Duplicate Machine SID’s are not an issue except when they are an issue.

I came across an article on InfoWorld about a blog post from a Microsoft tech regarding Windows Machine SID’s and the myths that surround them.  The InfoWorld article is mostly fluff, but the blog post is well worth the read.  Basically, machines that are imaged without being sysprepped usually have the same machine SID’s.  It’s long been believed that this is a security issue.  It turns out that’s not the case.  Machines can be on the same network with the same SID”s if the machine is not already connected to a domain, not going to be promoted to a Domain Controller, and if there isn’t an application that reacts badly to it.  (The example given is applications that use the Machine SID as their own ID’s.)  The bottom line is that machines SHOULD BE SYSPREPED to prevent any known and unknown issues.  Also, Microsoft will not support machines that don’t have unique SID’s.  Sysprep is easy to run.  Don’t slack off just because it might not cause a problem.

The reason that I began considering NewSID for retirement is that, although people generally reported success with it on Windows Vista, I hadn’t fully tested it myself and I got occasional reports that some Windows component would fail after NewSID was used. When I set out to look into the reports I took a step back to understand how duplicate SIDs could cause problems, a belief that I had taken on faith like everyone else. The more I thought about it, the more I became convinced that machine SID duplication – having multiple computers with the same machine SID – doesn’t pose any problem, security or otherwise. I took my conclusion to the Windows security and deployment teams and no one could come up with a scenario where two systems with the same machine SID, whether in a Workgroup or a Domain, would cause an issue. At that point the decision to retire NewSID became obvious.

I realize that the news that it’s okay to have duplicate machine SIDs comes as a surprise to many, especially since changing SIDs on imaged systems has been a fundamental principle of image deployment since Windows NT’s inception. This blog post debunks the myth with facts by first describing the machine SID, explaining how Windows uses SIDs, and then showing that – with one exception – Windows never exposes a machine SID outside its computer, proving that it’s okay to have systems with the same machine SID. Note that Sysprep resets other machine-specific state that, if duplicated, can cause problems for certain applications like Windows Server Update Services (WSUS), so MIcrosoft’s support policy will still require cloned systems to be made unique with Sysprep.

You can read the full article here.

You should also read this follow-up post by Microsoft tech Aaron Margosis that explains the difference between Machine SID’s and Domain SID’s.  The key statement in his post: “So while it’s OK to clone a system before it joins a domain, doing so after it joins a domain (and is assigned a domain computer account and a corresponding domain SID) will cause problems.”

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One response to “Duplicate Machine SID’s are not an issue except when they are an issue.

  1. I am regular reader, how are you everybody? This paragraph posted at this website is actually fastidious.

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