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Microsoft issues out-of-band .NET patches for critical Brotli flaw
Microsoft released emergency .NET 8 through 11 updates Sunday for CVE-2026-111497, a critical Brotli decoder out-of-bounds write that Imperva found and that BleepingComputer said is not exploited in the wild.
Microsoft released out-of-band security updates for.NET on Sunday to patch a Brotli decoder vulnerability tracked as CVE-2026-111497, also called Broccoli2Shell, Node2Shell and HTTP2Shell, BleepingComputer reported Monday1.
The flaw is an out-of-bounds write in the decoder that expands data compressed with Google's Brotli algorithm and was found by Imperva Red Team, according to the report1. BleepingComputer said successful exploitation can crash processes, leak memory or, in some cases, allow remote code execution on systems with vulnerable Brotli binaries or libraries1. Microsoft's.NET blog describes a buffer overflow that may allow an unauthenticated attacker to read or corrupt process memory, which it said could lead to information disclosure, denial of service or elevation of privilege1.
A Microsoft advisory published Sunday also said attackers can remotely escalate privileges on unpatched.NET installs that use non-default configurations, the outlet reported1. Microsoft said applications may be affected if they process Brotli-compressed data through ASP.NET Core request decompression, HttpClient automatic response decompression or System.IO.Compression Brotli APIs1.
"Brotli decompression is not enabled by default in an out-of-the-box.NET or ASP.NET Core install," Microsoft said, as quoted by BleepingComputer1. Apps must opt in to those features or call the Brotli APIs directly1.
The company shipped updates for.NET 8 (8.0.33),.NET 9 (9.0.22),.NET 10 (10.0.14) and.NET 11 Release Candidate 3 and told customers to update as soon as possible1.
Proof-of-concept exploits for Broccoli2Shell have been published on GitHub, BleepingComputer said, and it had seen no reports of malicious exploitation in the wild1. A tracking site cited in the report warned that updating a system Brotli library does not fix apps that bundle their own copy1.
Until patches are applied, Microsoft and the report advised blocking incoming Brotli-compressed requests at a proxy or firewall or disabling Brotli decoding1. It is not independently confirmed here whether default.NET deployments are reachable without those opt-in settings1.
