starlette 'access-control-allow-origin' header seems to

The Fix

Upgrade to version 0.13.4 or later.

Based on closed Kludex/starlette issue #510 · PR/commit linked

Jump to Verify Open PR/Commit

@@ -98,7 +98,6 @@ def preflight_response(self, request_headers: Headers) -> Response:
         requested_method = request_headers["access-control-request-method"]
         requested_headers = request_headers.get("access-control-request-headers")
-        requested_cookie = "cookie" in request_headers
 
         headers = dict(self.preflight_headers)

repro.py

import uvicorn

from starlette.applications import Starlette
from starlette.responses import PlainTextResponse
from starlette.middleware.cors import CORSMiddleware


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


@app.route("/")
def homepage(request):
    return PlainTextResponse("Homepage", status_code=200)


if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=5100)

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

fix.md

Option A — Upgrade to fixed release\nUpgrade to version 0.13.4 or later.\nWhen NOT to use: Do not apply this fix if the application requires retaining the origin for specific requests.\n\n

Why This Fix Works in Production

Trigger: 'access-control-allow-origin' header seems to "leak" between requests
Mechanism: The 'Access-Control-Allow-Origin' header incorrectly retains the origin from previous credentialed requests
Why the fix works: Fixes a CORS issue where the 'Access-Control-Allow-Origin' header leaks between credentialed and non-credentialed requests. (first fixed release: 0.13.4).

Production impact:

If left unfixed, this can cause silent data inconsistencies that propagate (bad cache entries, incorrect downstream decisions).

Why This Breaks in Prod

The 'Access-Control-Allow-Origin' header incorrectly retains the origin from previous credentialed requests
Production symptom (often without a traceback): 'access-control-allow-origin' header seems to "leak" between requests

Proof / Evidence

GitHub issue: #510
Fix PR: https://github.com/encode/starlette/pull/513
First fixed release: 0.13.4
Reproduced locally: No (not executed)
Last verified: 2026-02-09
Confidence: 0.75
Did this fix it?: Yes (upstream fix exists)
Own content ratio: 0.57

Discussion

High-signal excerpts from the issue thread (symptoms, repros, edge-cases).

Jump to Sources Open on GitHub

“Re "cookies": the var here is not used: https://github.com/blueyed/starlette/blob/1fc5cee3d603fa917facf66cf8259c5e05635c10/starlette/middleware/cors.py#L101”

@blueyed · 2019-05-13 · source

“@steinnes Can you check https://github.com/encode/starlette/pull/513, please?”

@blueyed · 2019-05-13 · source

Failure Signature (Search String)

'access-control-allow-origin' header seems to "leak" between requests

Copy-friendly signature

signature.txt

Failure Signature
-----------------
'access-control-allow-origin' header seems to "leak" between requests

Error Message

Signature-only (no traceback captured)

error.txt

Error Message
-------------
'access-control-allow-origin' header seems to "leak" between requests

Minimal Reproduction

repro.py

import uvicorn

from starlette.applications import Starlette
from starlette.responses import PlainTextResponse
from starlette.middleware.cors import CORSMiddleware


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


@app.route("/")
def homepage(request):
    return PlainTextResponse("Homepage", status_code=200)


if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=5100)

What Broke

Non-credentialed requests receive incorrect CORS headers, leading to potential security issues.

Why It Broke

The 'Access-Control-Allow-Origin' header incorrectly retains the origin from previous credentialed requests

Fix Options (Details)

Option A — Upgrade to fixed release Safe default (recommended)

Upgrade to version 0.13.4 or later.

When NOT to use: Do not apply this fix if the application requires retaining the origin for specific requests.

Use when you can deploy the upstream fix. It is usually lower-risk than long-lived workarounds.

Fix reference: https://github.com/encode/starlette/pull/513

First fixed release: 0.13.4

Last verified: 2026-02-09. Validate in your environment.

When NOT to Use This Fix

Do not apply this fix if the application requires retaining the origin for specific requests.

Verify Fix

verify

Re-run the minimal reproduction on your broken version, then apply the fix and re-run.

Did This Fix Work in Your Case?

Quick signal helps us prioritize which fixes to verify and improve.

Prevention

Capture the exact failing error string in logs and tests so you can reproduce via a minimal script.
Pin production dependencies and upgrade only with a reproducible test that hits the failing path.

Version Compatibility Table

Version	Status
0.13.4	Fixed

Related Issues

No related fixes found.

Cluster: starlette:data-consistency All hubs All clusters

Related clusters: Configuration error SSL handshake Other

Sources

We don’t republish the full GitHub discussion text. Use the links above for context.

starlette 'access-control-allow-origin' header seems to "leak" between requests (Fix)