What is the BROWSER file format?
A .browser file is an XML configuration file used by ASP.NET (versions 2.0 through 4.x) to define browser capabilities. It maps User-Agent string patterns to a set of capability flags - JavaScript support, CSS level, cookie handling, screen dimensions - that the ASP.NET runtime uses to adapt server-rendered output to specific clients.
The file follows a hierarchical schema with a <browsers> root element containing <browser> child elements. Each element declares a User-Agent regex pattern, optionally inherits from a parent browser definition via parentID, and lists key-value capability pairs. Files are stored in App_Browsers/ within the web project (application-level overrides) or in the machine-wide directory at %windir%\Microsoft.NET\Framework\v4.0.30319\CONFIG\Browsers\.
At application startup, ASP.NET compiles all .browser files into a BrowserCapabilitiesFactory class. The mechanism replaced the older <browserCaps> section in web.config when ASP.NET 2.0 launched in November 2005.
This system became obsolete as User-Agent sniffing proved unreliable across the proliferation of browsers and devices. ASP.NET Core and .NET 5+ removed BrowserCapabilities entirely in favour of client-side feature detection. Existing .browser files remain relevant only for applications still targeting .NET Framework 4.x on IIS.
Security & safety
RISK: LOWXML text file with no executable code; configuration data only. No known security risk from viewing or editing. Malformed entries would affect server-side browser detection at worst.
Format details
in a nutshellPrograms that open BROWSER files
Windows3 apps
Microsoft Visual Studio
Freemium Open the .browser file in Solution Explorer or File → Open. IntelliSense and schema validation available. 
Visual Studio Code
Open-source Open .browser file directly; XML syntax highlighting with XML extension. 
Technical details
deep spec| Format | XML (UTF-8 plain text) |
| Root element | <browsers> |
| Child element | <browser> - attributes: id, parentID, defaultStateID |
| User-Agent matching | Regex patterns declared in <identification> child elements |
| Capability storage | Key-value pairs in <capabilities> child elements (JavaScript, CSS, cookies, screen dimensions, etc.) |
| Inheritance | Browser entries inherit parent definitions via parentID attribute |
| Machine-level path | %windir%\Microsoft.NET\Framework\v4.0.30319\CONFIG\Browsers\ |
| App-level path | App_Browsers\ folder inside the ASP.NET web project |
| Compilation | ASP.NET compiles .browser files into BrowserCapabilitiesFactory class at application startup |
| Schema version | ASP.NET 2.0+ uses .browser files; 1.x predecessor used <browserCaps> in web.config |
| Associated OS | Windows (IIS + ASP.NET .NET Framework) |
| Typical file size | 1 KB - 50 KB |
| Deprecated in | .NET Core / .NET 5+ - BrowserCapabilities mechanism fully removed |
| Replacement | Client-side feature detection via browser native APIs and libraries such as Modernizr |
| Released | ASP.NET 2.0 (2005) |
| Latest version | ASP.NET 4.x (last significant update; deprecated in .NET Core/.NET 5+) |
| Specification | learn.microsoft.com |
BROWSER conversions
Community Q&A
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