WAF rule statement unable to match Header

Question

Hello,  
I'd like to block requests that contain a specific Header "X-Forwarded-Host" using a rule in my Web ACL by using a "Size match condition" (like it is also demonstrated in Option 2 from this guide:  )  
  
I tired four scenarios but none was able to match the requests where the Header was set.  
1. If matches: Size greater than or equal to 0 _-> matches nothing_  
2. if matches: Size less than or equal to 0 _-> matches nothing_  
3. If NOT matches: Size greater than or equal to 0 _-> matches every request without the header_  
4. if NOT matches: Size less than or equal to 0 _-> matches every request without the header_  
  
Expected behavior would be that 1. or 4. would match the requests.  
Am I overlooking something here or is there a different solution?  
  
Thank you for your time

Answer

OP here (somehow I had to set a new username)  
  
I was able to solve this issue. The rule was not working because of it's priority in respect to another whitelisting rule. Once it was moved before the whitelisting the filtering works as expected.

WAF rule statement unable to match Header

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