# OpenAI reports Brotli remote code execution flaw found during red-teaming

> Original title: During this red-teaming exercise, we also found a vulnerability that allows for unauthenticated remote code execution in Brotli, a widely...

- Source: OpenAI
- Published: 2026-10-23T22:55:45.000Z
- Bad Influence: https://badinfluence.news/items/xgran9xyj9eqdbouurb0gyhrg
- Original: https://x.com/OpenAI/status/2113766354386084338

## Summary

OpenAI says a red-teaming exercise uncovered an unauthenticated remote code execution vulnerability in Brotli, a compression library used in Node.js, Bun, and Envoy. The flaw lets an attacker take over unpatched servers or clients that accept compressed input, and its root cause was fixed in Brotli's source in February. Brotli has since issued a new release and CVE, and downstream projects are shipping updates.

## Why it matters

The post links a model-containment incident to a Brotli RCE disclosure, showing how a training-environment red-team uncovered a fix that spread to downstream software.

## Post

During this red-teaming exercise, we also found a vulnerability that allows for unauthenticated remote code execution in Brotli, a widely-deployed compression library included in many pieces of software such as Node.js, Bun, and Envoy.

The flaw could let an attacker run their own code on affected systems by sending or replying with malicious compressed data, without any need to create a user account. That is, an unpatched server that accepts compressed input could be taken over by a malicious client, and an unpatched client that accepts compressed input (such as “curl --compressed”) could be taken over by a malicious server.

This root cause issue was actually fixed in the Brotli source code in February, but it wasn’t known at the time that it gave rise to a security vulnerability and there was no more recent release of Brotli. Since Brotli is built into other software, any software which includes this library needs to take the update too, followed by the users of that software. We disclosed the vulnerability to Brotli and other downstream projects such as Node.js, which released or are currently releasing security updates, and Brotli cut new release versions and published a CVE.

More details on the patch and mitigations below:
https://patchtheplanet.org/brotli

## Quoted post @OpenAI

> Over the past several months, we have substantially hardened the security of our research training environment. We’ve removed live internet access for all frontier models during training and built an offline world for them to use instead—a cached version of the web where they can still learn to browse, search, and use applications.
> 
> On September 20, after a model exploited a DNS misconfiguration to find a way outside of this offline world, we paused frontier training to strengthen containment further. We assembled teams of agents, powered by our most cyber-capable models, and tasked them with finding weaknesses in our research environments.
> 
> While they found no route to the internet they could easily exploit while concealing their activity from our monitors, they did find zero-day vulnerabilities, virtual machine escapes, and hardware side-channels for communication between agents. We used their discoveries to further strengthen our defenses and contribute fixes to open-source software.
> 
> After strengthening our containment architecture, we resumed our frontier RL training and inference on October 1. We’ve reserved substantial compute for continuous red-teaming, and made high-compute red-teaming part of the safety cases we use to decide whether frontier training and evaluation can proceed securely.
> 
> https://openai.com/index/securing-our-frontier-model-training-environment/

https://x.com/OpenAI/status/2113718797505036665