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

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.

8.5.22

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

8.5.21

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

8.5.20

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

8.5.19

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

8.5.18

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

8.5.17

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

Does any of this look wrong? Please let us know.

See the full diff on Github. The new version differs by 37 commits:

• Release 8.5.23 version
• Update dependencies
• Does no load source map file without opts.from
• Typo
• Update EM banner
• Update EM banner
• Release 8.5.22 version
• Fix custom property losing its semicolon before a comment (#2117)
• Release 8.5.21 version
• Update dependencies
• Fix childless at-rule losing its semicolon before a comment (#2115)
• Fix mismatched JSDoc comments on Position (#2114)
• Release 8.5.20 version
• Fix missing space when AtRule#params is set after parsing (#2113)
• Update dependencies
• Move back to latest 11 pnpm
• Update pnpm to check that CI is fixed
• Rebuild nodes from another PostCSS copy in Warning (#2112)
• Improve CI security
• Remove patch after updating dependency
• Release 8.5.19 version
• Fix CI on Windows too
• Keep explicitly set raws.before when inserting nodes into root (#2111)
• Temporary fix CI
• Release 8.5.18 version
• Update dependencies
• Limit where source map can be loaded for security reasons
• Release 8.5.17 version
• Fix Maximum call stack size exceeded error
• Fix linter
• Fix CI
• Prevent prototype hijacking in fromJSON
• Update dependencies
• Fix non-closed `<div align="center">` in README (#2110)
• Remove old sponsors
• docs: fix invalid JS in writing-a-plugin examples (#2107)
• Fix `Input#origin()` mixing null and undefined for unmapped end position (#2106)

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