CORS middleware setup in FastAPI - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the effect of this CORS middleware configuration?

Consider this FastAPI app snippet with CORS middleware added. What will be the behavior regarding cross-origin requests?

FastAPI

from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware

app = FastAPI()

app.add_middleware(
    CORSMiddleware,
    allow_origins=["https://example.com"],
    allow_methods=["GET", "POST"],
    allow_headers=["X-Custom-Header"],
)

@app.get("/")
async def root():
    return {"message": "Hello"}

A Only GET and POST requests from https://example.com with header X-Custom-Header are allowed cross-origin.

B All origins can send any method requests with any headers cross-origin.

C No cross-origin requests are allowed because allow_credentials is not set.

D Only GET requests from any origin are allowed cross-origin.

💡 Hint

Look at the allow_origins, allow_methods, and allow_headers parameters.

The CORS middleware is configured to allow only requests from https://example.com origin. It permits only GET and POST methods and only the header X-Custom-Header. Other origins or methods will be blocked.

📝 Syntax

intermediate

Which option correctly adds CORS middleware allowing all origins and methods?

Select the correct code snippet to add CORS middleware in FastAPI that allows all origins and all HTTP methods.

FastAPI

from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware

app = FastAPI()

# Add CORS middleware here

A app.add_middleware(CORSMiddleware, allow_origins=["*"], allow_methods=["GET", "POST", "PUT", "DELETE", "PATCH", "OPTIONS"])

B app.add_middleware(CORSMiddleware, allow_origins=["*"], allow_methods=["*"])

C app.add_middleware(CORSMiddleware, allow_origins=["*"], allow_methods=["GET", "POST", "PUT", "DELETE"])

D app.add_middleware(CORSMiddleware, allow_origins="*", allow_methods="*")

💡 Hint

Check the types of allow_origins and allow_methods parameters and what values they accept.

The allow_origins and allow_methods parameters both accept lists of strings. Using ["*"] for both allows all origins and all HTTP methods, respectively. This is the standard way recommended in the FastAPI documentation. Option B only allows the listed methods (not truly all), option B misses some common methods, and option B passes strings instead of lists, causing a type error.

🔧 Debug

advanced

Why does this FastAPI app still block cross-origin requests despite adding CORS middleware?

Review the code and identify why cross-origin requests are blocked even though CORS middleware is added.

FastAPI

from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware

app = FastAPI()

app.add_middleware(
    CORSMiddleware,
    allow_origins=["https://allowed.com"],
    allow_methods=["GET", "POST"],
    allow_headers=["*"],
    allow_credentials=True
)

@app.get("/")
async def root():
    return {"message": "Hello"}

A allow_credentials=True requires allow_origins to be set to ["*"] to work properly.

B The allow_headers parameter set to ["*"] causes a syntax error, blocking requests.

C The frontend is sending requests from a different origin than https://allowed.com, so they are blocked.

D The middleware must be added after all route definitions to work.

💡 Hint

Check the origin of the requests compared to allow_origins list.

The CORS middleware only allows requests from origins listed in allow_origins. If the frontend origin is not exactly https://allowed.com, the request is blocked. The other options are incorrect because ["*"] is valid for allow_headers, allow_credentials does not require allow_origins to be ["*"], and middleware order does not affect CORS behavior.

🧠 Conceptual

advanced

What is the purpose of the allow_credentials parameter in FastAPI's CORS middleware?

Select the best explanation for what setting allow_credentials=True does in FastAPI's CORS middleware.

A It automatically adds Access-Control-Allow-Origin: * header to all responses.

B It disables all CORS protections and allows any cross-origin request.

C It restricts cross-origin requests to only those using HTTPS protocol.

D It allows cookies, authorization headers, or TLS client certificates to be included in cross-origin requests.

💡 Hint

Think about what credentials mean in web requests.

Setting allow_credentials=True allows the browser to send cookies, authorization headers, or TLS client certificates with cross-origin requests. This is necessary for authenticated requests across origins. It does not disable CORS or restrict protocols.

❓ state_output

expert

What is the value of the Access-Control-Allow-Methods header in this FastAPI response?

Given this FastAPI app with CORS middleware, what will be the exact value of the Access-Control-Allow-Methods header in the preflight OPTIONS response?

FastAPI

from fastapi import FastAPI
from fastapi.middleware.cors import CORSMiddleware

app = FastAPI()

app.add_middleware(
    CORSMiddleware,
    allow_origins=["https://site1.com", "https://site2.com"],
    allow_methods=["GET", "POST"],
    allow_headers=["Content-Type", "Authorization"]
)

@app.options("/")
async def options():
    return {"status": "ok"}

A "GET, POST"

B "GET,POST"

C "GET"

D "*"

💡 Hint

Check how FastAPI's CORS middleware formats the allow_methods header.

The Access-Control-Allow-Methods header lists allowed methods separated by commas and spaces. Since allow_methods is ["GET", "POST"], the header value will be "GET, POST". It is not joined without spaces or replaced by a wildcard.