Prefix reservation is a nuget.org feature where an owner reserves a package ID prefix, such as a company name followed by a dot. New packages under that prefix are rejected unless they come from the owner, and matching packages show a visual indicator. It helps you judge a package, but it does not prove the code is safe.
What happens when a prefix is reserved
According to NuGet's documentation:
- The reservation is tied to one or more owners on nuget.org.
- A new package whose ID matches the prefix is rejected unless it comes from those owners.
- Packages from the owners under that prefix get an indicator on nuget.org and in Visual Studio 2017 version 15.4 or later. In Visual Studio it appears only when a single feed is selected.
- Existing packages that match the prefix but belong to other owners are left unchanged, without the indicator.
There are advanced options too. An owner can delegate a sub prefix to another account, or mark a prefix as public so it shows the indicator without blocking other contributors.
What it tells you
If a package you are about to add carries the indicator, the identifying properties were checked against a reserved namespace. That makes a lookalike package, which uses a similar name from a different account, easier to spot.
What it does not tell you
- It does not scan the package for malware or flaws.
- It does not protect an account that gets taken over. An attacker with the owner's credentials publishes under the same reserved prefix.
- It does not remove older packages owned by someone else under the same prefix.
- It does not cover feeds other than nuget.org, unless the feed implements the same idea.
Treat it as one signal among several, next to signing, download history, repository links and maintainer activity.
How owners qualify
The NuGet team reviews applications against three questions: does the prefix clearly identify the owner, is it something common that should not belong to one owner, and would not reserving it cause confusion or harm. Prefixes shorter than four characters and generic words are discouraged. Applicants are also expected to publish packages with clear authorship, a license expression, and an embedded icon.
A consumer checklist
- Look for the indicator on any new dependency from a name you do not recognize.
- Check that the package ID matches the project or organization you intend.
- Compare spelling against the real project, because typosquats use near matches.
- Pair this with source mapping so internal names come only from your feed.
- Re-check packages whose ownership changes.
Try it yourself
Click through it at your own pace, or open it full screen. Notice that a trusted name and a vulnerable version can coexist, which is why identity checks and vulnerability checks are separate steps.
Safeguard's dependency scanning flags known vulnerabilities in the packages you use, and a free scan shows them.
Quick check
Does a reserved prefix indicator mean a package is malware free?
No. It speaks to ownership of the namespace, not to the contents of the package.
Can anyone reserve any prefix?
No. Applications are reviewed, and very short or generic prefixes are discouraged.
Sources
- ID prefix reservation, Microsoft Learn
- Best practices for a secure software supply chain, Microsoft Learn
Last reviewed: 19 October 2026.
