Check your Microsoft.TeamFoundation.Framework.Server.dll version in the Application Tier\Web Services\bin folder to confirm your current patch level.
TFS 2015 Update 4 has been a stable "long-tail" version for many enterprises. However, older software is a prime target for vulnerabilities. Microsoft has released several patches—often referred to in documentation as or Update 4.2 —specifically to fix: TFS-Update_4a-pc.zip
This is non-negotiable. Ensure you have a consistent set of backups for all Azure DevOps/TFS databases. Check your Microsoft
If you are already patching Update 4, you are in a great position to perform a direct upgrade to the latest Azure DevOps Server. Use the command line with the /CheckInstall flag
Use the command line with the /CheckInstall flag (if available) or verify the file version in your installation directory to ensure the patch was applied correctly. The Path Forward: Time to Upgrade?