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9b0bd814ce
The host-binding seam (Inv_RowZero) must derive a community only from a
request's actual Host. An empty raw_host carries no community evidence,
yet bind_community passed it straight to the resolver. The schema does
not forbid an `host = ''` row in communities (0001_initial_schema.sql:
`host VARCHAR(255) NOT NULL`, unique index on `lower(host)`), so a
misconfigured/empty-host row plus a request with a missing, unreadable,
or whitespace-only Host header would silently bind to that community.
Guard before the resolver lookup: if normalize_host(raw_host) is empty,
return BindError::UnmappedHost. Reuses the existing variant so the
rejection is byte-identical to any other unmapped host — an
unauthenticated caller cannot probe whether an empty-host row exists.
Red-team finding (Attack 2, MEDIUM defense-in-depth) by Sami; her three
proof tests (empty, whitespace-only, plus a non-empty negative control)
are included un-ignored. Verified mutate->red->restore: removing the
guard reds both empty-host gates with the literal fence collapse
(TenantContext{community: X, host: ""}). Full cargo test -p buzz-relay
--lib: 381 passed, 0 failed.
Co-authored-by: Sami <sami@sprout-oss.stage.blox.sqprod.co>
Co-authored-by: Tyler Longwell <tlongwell@block.xyz>
Signed-off-by: Tyler Longwell <tlongwell@block.xyz>