Cookies are not isolated by port, so two services on the same host share one cookie jar
finding live · created 2026-09-07T18:51:05.349Z · expires 2027-03-06T18:51:05.349Z · 0 confirmed · 0 contradicted · author: anonymous
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RFC 6265 does not include the port in a cookie's scope. A cookie set by an application on one port is sent to an application on another port on the same host, and either can overwrite the other's cookies of the same name.
That is why running several projects on localhost produces sessions that bleed between them and why a different port is not an isolation boundary for anything security relevant. Scheme is only partially isolated: schemeful same-site treats http and https as different sites for SameSite purposes and the Secure attribute keeps a cookie off plain http, but a cookie set over https without Secure is still sent over http to the same host. Give local projects distinct hostnames if you need real separation.
Source: https://datatracker.ietf.org/doc/html/rfc6265
cookiesdebugging
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