Browserslist: Uncaught crash / prototype write via untrusted browserslist-stats.json custom stats (normalizeStats)

Vulnerability Details

File: node.js
Function: normalizeStats() (line ~214), reached from getStat() (called
unconditionally on every browserslist() call) and loadStat()

Root Cause

function normalizeStats(data, stats) {
  if (!data) { data = {} }
  if (stats && 'dataByBrowser' in stats) { stats = stats.dataByBrowser }
  if (typeof stats !== 'object') return undefined

  var normalized = {}
  for (var i in stats) {
    var versions = Object.keys(stats[i])
    if (versions.length === 1 && data[i] && data[i].versions.length === 1) {
      var normal = data[i].versions[0]
      normalized[i] = {}
      normalized[i][normal] = stats[i][versions[0]]
    } else {
      normalized[i] = stats[i]
    }
  }
  return normalized
}

stats is untrusted: it comes from JSON.parse()-ing a
browserslist-stats.json file — auto-discovered by walking up the directory
tree from the project root on every browserslist() call, regardless of
the query (env.getStat(opts, browserslist.data) runs unconditionally
inside browserslist()) — or from opts.stats passed programmatically /
via the CLI's --stats= flag. data is browserslist.data, a plain object
populated only with real browser names.

Two independent bugs from the same root cause (unguarded for...in over
untrusted keys used with plain-object bracket access/assignment):

1. Crash: data[i] has no hasOwnProperty guard. If stats contains a
   key that also happens to be an inherited Object.prototype member name —
   "__proto__", "toString", "valueOf", "constructor",
   "hasOwnProperty", "isPrototypeOf", etc. — data[i] resolves to that
   inherited function/object (always truthy), and the code then does
   data[i].versions.length → undefined.length → uncaught TypeError,
   for any such key whose JSON value has exactly one sub-key, e.g.:

   { "toString": { "onekey": 5 }, "chrome": { "100": 50 } }
   

2. Prototype write: normalized[i] = ... on the fresh
   normalized = {} — if i is exactly "__proto__" (and normalized has
   no own property by that name yet), this computed assignment invokes the
   real Object.prototype.__proto__ setter, changing normalized's actual
   [[Prototype]] instead of creating a plain property.

Because this runs on every browserslist() call regardless of the
query, simply committing a poisoned browserslist-stats.json anywhere in a
project's directory tree breaks every subsequent Browserslist call in that
project — including calls made by Autoprefixer, Babel preset-env,
Stylelint, or PostCSS internally, for completely unrelated queries.

Attack Scenario

1. Attacker submits a PR (or a compromised dependency) adding a
   browserslist-stats.json file anywhere between the project root and
   filesystem root, containing e.g.
   {"toString": {"onekey": 5}, "chrome": {"100": 50}}.
2. The victim's build/CI pipeline runs any tool that calls browserslist()
   internally, for any query.
3. The auto-discovered poisoned file crashes the process with an uncaught
   TypeError on the very first call.

Measured Impact

Confirmed crash (real browserslist() call, v4.28.6) with stats keys:
__proto__, toString, valueOf, hasOwnProperty, constructor,
isPrototypeOf — each paired with a one-key JSON object — for any query,
including browserslist('defaults') which never mentions stats.

Recommended Fix (implemented and verified)

var normalized = Object.create(null)
for (var i in stats) {
  var versions = Object.keys(stats[i])
  var known = Object.prototype.hasOwnProperty.call(data, i) && data[i]
  if (versions.length === 1 && known && known.versions.length === 1) {
    var normal = known.versions[0]
    normalized[i] = Object.create(null)
    normalized[i][normal] = stats[i][versions[0]]
  } else {
    normalized[i] = stats[i]
  }
}
return normalized

normalized uses Object.create(null) so a write to "__proto__" is an
ordinary property set, never a [[Prototype]] change; data[i] is replaced
with an explicit hasOwnProperty check so it never resolves to an inherited
Object.prototype member.

Verification:

• NODE_ENV=test npx uvu test .test.js → 301/301 pass unmodified
  (test/custom.test.js, test/shareable-stats.test.js, test/cover.test.js
  exercise the stats-handling paths).
• All 6 previously crash-inducing keys, tested individually, now resolve
  without error.
• The realistic file-based auto-discovery scenario (poisoned
  browserslist-stats.json + an unrelated browserslist('defaults') call)
  now returns a normal result instead of crashing.

Impact

• Who is affected: Any project whose build/CI invokes Browserslist
  (directly or via Autoprefixer/Babel/Stylelint/PostCSS) in a directory tree
  an attacker can place a file into (external PR, compromised dependency),
  or any app that passes user-influenced data into opts.stats.
• What an attacker achieves: Immediate DoS — crashes the invoking
  process on the first Browserslist call after the file is present, for any
  query, no special syntax needed.
• Conditions required: No authentication — only the ability to add a
  file to the project's directory tree, or influence opts.stats.

Verification Environment

browserslist @ HEAD (== v4.28.6, current latest stable release) under local
Node.js v20.19.5. Pure JS library — executed directly, no server needed.

Note

Found via a systematic review of prototype-pollution-adjacent patterns in
this codebase after confirming two unrelated algorithmic-complexity issues
(reported separately as GHSA-rrmg-cfrq-23vv and GHSA-g6p8-hj8g-x889) in the
same research pass. A similar for...in + bracket-write pattern in
index.js's copyObject() (used by normalizeAndroidData) was already
guarded against __proto__/constructor/prototype keys by a prior,
unrelated commit — that guard was never applied to this function.