The bug first came to light on October 21, when the Node.js team disclosed fixes for ten vulnerabilities, the most serious being this one, crediting Khai Tran and Sylvie Mayer of OpenAI's Codex Security with the discovery. Microsoft's advisory says working exploits have been assembled by experts, but there is nothing to suggest criminals are using them in successful attacks. Its exploitability assessment nonetheless reads "Exploitation More Likely".

There is a wrinkle in the timeline: the underlying issue was actually fixed in Brotli's source code in February, but OpenAI explained that it was not known at the time to be a security vulnerability and no more recent Brotli release had followed, so software embedding the library needed to take the update before its users could be protected. Attackers can exploit vulnerable systems by sending them maliciously crafted Brotli-compressed data in a request.

The reach is broad: alongside .NET, Node.js and Python, patches have shipped for Bun, Envoy and Istio, and JFrog has built an exploit affecting FFmpeg via a crafted JPEG XL image, extending the attack path beyond HTTP. The same researchers found Apache Arrow affected, with denial of service possible. Merav Bar, a threat researcher at Wiz, said 91 percent of cloud environments rely on components vulnerable to the flaw and 82 percent have publicly exposed Node.js instances, spanning containerised applications, Kubernetes clusters, API gateways and serverless workloads. OpenAI's advisory recommends blocking untrusted Brotli-compressed data where practical until patches are applied, and rebuilding affected deployments and restarting services so fixes take effect.