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A forbidden header name is the name of any HTTP header that cannot be modified programmatically; specifically, an HTTP request header name (in contrast with a Forbidden response header name).

Modifying such headers is forbidden because the user agent retains full control over them. Names starting with

`Sec-`

are reserved for creating new headers safe from APIs using Fetch that grant developers control over headers, such as

XMLHttpRequest

Forbidden header names start with

Proxy-

Sec-

, or are one of the following names:

```
Accept-Charset
```
```
Accept-Encoding
```
```
Access-Control-Request-Headers
```
```
Access-Control-Request-Method
```
```
Connection
```
```
Content-Length
```
```
Cookie
```
```
Cookie2
```
```
Date
```
```
DNT
```
```
Expect
```
```
Feature-Policy
```
```
Host
```
```
Keep-Alive
```
```
Origin
```
```
Proxy-
```
```
Sec-
```
```
Referer
```
```
TE
```
```
Trailer
```
```
Transfer-Encoding
```
```
Upgrade
```
```
Via
```

Note: The
User-Agent
header is no longer forbidden, as per spec — see forbidden header name list (this was implemented in Firefox 43) — it can now be set in a Fetch Headers object, or via XHR setRequestHeader(). However, Chrome will silently drop the header from Fetch requests (see Chromium bug 571722).

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Source: https://developer.mozilla.org/en-US/docs/Glossary/Forbidden_header_name
Published under Open CC Attribution ShareAlike 3.0 license

Glossary of Security Terms: Forbidden Header Name

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