safe-buffer

Safer Node.js Buffer API

install

npm install safe-buffer

usage

The goal of this package is to provide a safe replacement for the node.js Buffer.

It's a drop-in replacement for Buffer. You can use it by adding one require line to the top of your node.js modules:

var Buffer = require('safe-buffer')

// Existing buffer code will continue to work without issues:

new Buffer('hey', 'utf8')
new Buffer([1, 2, 3], 'utf8')
new Buffer(obj)

// But this potentially unsafe operation now returns zeroed out memory:

new Buffer(16) // this is safe now!

// Going forward, use an explicit API to make clear what you want.

Buffer.from('hey', 'utf8') // convert from many types to a Buffer
Buffer.zalloc(16) // create a zero-filled buffer (safe)
Buffer.alloc(16) // create an uninitialized buffer (potentially unsafe)

api

Buffer.from(value, [encoding])

Creates a new Buffer containing the given value. value can be an Array, Buffer, ArrayBuffer, string, or TypedArrayView (Uint8Array, etc.). If value is a string, an encoding parameter can also be provided to identify the string character encoding.

Buffer.zalloc(size)

Allocate a zero-filled buffer with the given size. This is safe and recommended to use.

Buffer.alloc(size)

Allocate an uninitialized buffer with the given size. This is potentially unsafe. The underlying memory for Buffer instances created in this way is not initialized. The contents are unknown and could contain sensitive data. The buffer must be overwritten before it's used.

more methods

The rest of the Buffer API is exactly the same as in node.js. See the docs.

Related links

• Node.js issue: Buffer(number) is unsafe
• Node.js Enhancement Proposal: Buffer.from/Buffer.alloc/Buffer.zalloc/Buffer() soft-deprecate

Why is Buffer unsafe?

Today, the node.js Buffer constructor is overloaded to handle many different argument types like String, Array, Object, TypedArrayView (Uint8Array, etc.), ArrayBuffer, and also Number.

The API is optimized for convenience: you can throw any type at it, and it will try to do what you want.

Because the Buffer constructor is so powerful, you often see code like this:

// Convert UTF-8 strings to hex
function toHex (str) {
  return new Buffer(str).toString('hex')
}

But what happens if toHex is called with a Number argument?

Remote Memory Disclosure

If an attacker can make your program call the Buffer constructor with a Number argument, then they can make it allocate uninitialized memory from the node.js process. This could potentially disclose TLS private keys, user data, or database passwords.

When the Buffer constructor is passed a Number argument, it returns an UNINITIALIZED block of memory of the specified size. When you create a Buffer like this, you MUST overwrite the contents before returning it to the user.

From the node.js docs:

new Buffer(size)

• size Number

The underlying memory for Buffer instances created in this way is not initialized. The contents of a newly created Buffer are unknown and could contain sensitive data. Use buf.fill(0) to initialize a Buffer to zeroes.

(Emphasis our own.)

Whenever the programmer intended to create an uninitialized Buffer you often see code like this:

var buf = new Buffer(16)

// Immediately overwrite the uninitialized buffer with data from another buffer
for (var i = 0; i < buf.length; i++) {
  buf[i] = otherBuf[i]
}

Would this ever be a problem in real code?

Yes. It's surprisingly common to forget to check the type of your variables in a dynamically-typed language like JavaScript.

Usually the consequences of assuming the wrong type is that your program crashes with an uncaught exception. But the failure mode for forgetting to check the type of arguments to the Buffer constructor is more catastrophic.

Here's an example of a vulnerable service that takes a JSON payload and converts it to hex:

// Take a JSON payload {str: "some string"} and convert it to hex
var server = http.createServer(function (req, res) {
  var data = ''
  req.setEncoding('utf8')
  req.on('data', function (chunk) {
    data += chunk
  })
  req.on('end', function () {
    var body = JSON.parse(data)
    res.end(new Buffer(body.str).toString('hex'))
  })
})

server.listen(8080)

In this example, an http client just has to send:

{
  "str": 1000
}

and it will get back 1,000 bytes of uninitialized memory from the server.

This is a very serious bug. It's similar in severity to the the Heartbleed bug that allowed disclosure of OpenSSL process memory by remote attackers.

Which real-world packages were vulnerable?

bittorrent-dht

Mathias Buus and I (Feross Aboukhadijeh) found this issue in one of our own packages, bittorrent-dht. The bug would allow anyone on the internet to send a series of messages to a user of bittorrent-dht and get them to reveal 20 bytes at a time of uninitialized memory from the node.js process.

Here's the commit that fixed it. We released a new fixed version, created a Node Security Project disclosure, and deprecated all vulnerable versions on npm so users will get a warning to upgrade to a newer version.

ws

That got us wondering if there were other vulnerable packages. Sure enough, within a short period of time, we found the same issue in ws, the most popular WebSocket implementation in node.js.

If certain APIs were called with Number parameters instead of String or Buffer as expected, then uninitialized server memory would be disclosed to the remote peer.

These were the vulnerable methods:

socket.send(number)
socket.ping(number)
socket.pong(number)

Here's a vulnerable socket server with some echo functionality:

server.on('connection', function (socket) {
  socket.on('message', function (message) {
    message = JSON.parse(message)
    if (message.type === 'echo') {
      socket.send(message.data) // send back the user's message
    }
  })
})

socket.send(number) called on the server, will disclose server memory.

Here's the release where the issue was fixed, with a more detailed explanation. Props to Arnout Kazemier for the quick fix. Here's the Node Security Project disclosure.

What's the solution?

It's important that node.js offers a fast way to get memory otherwise performance-critical applications would needlessly get a lot slower.

But we need a better way to signal our intent as programmers. When we want uninitialized memory, we should request it explicitly.

Sensitive functionality should not be packed into a developer-friendly API that loosely accepts many different types. This type of API encourages the lazy practice of passing variables in without checking the type very carefully.

A new API: Buffer.alloc(number)

The functionality of creating buffers with uninitialized memory should be part of another API. We propose Buffer.alloc(number). This way, it's not part of an API that frequently gets user input of all sorts of different types passed into it.

var buf = Buffer.alloc(16) // careful, uninitialized memory!

// Immediately overwrite the uninitialized buffer with data from another buffer
for (var i = 0; i < buf.length; i++) {
  buf[i] = otherBuf[i]
}

How do we fix node.js core?

We sent a PR to node.js core (merged as semver-major) which defends against one case:

var str = 16
new Buffer(str, 'utf8')

In this situation, it's implied that the programmer intended the first argument to be a string, since they passed an encoding as a second argument. Today, node.js will allocate uninitialized memory in the case of new Buffer(number, encoding), which is probably not what the programmer intended.

But this is only a partial solution, since if the programmer does new Buffer(variable) (without an encoding parameter) there's no way to know what they intended. If variable is sometimes a number, then uninitialized memory will sometimes be returned.

What's the real long-term fix?

We could deprecate and remove new Buffer(number) and use Buffer.alloc(number) when we need uninitialized memory. But that would break 1000s of packages.

We believe the best solution is to:

1. Change new Buffer(number) to return safe, zeroed-out memory

2. Create a new API for creating uninitialized Buffers. We propose: Buffer.alloc(number)

Update

Upon further consideration, we think that returning zeroed out memory is a separate issue. The core issue is: unsafe buffer allocation should be in a different API.

We now support adding two APIs:

• Buffer.from(value) - convert from any type to a buffer
• Buffer.alloc(size) - create an uninitialized buffer with given size

This solves the core problem that affected ws and bittorrent-dht which is Buffer(variable) getting tricked into taking a number argument.

This way, existing code continues working and the impact on the npm ecosystem will be minimal. Over time, npm maintainers can migrate performance-critical code to use Buffer.alloc(number) instead of new Buffer(number).

Conclusion

We think there's a serious design issue with the Buffer API as it exists today. It promotes insecure software by putting high-risk functionality into a convenient API with friendly "developer ergonomics".

This wasn't merely a theoretical exercise because we found the issue in some of the most popular npm packages.

Fortunately, there's an easy fix that can be applied today. Use safe-buffer in place of buffer.

var Buffer = require('safe-buffer')

Eventually, we hope that node.js core can switch to this new, safer behavior. We believe the impact on the ecosystem would be minimal since it's not a breaking change. Well-maintained, popular packages would be updated to use Buffer.alloc quickly, while older, insecure packages would magically become safe from this attack vector.

links

• Node.js PR: buffer: throw if both length and enc are passed
• Node Security Project disclosure for ws
• Node Security Project disclosure forbittorrent-dht

credit

The original issues in bittorrent-dht (disclosure) and ws (disclosure) were discovered by Mathias Buus and Feross Aboukhadijeh.

Thanks to Adam Baldwin for helping disclose these issues and for his work running the Node Security Project.

Thanks to John Hiesey for proofreading this README and auditing the code.

license

MIT. Copyright (C) Feross Aboukhadijeh