Summary

An attacker can make a Kitsune server recognize that they have an account with arbitrary content on a victim server's domain, if the victim server allows arbitrary users to upload documents.

Details

Kitsune's ActivityPub fetcher doesn't check the response's Content-Type, and accepts it even if the server doesn't mean that it's an Activity Streams document.

Affected code:

• kitsune/crates/kitsune-core/src/activitypub/fetcher.rs

  Line 92 in 738d21c

  pub async fn fetch_actor(&self, opts: FetchOptions<'_>) -> Result<Account> {

There are some possible solutions to this problem:

• Check Content-Type of the response
• Mandate WebFinger support on the federating servers (like Mastodon does. Possibly as part of #349)

I prefer the former option because it seems to be more faithful to the ActivityPub standard, but the ActivityPub recommendation doesn't state that a federating server MAY reject a response without proper Content-Type either, AFAICT.

PoC

None yet.

Impact

For example, an attacker can create a GitHub Gist containing an Activity Streams document with preferredUsername being GitHub, so that they can assume a false handle name of @GitHub@gist.github.com.

AT Protocol has (or had?) a similar problem. Mastodon isn't affected by this because it mandates WebFinger support.