User-Agent strings are added to almost all HTTP queries on the Internet. They are designed to uniquely identify the particular browser, tool, or other piece of software generating the query. BlueCoat Proxy recently ran into a scenario where it was rewriting all client User-agent strings to “Mozilla/4.0 (compatible;)” which was generating false positives with SID 21444. If you’ve got an IDS inspecting client traffic headed out to the Internet, you’re going to be missing valuable data.”]
Source: https://blog.talosintelligence.com/2012/11/web-proxies-user-agent-strings-and.html