WebKit accepts SameSite=Lax and SameSite=Strict cookies in a same-site response if the request was made by a cross-site iframe

• Component: Frames
• Status: RESOLVED MOVED

[Jakob L]

Created attachment 438351 [details]
Safari accetps cookie loaded via 3rd-party frame, see https://github.com/jaylinski/safari-same-site-cookie for more screenshots

## Description of the issue

Safari doesn't send "SameSite=Lax" and "SameSite=Strict" cookies to a same-site if the same-site was loaded by a cross-site iframe (Which is the correct behavior.)
But: Safari *accepts* "SameSite=Lax" and "SameSite=Strict" cookies from a same-site if the same-site was loaded by a cross-site iframe. (Which is probably the wrong behavior.)

The behavior from Safari differs from the behavior of Chrome and Firefox.
Both Chrome and Firefox block "SameSite=Lax" and "SameSite=Strict" cookies if they came by a same-site loaded by a cross-site iframe.

### Example

|- a.tld
|-- [iframe] b.tld
|--- [iframe] a.tld (Set-Cookie: x=y; path=/; SameSite=Lax)

Safari will accept the `x`-cookie, while Chrome and Firefox reject it, because "it came from a cross-site response".

## Expected behavior

Safari does not accept "SameSite=Lax"-cookies loaded via a cross-site iframes and behaves like Chrome and Firefox.

## Actual behavior

Safari accepts "SameSite=Lax"-cookies loaded via a cross-site iframes.

## Relevant spec

https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-rfc6265bis-05#section-5.3.7.1

The spec only defines what to *send*, not what to *set*. So I guess Safari doesn't violate the spec, but the current behavior is still confusing.

## Additional information

I created a test case reduction in this repository:
https://github.com/jaylinski/safari-same-site-cookie
Please refer to the `readme.md` for how to set it up.

This was tested on latest Safari Technology Preview 131.

This is probably not a security issue, but it can create undesired side-effects.
In my case, the current behavior caused issues with overwritten session-cookies.

[Radar WebKit Bug Importer]

<rdar://problem/83447688>

[Brent Fulgham]

The cause of this bug is outside of WebKit, so resolving as MOVED. We believe we understand the cause, and hope to have news about a fix in the near future.