CVE-2026-76216: Vikunja: Link-share principal-type confusion enables cross-account team removal, bot takeover, and roster disclosure

Vikunja’s web.Auth interface (pkg/web/web.go, single method GetID() int64) is satisfied by BOTH *user.User and *models.LinkSharing. A link-share’s GetID() returns the raw positive share.ID (pkg/models/link_sharing.go:83-85), which lives in the same positive autoincrement ID space as users.id. The safe negated form getUserID() = share.ID * -1 (link_sharing.go:126-128) exists but is NOT used at three permission sinks. As a result, a link-share principal with id N — which should have zero authority over teams or bot users — is treated as the user whose users.id == N at three permission checks that lack the a.(*LinkSharing) guard their sibling methods have. This is the same principal-type-confusion class as CVE-2026-68581 (GHSA-vvcv-vpph-h844), but at three code paths that advisory/fix never touched.