SameSite cookie attribute

Cookies are text strings sent from websites and stored on a computer by the web browser. They're used for authentication and personalization. For example, cookies are used to recall stateful information, preserve user settings, record browsing activity, and display relevant ads. Cookies are always linked to a particular domain and are installed by various parties.

Types of cookies

The cookie types and their corresponding scopes are as follows:

Cookie Scope
First party cookie A first party cookie is created by websites that a user visits. It's used to save data, such as shopping cart items, sign in credentials. For example, authentication cookies, and other analytics.
Second party cookie A second party cookie is technically the same as a first party cookie. The difference is that data is shared with a second party through a data partnership agreement. For example, Microsoft Teams analytics and reporting.
Third party cookie A third party cookie is installed by a domain other than the one the user explicitly visited and is used for tracking. For example, Like buttons, ad serving, and live chats.

Cookies and HTTP requests

Before the introduction of SameSite restrictions, the cookies were stored on the browser. They were attached to every HTTP web request and sent to the server by the Set Cookie HTTP response header. This method introduced security vulnerabilities, such as Cross Site Request Forgery, called CSRF attacks. The SameSite component reduced the exposure through its implementation and management in the SetCookie header.

Google Chrome version 51 introduced the SetCookie SameSite specification as an optional attribute. Starting with Build 17672, Windows 10 introduced SameSite cookie support for the Microsoft Edge browser.

You can opt out of adding the SameSite cookie attribute to the SetCookie header or add it with one of two settings, Lax and Strict. An unimplemented SameSite attribute was considered the default state.

Chrome 80, released in February 2020, introduces new cookie values and imposes cookie policies by default. Three values are passed into the updated SameSite attribute: Strict, Lax, or None. If not specified, cookies SameSite attribute takes the value SameSite=Lax by default.

SameSite cookie attributes are as follows:

Setting Enforcement Value Attribute Specification
Lax Cookies are sent automatically only in a first party context and with HTTP GET requests. SameSite cookies are withheld on cross site sub requests, such as calls to load images or iframes. They sent when a user navigates to the URL from an external site, for example, by following a link. Default Set-Cookie: key=value; SameSite=Lax
Strict The browser only sends cookies for first party context requests. These are requests originating from the site that set the cookie. If the request originated from a different URL than that of the current location, none of the cookies tagged with the Strict attribute are sent. Optional Set-Cookie: key=value; SameSite=Strict
None Cookies are sent in both first party context and cross origin requests; however, the value must be explicitly set to None and all browser requests must follow the HTTPS protocol and include the Secure attribute, which requires an encrypted connection. Cookies that don't adhere to that requirement are rejected.
Both attributes are required together. If None is specified without Secure or if the HTTPS protocol isn't used, then the third party cookies are rejected.
Optional, but, if set, the HTTPS protocol is required. Set-Cookie: key=value; SameSite=None; Secure

Teams implications and adjustments

  1. Enable the relevant SameSite setting for your cookies and validate that your apps and extensions continue to work in Teams.
  2. If your apps or extensions fail, make the necessary fixes prior to the Chrome 80 release.
  3. Microsoft internal partners can join the following team for more information or help with this issue: https://teams.microsoft.com/l/team/19%3A08b594cd465e4c0491fb751e823802e2%40thread.skype/conversations?groupId=4d6d04cd-dbf0-43c8-a2ff-f80dd38be034&tenantId=72f988bf-86f1-41af-91ab-2d7cd011db47.

Note

You must set SameSite attributes to reflect the intended use for your cookies. Do not rely on default browser behavior. For more information, see Developers: Get Ready for New SameSite=None; Secure Cookie Settings.

Tabs, task modules, and message extensions

  • Teams tabs use <iframes> to embed content that is viewed at a top level or first party context.
  • Task modules allow you to create modal popup experiences in your Teams application. Similar to a tab, a modal window opens inside the current page.
  • Message extensions allow you to insert enriched content into a chat message from external resources.

Any cookies used by embedded content are considered as third party when the site is displayed in an <iframe>. In addition, if any remote resources on a page rely on cookies being sent with a request <img> and <script> tags, external fonts, and personalized content, you must ensure those are marked for cross site usage, such as SameSite=None; Secure or ensure that a fallback is in place.

Authentication

You must use the web based authentication flow for the following:

  • Embedded content pages in tabs.
  • Configuration page, task module, and message extension.
  • Conversational bot with a task module.

According to the updated SameSite restrictions, a browser doesn't add a cookie to an already authenticated web site if the link derives from an external site. You must ensure your authentication cookies are marked for cross site usage SameSite=None; Secure or ensure that a fallback is in place.

Android System WebView

Android WebView is a Chrome system component that allows Android apps to display the web content. While the new restrictions are default, starting with Chrome 80, they aren't immediately enforced on WebViews. They'll be applied in the future. To prepare, Android allows native apps to set cookies directly through the CookieManager API.

Note

  • You must declare first party cookies as SameSite=Lax or SameSite=Strict, as appropriate.
  • You must declare third party cookies as SameSite=None; Secure.

See also