Microsoft have recently had a bit of a nightmare with a recent update. In update KB5066835, they have broken connectivity to localhost. This might not seem that important and you may have never heard of localhost. However, chances are your business relies on it more than you think. It’s one of those behind-the-scenes pieces of technology that keeps everyday systems running smoothly. Common examples include locally developed apps and dashboards, but interestingly we have heard of software installs failing and even peripherals not fully working due to this bug.
In simple terms, localhost refers to your own computer. It’s a special network address, usually 127.0.0.1 (though technically 127.anything.anything.anything can be localhost), that points back to the same machine you’re using. When something runs on localhost, it isn’t reaching out across the internet; it’s staying entirely on your device or your internal network. That makes it fast, secure, and ideal for local operations that don’t need to be exposed online.
Many business applications quietly use localhost to handle tasks in the background. For example, software that needs to store or process data locally might spin up a tiny server on localhost to manage communication between different parts of the program. That’s common in modern business tools like database management systems, internal dashboards, or even point-of-sale software.
IT teams also make heavy use of localhost for testing and troubleshooting. Before deploying a new system update or launching a website, they can safely test everything on localhost without risking downtime or data loss. It’s effectively a sandbox or a safe place to experiment, fix problems, and confirm that things work properly before rolling them out to your live environment.
Even cloud-connected services sometimes rely on localhost. Backup agents, monitoring tools and security scanners often run local services on your computers or servers, using localhost to handle communications internally before sending results to the cloud. It’s an efficient way to keep things secure and responsive.
Whilst Microsoft have acknowledged the issue and are going to push out a fix, the risk remains of people trying to either fix it themselves, risking further damage, or having to live with the issue until Microsoft do manage to push out the update! Our advice would be to hold off that specific update if you have the ability and await the fix from Microsoft.
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