Friday, August 11, 2017

About that hash flooding vulnerability in Node.js…

Early July this year, Node.js released a security update for all currently maintained branches to address a hash flooding vulnerability. This intermediate fix comes at the cost of a significant startup performance regression. In the meantime, V8 has implemented a solution which avoids the performance penalty.

In this post, we want to give some background and history on the vulnerability and the eventual solution.

Hash flooding attack

Hash tables are one of the most important data structures in computer science. They are widely used in V8, for example to store an object’s properties. On average, inserting a new entry is very efficient at O(1). However, hash collisions could lead to a worst case of O(n). That means that inserting n entries can take up to O(n²).

In Node.js, HTTP headers are represented as JavaScript objects. Pairs of header name and values are stored as object properties. With cleverly prepared HTTP requests, an attacker could perform a denial-of-service attack. A Node.js process would become unresponsive, being busy with worst-case hash table insertions.

This attack has been disclosed as early as December of 2011, and shown to affect a wide range of programming languages. How come it took this long for V8 and Node.js to finally address this issue?

In fact, very soon after the disclosure, V8 engineers worked with the Node.js community on a mitigation. From Node.js v0.11.8 onwards, this issue had been addressed. The fix introduced a so-called hash seed value. The hash seed is randomly chosen at startup and used to seed every hash value in a particular V8 instance. Without the knowledge of the hash seed, an attacker has a hard time to hit the worst-case, let alone come up with an attack that targets all Node.js instances.

This is part of the commit message of the fix:

This version only solves the issue for those that compile V8 themselves or those that do not use snapshots. A snapshot-based precompiled V8 will still have predictable string hash codes.

Startup snapshot

Startup snapshots are a mechanism in V8 to dramatically speed up both engine startup and creating new contexts (i.e. via the vm module in Node.js). Instead of setting up initial objects and internal data structures from scratch, V8 deserializes from an existing snapshot. An up-to-date build of V8 with snapshot starts up in less than 3ms, and requires a fraction of a millisecond to create a new context. Without the snapshot, startup takes more than 200ms, and a new context more than 10ms. This is a difference of two orders of magnitude.

We covered how any V8 embedder can take advantage of startup snapshots in previous posts.

A pre-built snapshot contains hash tables and other hash-value-based data structures. Once initialized from snapshot, the hash seed can no longer be changed without corrupting these data structures. A Node.js release that bundles the snapshot has a fixed hash seed, making the mitigation ineffective.

That is what the explicit warning in the commit message was about.

Almost fixed, but not quite

Fast-forward to 2015, a Node.js issue reports that creating a new context has regressed in performance. Unsurprisingly, this is because the startup snapshot has been disabled as part of the mitigation. But by that time not everyone participating in the discussion was aware of the reason.

As explained in this post, V8 uses a pseudo-random number generator to generate Math.random results. Every V8 context has its own copy of the random number generate state. This is to prevent Math.random results from being predictable across contexts.

The random number generator state is seeded from an external source right after the context is created. It does not matter whether the context is created from scratch, or deserialized from snapshot.

Somehow, the random number generator state has been confused with the hash seed. As result, a pre-built snapshot started being part of the official release since io.js v2.0.2.

Second attempt

It was not until May 2017, during some internal discussions between V8, Google’s Project Zero, and Google’s Cloud Platform, when we realized that Node.js was still vulnerable to hash flooding attacks.

The initial response came from our colleagues Ali and Myles from the team behind Google Cloud Platform's Node.js offerings. They worked with the Node.js community to disable startup snapshot by default, again. This time around, they also added a test case.

But we did not want to leave it at that. Disabling startup snapshot has significant performance impacts. Over the years, we have added many new language features and sophisticated optimizations to V8. Some of these additions made starting up from scratch even more expensive. Immediately after the security release, we started working on a long-term solution. The goal is to be able to re-enable startup snapshot without becoming vulnerable to hash flooding.

From proposed solutions, we chose and implemented the most pragmatic one. After deserializing from snapshot, we would choose a new hash seed. Affected data structures are then rehashed to ensure consistency.

As it turns out, in an ordinary startup snapshot few data structures are actually affected. And to our delight, rehashing hash tables have been made easy in V8 in the meantime. The overhead this adds is insignificant.

The patch to re-enable startup snapshot has been merged into Node.js. It is part of the recent Node.js v8.3.0 release.

Posted by Yang Guo, aka @hashseed

Thursday, August 3, 2017

V8 Release 6.1

Every six weeks, we create a new branch of V8 as part of our release process. Each version is branched from V8’s git master immediately before a Chrome Beta milestone. Today we’re pleased to announce our newest branch, V8 version 6.1, which is in beta until its release in coordination with Chrome 61 Stable in several weeks. V8 v6.1 is filled with all sorts of developer-facing goodies. We’d like to give you a preview of some of the highlights in anticipation of the release.

Performance improvements

Visiting all the elements of the Maps and Sets — either via iteration or the Map.prototype.forEach / Set.prototype.forEach methods — became significantly faster, with a raw performance improvement of up to 11× since V8 version 6.0. Check the dedicated blog post for additional information.
In addition to that, work continued on the performance of other language features. For example, the Object.prototype.isPrototypeOf method, which is important for constructor-less code using mostly object literals and Object.create instead of classes and constructor functions, is now always as fast and often faster than using the instanceof operator.
Function calls and constructor invocations with variable number of arguments also got significantly faster. Calls made with Reflect.apply and Reflect.construct received an up to 17× performance boost in the latest version.

Array.prototype.forEach is now inlined in TurboFan and optimized for all major non-holey elements kinds.

Binary size reduction

The V8 team has completely removed the deprecated Crankshaft compiler, giving a significant reduction in binary size. Alongside the removal of the builtins generator, this reduces the deployed binary size of V8 by over 700 KB, depending on the exact platform.

asm.js is now validated and compiled to WebAssembly

If V8 encounters asm.js code it now tries to validate it. Valid asm.js code is then transpiled to WebAssembly. According to V8’s performance evaluations, this generally boosts throughput performance. Due to the added validation step, isolated regressions in startup performance might happen.

Please note that this feature was switched on by default on the Chromium side only. If you are an embedder and want to leverage the asm.js validator, enable the flag --validate-asm.


When debugging WebAssembly, it is now possible to display local variables in DevTools when a breakpoint in WebAssembly code is hit.

Please check out our summary of API changes. This document is regularly updated a few weeks after each major release.

Developers with an active V8 checkout can use git checkout -b 6.1 -t branch-heads/6.1 to experiment with the new features in V8 v6.1. Alternatively you can subscribe to Chrome’s Beta channel and try the new features out yourself soon.

Posted by the V8 team