How can I send multiple Set-Cookie headers from API Gateway using a proxied Lambda

Question

I'm using API Gateway's Proxy integration to call a Lambda. The output format specification is this follow JSON format:

{
  "statusCode": httpStatusCode,
  "headers": { "headerName": "headerValue", ... },
  "body": "..."
}

In one response I wish to set two cookies (two different auth cookies) but JSON doesn't allow having two identical keys in the headers object (OK, technically the spec does but most libraries do not).

RFC 7230 notes that Set-Cookie should be handled specially but I can't see how I can send multiple Set-Cookie values through API gateway.

Does anyone know whether this is possible?

Answer 1

Note: API gateway now has a version 2 payload that is fundamentally different from that described here (and the default for new APIs). Documentation on the differences here. See other accepted answer from Samuel for some more details.

As of November 2018 this is possible using the multiValueHeaders field in the response instead of headers (see announcement).

As an example instead of:

{
  "statusCode": 200,
  "body": "testing multiple set-cookie headers",
  "headers": {
    "X-Test-Header": "baking experiment",
    "Set-Cookie": "cookie1=chocolate-chip",
    "Set-Cookie": "cookie2=oatmeal",
    "Content-Type": "text/plain"
  }
}

You can respond with:

{
  "statusCode": 200,
  "body": "testing multiple set-cookie headers",
  "multiValueHeaders": {
    "X-Test-Header": ["baking experiment"],
    "Set-Cookie": ["cookie1=chocolate-chip", "cookie2=oatmeal"],
    "Content-Type": ["text/plain"]
  }
}

Note that you can use a mix of headers and multiValueHeaders:

{
  "statusCode": 200,
  "body": "testing multiple set-cookie headers",
  "headers": {
    "X-Test-Header": "baking experiment",
    "Content-Type": "text/plain"
  },
  "multiValueHeaders": {
    "Set-Cookie": ["cookie1=chocolate-chip", "cookie2=oatmeal"]
  }
}

However using the same header in both will mean that the value under headers is dropped.

See the documentation for more details.

When using only the header field (as available prior to Nov 2018) I tried sending the following manually curated JSON as a response:

{
  "statusCode": 200,
  "body": "testing multiple set-cookie headers",
  "headers": {
    "X-Test-Header": "baking experiment",
    "Set-Cookie": "cookie1=chocolate-chip",
    "Set-Cookie": "cookie2=oatmeal",
    "Content-Type": "text/plain"
  }
}

The cookies that API gateway returns in response to a CURL request are:

< Content-Type: text/plain
< Content-Length: 35
< Connection: keep-alive
< Date: Thu, 29 Sep 2016 11:22:09 GMT
< Set-Cookie: cookie2=oatmeal
< X-Test-Header: baking experiment
< X-Cache: Miss from cloudfront

As you can see the first Set-Cookie is dropped on the floor.

Answer 2

Please note that in 09/2022 AWS Api Gateway by default uses a new payload format (version 2.0) that no longer recognizes multiValueHeaders as proposed by all the top answers, including the accepted answer. I just spent 40 minutes trying to figure out why it doesn't return my cookies :).

From AWS Documentation:

Format 2.0 doesn't have multiValueHeaders or multiValueQueryStringParameters fields. Duplicate headers are combined with commas and included in the headers field. Duplicate query strings are combined with commas and included in the queryStringParameters field.

Format 2.0 includes a new cookies field. All cookie headers in the request are combined with commas and added to the cookies field. In the response to the client, each cookie becomes a set-cookie header.

https://docs.aws.amazon.com/apigateway/latest/developerguide/http-api-develop-integrations-lambda.html

So the actual solution should now be:

    return {
      statusCode: 302,
      headers: {
        Location: location,
      },
      cookies: [
         "cookie1=value1;",
         "cookie2=value2;"
      ]
    };

Answer 3

As answered, to date, API Gateway will drop identical keys, only setting one of the cookies.

However, a workaround exists. You can change the casing of the string 'Set-Cookie' so the keys are not unique. For example, you could use the keys set-cookie, Set-cookie, sEt-cookie, and the headers will be preserved and 3 different cookies would be set.

Because the RFC standard makes headers case-insensitive this should work with all RFC-compliant clients.

So, you could rewrite your set-cookie headers, permuting all the possible casings of "Set-Cookie" to get around this.

This technique (hack) is employed by Zappa, a popular serverless framework written in Python.

Answer 4

Use multiValueHeaders:

response.multiValueHeaders = {
  "Set-Cookie": [
    'cookie1=value1',
    'cookie1=value1'
  ]
}

or:

{
    "statusCode": httpStatusCode,
    "headers": { "headerName": "headerValue", ... },
    "multiValueHeaders": { "headerName": ["headerValue", "headerValue2",...], ... },
    "body": "..."
}

https://docs.aws.amazon.com/apigateway/latest/developerguide/set-up-lambda-proxy-integrations.html#api-gateway-simple-proxy-for-lambda-output-format

Answer 5

As Mark B pointed out, you can/should achieve this by setting multiple cookie name/value pairs in a single Set-Cookie header. The browser should interpret this correctly.

Cookie: a=1; b=2

Edit: as pointed out by OP, there are use cases that require multiple instances of the header. We've added it our backlog along with supporting multiple header names on incoming requests.

Answer 6

Couple of years late, but I just required to implement something like this and this is how I was able to make it work:

...

//15 minutes
var expirationTime = new Date(new Date().getTime() + 15 * 60 * 1000);
//30 minutes
var expirationTime2 = new Date(new Date().getTime() + 30 * 60 * 1000);

var response = {};

var cookies = [];
cookies.push("testCookie={'keyX':'valx', 'keyy':'valy'}; Expires=" + expirationTime + ";");
cookies.push("testCookie2={'key1':'val1', 'key2':'val2'}; Expires=" + expirationTime2 + ";");

response.headers["Set-Cookie"] =  cookies;

...

Each array item will be processed independently, so you can add as many cookies to the array with different settings.

i.e.

cookies.push("testCookie3={'key1':'val1', 'key2':'val2'}; Expires=" + expirationTime2 + "; Max-Age=...");

cookies.push("testCookie4={'key1':'val1', 'key2':'val2'}; Expires=" + expirationTime2 + "; Domain=<domain-value>; Path=<path-value>");