If I have a site (e.g. foo.com) and on the home page of foo.com, there is an image request where the src=bar.com..., will the cookies on the bar.com domain be sent to the bar.com servers?
6 Answers
Yes. HTTP doesn't distinguish between one kind of resource or another (image vs html).
The cookie will typically be included in any type of request, but the scenario you describe is what's known as a third-party cookie (that is, the cookie is set on a domain that is different than the domain of the loaded page) and most browsers offer a privacy setting to block third-party cookies.
A third-party cookie allows the owners of bar.com to place an image (say a banner ad) on foo.com and track the users of foo.com even though those users have never visited bar.com. This is a privacy concern and many users elect to block such cookies.
This question is old, but was the first result on Google for me, so I think it's worth clarifying how this works nowadays (2021).
When bar.com sets the cookie, they can specify a SameSite attribute.
If the cookie is set with SameSite=Lax (or the SameSite attribute is not specified), then the cookie will not be sent for requests for images/iframes/etc hosted on bar.com, but will be sent if the user clicks a link on your foo.com homepage that takes them to bar.com
If the cookie is set with SameSite=Strict, the cookie will not be included in requests to bar.com that originate from another webiste, including if the user clicks a bar.com link on foo.com.
If the cookie is set with SameSite=None, the cookie will be sent to bar.com, including requests for images.
Yes cookies are sent on all requests. This includes "img" and "script" as well as XMLHttpRquest calls from javascript and can be a security issue on script tags as scripts loaded by one website can load scripts from another site and will send their authentication cookies too. This can be exploited to steal data.
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