Article ID: 294217
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IIS cannot retrieve custom HTTP header variables that contain an underscore following "HTTP_" (for example, "HTTP_SM_TESTVARIABLE"). The value returned for the header variable is NULL.
Any underscore ( _ ) that follows "HTTP_" is incorrectly converted to a dash when IIS attempts to retrieve the custom header value for the variable, and therefore, the variable is not found.
Do not use an underscore in the custom HTTP header variable name.
Retrieve and parse the HTTP_ALL header variable for the custom HTTP header variable.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed at the beginning of this article.