CORS configuration in Supabase - Step-by-Step Execution

Process Flow - CORS configuration

Client sends request

↓

Browser sets Origin

↓

Request reaches Supabase API

↓

Supabase checks allowed origins

↓

Response sent

↓

Browser accepts

When a client sends a request, the browser includes the Origin header. Supabase checks if the origin is allowed. If allowed, it responds with CORS headers; if not, the browser blocks the response due to missing CORS headers.

Execution Sample

Supabase

// Origin: https://example.com (automatically set by browser)
await supabase.from('test').select('*');

This simulates a client request from 'https://example.com' to Supabase with an Origin header.

Process Table

Step	Action	Origin Header	CORS Allowed Origins	CORS Check Result	Request Outcome
1	Client sends request	https://example.com	https://example.com, https://app.example.com	Origin matches allowed list	Request proceeds
2	Supabase receives request	https://example.com	https://example.com, https://app.example.com	Origin allowed	Response sent with CORS headers
3	Browser receives response	https://example.com	https://example.com, https://app.example.com	CORS headers present	Browser accepts response
4	Client sends request	https://malicious.com	https://example.com, https://app.example.com	Origin not in allowed list	Request blocked
5	Browser blocks response	https://malicious.com	https://example.com, https://app.example.com	No CORS headers	Browser blocks response

💡 Requests from origins not in the allowed list are blocked by the browser due to missing CORS headers.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
Origin Header	None	https://example.com	https://example.com	https://example.com	https://malicious.com	https://malicious.com
CORS Allowed Origins	https://example.com, https://app.example.com	https://example.com, https://app.example.com	https://example.com, https://app.example.com	https://example.com, https://app.example.com	https://example.com, https://app.example.com	https://example.com, https://app.example.com
CORS Check Result	None	Allowed	Allowed	Allowed	Not Allowed	Not Allowed
Request Outcome	None	Proceed	Response Sent	Accepted	Blocked	Blocked

Key Moments - 2 Insights

Why does the browser block the request from 'https://malicious.com' even though the request reaches Supabase?

Because 'https://malicious.com' is not in the allowed origins list, Supabase does not send CORS headers. The browser sees this and blocks the response as shown in steps 4 and 5.

What happens if the Origin header matches one of the allowed origins?

Supabase includes CORS headers in the response, so the browser accepts the response and allows the client to use it, as shown in steps 1 to 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the CORS Check Result at Step 4?

A Allowed

B Not Allowed

C Pending

D Unknown

💡 Hint

Check the 'CORS Check Result' column for Step 4 in the execution table.

At which step does the browser accept the response?

A Step 3

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Request Outcome' column to find when the browser accepts the response.

If we add 'https://malicious.com' to the allowed origins, what changes in the execution table?

A Step 1 Origin Header changes

B Step 5 Request Outcome remains Blocked

C Step 4 CORS Check Result becomes Allowed

D No changes occur

💡 Hint

Adding an origin to allowed origins affects the CORS Check Result for requests from that origin.

Concept Snapshot

CORS configuration controls which website origins can access Supabase APIs.
Set allowed origins in Supabase settings.
Browser sends Origin header with requests.
Supabase responds with CORS headers if origin allowed.
Browser blocks responses without matching CORS headers.
Proper CORS setup prevents unauthorized cross-site requests.

Full Transcript

CORS configuration is a security feature that controls which websites can access Supabase APIs. When a client sends a request, the browser includes the Origin header. Supabase checks if this origin is in its allowed list. If yes, it sends back CORS headers allowing the browser to accept the response. If not, the browser blocks the response to protect the user. This flow ensures only trusted websites can interact with Supabase resources.