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.

Affected: npm: tmp

### 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…

Affected: Ubuntu:16.04:LTS: node-tmp, Ubuntu:18.04:LTS: node-tmp, Ubuntu:20.04:LTS: node-tmp, Ubuntu:22.04:LTS: node-tmp, Ubuntu:24.04:LTS: node-tmp, Ubuntu:25.10: node-tmp, Ubuntu:26.04:LTS: node-tmp

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.