stigmem-node: decay sweep expires and counts facts across all tenants (cross-tenant BOLA)

Summary

On a multi-tenant stigmem node, a caller holding a write credential for one tenant can run a decay sweep that acts on every tenant's facts. The candidate-selection queries in lifecycle/decay.py (_select_ttl_candidates, _select_confidence_candidates) carried no tenant_id predicate, and the caller's tenant was not threaded into the sweep or its async worker (run_decay_sweep / _decay_job_worker), reached via POST /v1/decay/sweep.

Impact

A sweep with ttl_seconds=0 expires all tenants' facts — cross-tenant data destruction (integrity and availability). A dry_run sweep returns a global candidate count, acting as a cross-tenant existence/volume oracle (information disclosure).

Affected configurations

This is a cross-tenant break. It is exploitable only on deployments running the opt-in stigmem-plugin-multi-tenant (multiple tenants on one node). A default single-tenant node has only tenant="default" — there is no second tenant to cross — so it is not exploitable on default deployments. The rating is HIGH for the multi-tenant deployments the plugin exists to isolate.

Patches

Fixed in 0.9.0a12 (PR #728): identity.tenant_id is threaded into run_decay_sweep and _decay_job_worker, and AND tenant_id = ? was added to the candidate selectors and the graph-sync. A tenant-B sweep now leaves tenant-A facts untouched, and dry_run counts only the caller's tenant. The check_fact_query_tenant_scope.py CI guard was extended to scan lifecycle/ so this class cannot silently regress.

Workarounds

None other than upgrading to 0.9.0a12. Single-tenant deployments are unaffected.