The "POST /api/forms/{formId}/actor/inbox" route - exposed publicly with "acl:"public"" - accepts an HTTP "Signature" header whose "keyId" parameter is a URL. "HttpSignatureService::verifySignature()" parses the header and immediately makes a server-side HTTP GET to that URL, before any cryptographic verification or URL validation. An unauthenticated remote attacker can therefore make YesWiki issue arbitrary outbound HTTP requests to any host the server can reach - internal services, cloud-metadata endpoints ("169.254.169.254"), intranet-only admin panels, etc. - and read enough back via timing and error-message oracles to scan ports, enumerate services, and (on a real cloud instance) reach IAM metadata. The only deployment-side precondition is that ActivityPub be enabled on at least one Bazar form ("bn_activitypub_enable = '1'").