ensure that epoch of attestation slot matches the target epoch

specs/core/0_beacon-chain.md

@@ -1482,6 +1482,7 @@ def process_attestation(state: BeaconState, attestation: Attestation) -> None:
     data = attestation.data
     assert data.index < get_committee_count_at_slot(state, data.slot)
     assert data.target.epoch in (get_previous_epoch(state), get_current_epoch(state))
+    assert data.target.epoch == compute_epoch_at_slot(data.slot)
     assert data.slot + MIN_ATTESTATION_INCLUSION_DELAY <= state.slot <= data.slot + SLOTS_PER_EPOCH
 
     committee = get_beacon_committee(state, data.slot, data.index)

specs/core/0_fork-choice.md

@@ -307,6 +307,7 @@ def on_attestation(store: Store, attestation: Attestation) -> None:
     # Use GENESIS_EPOCH for previous when genesis to avoid underflow
     previous_epoch = current_epoch - 1 if current_epoch > GENESIS_EPOCH else GENESIS_EPOCH
     assert target.epoch in [current_epoch, previous_epoch]
+    assert target.epoch == compute_epoch_at_slot(attestation.data.slot)
 
     # Attestations target be for a known block. If target block is unknown, delay consideration until the block is found
     assert target.root in store.blocks

test_libs/pyspec/eth2spec/test/fork_choice/test_on_attestation.py

@@ -84,6 +84,29 @@ def test_on_attestation_past_epoch(spec, state):
     run_on_attestation(spec, state, store, attestation, False)
 
 
+@with_all_phases
+@spec_state_test
+def test_on_attestation_mismatched_target_and_slot(spec, state):
+    store = spec.get_genesis_store(state)
+    spec.on_tick(store, store.time + spec.SECONDS_PER_SLOT * spec.SLOTS_PER_EPOCH)
+
+    block = build_empty_block_for_next_slot(spec, state)
+    signed_block = state_transition_and_sign_block(spec, state, block)
+
+    # store block in store
+    spec.on_block(store, signed_block)
+
+    attestation = get_valid_attestation(spec, state, slot=block.slot)
+    attestation.data.target.epoch += 1
+    sign_attestation(spec, state, attestation)
+
+    assert attestation.data.target.epoch == spec.GENESIS_EPOCH + 1
+    assert spec.compute_epoch_at_slot(attestation.data.slot) == spec.GENESIS_EPOCH
+    assert spec.compute_epoch_at_slot(spec.get_current_slot(store)) == spec.GENESIS_EPOCH + 1
+
+    run_on_attestation(spec, state, store, attestation, False)
+
+
 @with_all_phases
 @spec_state_test
 def test_on_attestation_target_not_in_store(spec, state):

test_libs/pyspec/eth2spec/test/phase_0/block_processing/test_process_attestation.py

@@ -177,6 +177,20 @@ def test_invalid_index(spec, state):
     yield from run_attestation_processing(spec, state, attestation, False)
 
 
+@with_all_phases
+@spec_state_test
+def test_mismatched_target_and_slot(spec, state):
+    next_epoch(spec, state)
+    next_epoch(spec, state)
+
+    attestation = get_valid_attestation(spec, state)
+    attestation.data.slot = attestation.data.slot - spec.SLOTS_PER_EPOCH
+
+    sign_attestation(spec, state, attestation)
+
+    yield from run_attestation_processing(spec, state, attestation, False)
+
+
 @with_all_phases
 @spec_state_test
 def test_old_target_epoch(spec, state):