CORS-safelisted request header

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A CORS-safelisted request header (also known as “simple header”) is one of the following HTTP headers:

• {{HTTPHeader("Accept")}} 
• {{HTTPHeader("Accept-Language")}} 
• {{HTTPHeader("Content-Language")}} 
• {{HTTPHeader("Content-Type")}} 
• {{HTTPHeader("Range")}} 

When containing only these headers (and values that meet the additional requirements laid out below), a request doesn’t need to send a {{glossary("preflight request")}}  in the context of {{Glossary("CORS")}} .

You can safelist more headers using the {{HTTPHeader("Access-Control-Allow-Headers")}}  header and also list the above headers there to circumvent the following additional restrictions.

Additional restrictions

CORS-safelisted headers must also fulfill the following requirements in order to be a CORS-safelisted request header:

• {{HTTPHeader("Accept-Language")}}  and {{HTTPHeader("Content-Language")}}  can only have values consisting of 0-9, A-Z, a-z, space or *,-.;=.
• {{HTTPHeader("Accept")}}  and {{HTTPHeader("Content-Type")}}  can’t contain a CORS-unsafe request header byte: 0x00-0x1F (except for 0x09 (HT), which is allowed), "():<>?@[\]{}, and 0x7F (DEL).
• {{HTTPHeader("Content-Type")}}  needs to have a MIME type of its parsed value (ignoring parameters) of either application/x-www-form-urlencoded, multipart/form-data, or text/plain.
• {{HTTPHeader("Range")}}  needs to have a value of a single byte range in the form of bytes=[0-9]+-[0-9]*. See the {{HTTPHeader("Range")}}  header documentation for more details.
• For any header: the value’s length can’t be greater than 128.

See also

• Related glossary terms:
  • {{Glossary("CORS-safelisted response header")}} 
  • {{Glossary("Forbidden header name")}} 
  • {{Glossary("Request header")}}