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silverpill@silverpill@mitra.social (2026-05-24 04:52:17)
FEP-fe34 (Origin-based security model) update : https://codeberg.org/fediverse/fep/pulls/849

I tried to better explain the assumptions on which the model is based, and clarified how exactly origins should enforce boundaries between actors:

Servers MUST ensure that activities published by a client do not represent unauthorized actions. This includes activities embedded within other activities and objects.

Servers MUST NOT allow clients to publish activities where embedded objects are owned by another actor.

Lemmy API and Mastodon API implementers don't have to worry about this, but one needs to be very careful when accepting arbitrary payloads from clients, for example, when implementing ActivityPub C2S API or FEP-ae97 API. Unfortunately, these security issues are completely ignored by people who push for wide deployment of ActivityPub C2S API.

Another addition is the recommendation to not use partially embedded objects, because that might lead to cache poisoning:

Embedded non-anonymous objects SHOULD NOT be partial representations. A server that relies on embedding for authentication might save a partial representation of an object to the cache, replacing the full object.

(see this issue for details: https://codeberg.org/silverpill/feps/issues/21)

#fep_fe34 #activitypub
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silverpill@silverpill@mitra.social (2026-05-24 04:56:26)
@phnt Following on one of our conversations, I now call out authorized fetch as ineffective when it is used on public objects:

Some servers require an HTTP signature in an attempt to limit access to public objects. In this scenario, the request is expected to be signed with a key that is owned by a server actor. However, this measure is ineffective and can easily be circumvented by changing the keyId parameter of a signature.

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Phantasm@phnt@fluffytail.org (2026-05-24 05:20:49)
@silverpill 
>this measure is ineffective and can easily be circumvented by changing the keyId parameter of a signature.

I never thought about it like that, but that too is a way to circumvent it. Although you would still need some way to publish that key and a valid Actor for verification, a server. If a remote server implementing restrictions on fetching based on signatures only disallows the instance Actor, then that is a way to bypass that restriction. Although I think there currently is no implementation that does that. Mastodon instead blocks everything on the domain including all subdomains and GTS probably does the same. No idea how the Misskey forks and Iceshrimp.NET do it though. Of course using different domains works as well.

>Servers MUST NOT allow clients to publish activities where embedded objects are owned by another actor.

Unrelated to the this FEP, but this came up when fixing the recent Pleroma security issues. There is no agreed upon way of federating moderation decisions to remote instances. It is logical when validating remote Update Activities to only allow Activities that update Objects owned by the same Actor, however that is never guaranteed when for example an admin on a remote instance forces a post to be NSFW. Similarly Delete Activities can have the Actor be the moderator, but Object actually owned by user.