-2026-73646 in the postcss package for Root:npm. Multiple fixed versions available.

Root has patched GHSA-r28c-9q8g-f849 in the postcss package for Root:npm. Multiple fixed versions available.