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Glossary of Security Terms: CORSby@mozilla
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Glossary of Security Terms: CORS

by Mozilla ContributorsAugust 19th, 2020
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CORS (Cross-Origin Resource Sharing) is a system, consisting of transmitting HTTP headers, that determines whether browsers block frontend JavaScript code from accessing responses for cross-origin requests. CORS gives web servers the ability to say they want to opt into allowing cross-Origin access to their resources. The same-origin security policy forbids cross-origest access to resources. Mozilla (stylized as moz://a) founded in 1998 by members of Netscape. by Mozilla Contributors.

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CORS (Cross-Origin Resource Sharing) is a system, consisting of transmitting HTTP headers, that determines whether browsers block frontend JavaScript code from accessing responses for cross-origin requests.

The same-origin security policy forbids cross-origin access to resources. But CORS gives web servers the ability to say they want to opt into allowing cross-origin access to their resources.

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General knowledge

CORS headers

Access-Control-Allow-Origin

Indicates whether the response can be shared.

Access-Control-Allow-Credentials

Indicates whether or not the response to the request can be exposed when the credentials flag is true.

Access-Control-Allow-Headers

Used in response to a preflight request to indicate which HTTP headers can be used when making the actual request.

Access-Control-Allow-Methods

Specifies the method or methods allowed when accessing the resource in response to a preflight request.

Access-Control-Expose-Headers

Indicates which headers can be exposed as part of the response by listing their names.

Access-Control-Max-Age

Indicates how long the results of a preflight request can be cached.

Access-Control-Request-Headers

Used when issuing a preflight request to let the server know which HTTP headers will be used when the actual request is made.

Access-Control-Request-Method

Used when issuing a preflight request to let the server know which HTTP method will be used when the actual request is made.

Origin

Indicates where a fetch originates from.

Technical reference

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