Problem Solved: The WSUS Export Bug

If you are managing updates for Windows Server 2012, you might have noticed that your WSUS server has been synchronizing a greater than usual number of updates these days. Windows Server 2012 is not only Microsoft’s most powerful, but also Microsoft’s most secure operating system to date. Since RTM, we’ve published dozens of updates that improve the security, reliability, and  functionality of Windows. Most notably, the Defender team has been publishing virus and malware definition updates 4 times a day, to ensure that your PC is never left unprotected from even the latest threats.

We’ve also added many new products to WSUS, including updates to Adobe Flash Player, Skype, Lync Server, and Office 2013. There are more WSUS updates than ever before.

All these updates certainly can take a toll on a WSUS server, especially on servers that have auto-approval rules. Many administrators were recently surprised to see that exporting updates from WSUS servers was failing, resulting in zero-size output files. It became clear to us rather quickly that the issue was due to a limitation in the CAB file format, which is an uncompressed file size limit of 2 GB.

I am pleased to announce that, as of today, a fix for this issue is available from Microsoft. An article describing how to get this update is available at:

Many people have been asking on the forums and even through this blog for this fix for some time, and were wondering why it took more than 4 months to complete. To that end, I thought it would be interesting to share some of the engineering story as well.

One of the unique challenges of working on the WSUS team, compared to other server role teams in Windows Server, is that our role actually ships, “out-of box,” all the way back to Windows Server 2003 R2. We had to be very careful to design a solution that would work on all versions of Windows Server since then. To keep everything as simple as possible, we constrained ourselves on using compression algorithms that are already publicly available in the .NET framework or public Win32 API. Here is what we considered:


Max Size




.NET 4.0

Unlimited, 4 GB prior to .NET4

Built in CRC

Results in 2 files

No support for signtool


.NET 4.0

Unlimited, not available until .NET4

Supports larger file sizes than CAB

Results in 2 files, harder to work with raw DEFLATE format vs. Gzip, no support for signtool


.NET 2.0

2 GB uncompressed file limit

Built in CRC

Same file format

Supported by signtool

Temp files needed (extra disk I/O), 2 GB limit per uncompressed file


.NET 4.5


Built in CRC

No CLR support for Windows Server 2003 R2; no support for signtool

We decided it wouldn’t be a great customer experience to require people to install .NET 4.5 on their servers. Plus, we wanted to preserve compatibility with Windows Server 2003 R2 if at all possible. .NET 4.5 is not available on Windows Server 2003 R2. We could have also adapted the CAB format to produce multiple output files, but we preferred to produce only a single output file, especially since the resulting files are comparatively small.  On systems running the latest version of .NET 4.0, the maximum file size is limited only by NTFS file size. While Gzip does store the length of the compressed content using 32 bits, the structure stores only the lowest 32-bits thereby allowing for larger file sizes, limited only by NTFS file size (prior to .NET 4.0, the length of the compressed content couldn’t exceed 32 bits, or about 4 GB). This does mean that there’s a limit of 4 GB on systems using .NET 3.5 (i.e., WSUS 3.0 SP2 (3.2) ). Windows Server 2012 is on .NET 4.5 and therefore not affected by the limit. Similar to CAB, GzipStream also includes built-in CRC error detection, which makes the export and import processes highly reliable. Best of all, the compressed output is generated on-the-fly, without the need for temporary uncompressed output files. This greatly decreases disk I/O and we found it reduces the time needed for import and export significantly.

Using our current export algorithm, the export process results in 2 files, a metadata.txt file and an file, which must be kept together. The CAB format archives these two files together. However, since Gzip is not an archive format, the end user would have needed to keep these files together manually. We would have preferred to produce only a single export file to ease distribution. Therefore, we changed the WSUS exported data XML schema to allow for both metadata and update data in a single compressed file.

In parallel, we also worked with our test engineers to formulate a test plan for the hotfix. Our test matrix is large, as we need to ensure that every supported version of Windows Server will be able to install this update. For Windows Server 2003 R2, that matrix is made even bigger by our support for both x86 and x64 architectures. We also worked with Customer Support Services (CSS) to gather validation data with regard to cases that were currently opened about this issue. And of course, nothing works perfectly the first time, and we’ve gone through several revisions to make sure that the latest WSUS update is more robust than ever!

At this point, we have a hotfix that we feel really good about releasing: well-informed by customer needs, developed by our esteemed software engineers, and thoroughly tested and validated. I hope that it will improve the experience of the many WSUS admins around the world, and I look forward to hearing your feedback about this update.

Thanks for being a valued Windows Server and WSUS customer!


The WSUS Team