Microsoft released out-of-band fixes, meaning updates outside its regular monthly schedule, for .NET, Visual Studio and PowerShell to close a memory-corruption flaw in Brotli, a Google data-compression library, SecurityWeek reported Monday. Microsoft's critical rating and its judgment that exploitation is more likely turn a bug first disclosed by OpenAI into a broad patching task for companies that build and run software on Microsoft's developer platform.
The Primary Wire reported Friday, citing Reuters, that OpenAI had found the flaw while testing defenses around its model training.
What Happened
Microsoft's security update for CVE-2026-111497, released Oct. 25, covers a flaw in the Brotli decoder in which code can write past the end of a block of memory. Microsoft rates it critical, classifies it as elevation of privilege and gives it a base score of 8.1 out of 10 on the Common Vulnerability Scoring System (CVSS) version 3.1, with an assessment of "Exploitation More Likely," SecurityWeek reported.
According to Microsoft's .NET blog post, the fixed runtimes are .NET 8.0.33, 9.0.22 and 10.0.14 plus .NET 11 Release Candidate 3. An unauthenticated attacker may be able to read or corrupt process memory. Apps are exposed if they process Brotli data through ASP.NET Core request decompression or HttpClient automatic response decompression, neither of which is on by default, or through the System.IO.Compression Brotli programming interfaces. The older .NET Framework is not affected. The same release fixes a Linux privilege-escalation flaw in .NET tarballs and reissues two fixes from the Oct. 13 Patch Tuesday that were incomplete.
SecurityWeek reported that Microsoft also updated Visual Studio 2019, 2022 and 2026 and PowerShell 7.4 through 7.6, rating the issue critical for .NET and Visual Studio and important for PowerShell.
The underlying bug affects Brotli 1.0.3 through 1.2.0 and is fixed in 1.0.10, 1.1.1 and 1.2.1; Google scores it 9.2 under CVSS version 4, SecurityWeek reported. Node.js fixed it in its Oct. 21 security release, rated high and credited to OpenAI Codex Security researchers. The Python packages Brotli 1.2.1 and brotlicffi 1.2.1.0 are patched, as are Bun, Envoy and Istio, according to SecurityWeek. PHP issued updated Windows builds Monday bundling Brotli 1.2.1.
How Dangerous It Is
No exploitation in the wild has been reported. Imperva, in vendor research described by SecurityWeek, achieved code execution against the official Node.js 24.22.0 image for Linux on arm64 processors using malicious data handled by the Express 5 or body-parser 2 web components, though not on every attempt; it has not published its exploit. A public proof of concept works only on a deliberately weakened build, and some researchers argue the bug is practical only for crashing services.
An attack needs malicious Brotli data to reach the decoder, for example in compressed web requests, responses from untrusted servers or compressed files. OpenAI said, according to SecurityWeek, that the root cause had been fixed in Brotli's source code in February without anyone recognizing it as a security issue.
Why It Matters
Brotli is embedded in web servers, runtimes and data tools, so the fix has to travel through many products, and installing it is not the end of the job. Microsoft says self-contained .NET apps must be rebuilt and republished, containers rebuilt on updated images and framework-dependent apps restarted; as SecurityWeek noted, running processes keep using the vulnerable code until then. For leaders, that makes the flaw a test of software inventory: whether the organization knows where its compression libraries live.
What to Watch
A public tracker cited by SecurityWeek still listed DuckDB, Apache Arrow, 7-Zip-zstd, p7zip and EDK II as unpatched, and its maintainers say the list is incomplete. Watch for fixes there, any published working exploit, reports of attacks and any action by U.S. cybersecurity authorities.
