fix(deps): update dependency postcss@<8.5.10 to >=8.5.26 [security]

[renovate]

This PR contains the following updates:

Package	Change	Age	Confidence
postcss@<8.5.10 (source)	>=8.5.10 → >=8.5.26

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

GHSA-r28c-9q8g-f849

More information

Details

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)) {
    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.

Verification

Dynamically confirmed on v8.5.16 (current npm release / repo HEAD) via a standalone Node.js harness against lib/postcss.js: a "secret" .map file placed two directories outside a simulated project directory was read via a crafted sourceMappingURL comment in otherwise-innocuous CSS, with its sourcesContent appearing verbatim in result.map.toString() — with no map option set by the caller. A second harness confirmed the simpler no-from case reads an absolute path directly. A third harness confirmed map: false is the only current workaround. The attached fix branch closes both vectors while keeping all 660 existing unit tests green.

Severity

• CVSS Score: 7.5 / 10 (High)
• Vector String: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

References

• https://github.com/postcss/postcss/security/advisories/GHSA-r28c-9q8g-f849
• https://github.com/postcss/postcss/commit/95663d3eb7ba26f4854dd19d3b4f4425760cf56c
• https://github.com/postcss/postcss/releases/tag/8.5.18
• https://github.com/advisories/GHSA-r28c-9q8g-f849

This data is provided by the GitHub Advisory Database (CC-BY 4.0).

⚠️ Renovate does not enforce Minimum Release Age for bump, lockfileUpdate, or rollback updates, so these are raised without a Minimum Release Age check. You will need to manually validate the Minimum Release Age for these package(s).

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Release Notes

postcss/postcss (postcss@<8.5.10)

v8.5.26

Compare Source

• Fixed list.split() regression (by @​lazerg).
• Track symlinks in path protection in source map loading (by @​drengir1).

v8.5.25

Compare Source

• Fixed 8.5.17 visitor regression.
• Fixed list.split() for non-string values (by @​amir-rezaei).

v8.5.24

Compare Source

• Preserve the BOM after the processing (by @​hdimer).

v8.5.23

Compare Source

• Do not load source map without opts.from for security reasons.

v8.5.22

Compare Source

• Fixed custom property losing semicolon before a comment (by @​sarathfrancis90).

v8.5.21

Compare Source

• Fixed childless at-rule losing semicolon before comment (by @​sarathfrancis90).
• Fixed docs (by @​isker).

v8.5.20

Compare Source

• Fixed missing space if AtRule#params is set after (by @​sarathfrancis90).
• Fixed mixing AST error on warnings (by @​MahinAnowar).

v8.5.19

Compare Source

• Fixed cleaning before for new nodes inserted to Root (by @​MahinAnowar).

v8.5.18

Compare Source

• Restricted loading previous source maps file to the opts.from folder for security reasons (use unsafeMap: true to disable the check).

v8.5.17

Compare Source

• Fixed Maximum call stack size exceeded error.
• Fixed Prototype hijacking for postcss.fromJSON().
• Fixed Input#origin() for unmapped end position (by @​chatman-media).

v8.5.16

Compare Source

• Fixed Input#origin() position (by @​mizdra).
• Fixed raws after rehydrating a JSON AST (by @​sarathfrancis90).
• Fixed putting parent-less node in nodes of new node (by @​MahinAnowar).
• Fixed computing offset in positionBy() (by @​greymoth-jp).
• Fixed rangeBy() on index: 0 (by @​sarathfrancis90).

v8.5.15

Compare Source

• Fixed declaration parsing performance (by @​homanp).

v8.5.14

Compare Source

• Fixed custom syntax regression (by @​43081j).

v8.5.13

Compare Source

• Fixed postcss-scss commend regression.

v8.5.12

Compare Source

• Fixed reading any file via user-generated CSS.
• Added opts.unsafeMap to disable checks.

v8.5.11

Compare Source

• Fixed nested brackets parsing performance (by @​offset).

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