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fix: tighten light-client zeroed-header checks to match spec #9419

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fix: tighten light-client zeroed-header checks to match spec #9419

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14 changes: 13 additions & 1 deletion packages/state-transition/src/lightClient/spec/processLightClientUpdate.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -5,7 +5,13 @@ import {pruneSetToMax} from "@lodestar/utils";
import {computeSyncPeriodAtSlot} from "../../util/epoch.js";
import {LightClientUpdateSummary, isBetterUpdate, toLightClientUpdateSummary} from "./isBetterUpdate.js";
import {type ILightClientStore, MAX_SYNC_PERIODS_CACHE, type SyncCommitteeFast} from "./store.js";
import {deserializeSyncCommittee, getSafetyThreshold, isSyncCommitteeUpdate, sumBits} from "./utils.js";
import {
deserializeSyncCommittee,
getSafetyThreshold,
isFinalityUpdate,
isSyncCommitteeUpdate,
sumBits,
} from "./utils.js";
import {validateLightClientUpdate} from "./validateLightClientUpdate.js";

export interface ProcessUpdateOpts {
Expand Down Expand Up @@ -44,7 +50,13 @@ export function processLightClientUpdate(
}

// Update finalized header
// Defense-in-depth: require isFinalityUpdate(update) so a non-finality update can never
// overwrite store.finalizedHeader, regardless of any future loosening of the zeroed-header
// checks in validateLightClientUpdate. Spec-faithful behavior also holds without this guard
// once update.finalizedHeader is guaranteed fully-zero in the non-finality case, but the
// guard makes the invariant local to this write site.
if (
isFinalityUpdate(update) &&
syncCommitteeTrueBits * 3 >= SYNC_COMMITTEE_SIZE * 2 &&
update.finalizedHeader.beacon.slot > store.finalizedHeader.beacon.slot
) {
Expand Down
12 changes: 7 additions & 5 deletions packages/state-transition/src/lightClient/spec/utils.ts
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Expand Up @@ -29,7 +29,6 @@ import type {LightClientStore, SyncCommitteeFast} from "./store.js";

export const GENESIS_SLOT = 0;
export const ZERO_HASH = new Uint8Array(32);
export const ZERO_PUBKEY = new Uint8Array(48);
export const ZERO_SYNC_COMMITTEE = ssz.altair.SyncCommittee.defaultValue();
export const ZERO_HEADER = ssz.phase0.BeaconBlockHeader.defaultValue();
/** From https://notes.ethereum.org/@vbuterin/extended_light_client_protocol#Optimistic-head-determining-function */
Expand Down Expand Up @@ -106,13 +105,16 @@ export function isFinalityUpdate(update: LightClientUpdate): boolean {
}

export function isZeroedHeader(header: BeaconBlockHeader): boolean {
// Fast return for when constructing full LightClientUpdate from partial updates
return header === ZERO_HEADER || byteArrayEquals(header.bodyRoot, ZERO_HASH);
// Spec requires the whole header to equal LightClientHeader() in the non-finality case
// (see altair/light-client/sync-protocol.md `process_light_client_update`). Checking only
// bodyRoot would let an attacker smuggle arbitrary slot/proposerIndex/parentRoot/stateRoot
// through the non-finality branch and overwrite store.finalizedHeader.
return header === ZERO_HEADER || ssz.phase0.BeaconBlockHeader.equals(header, ZERO_HEADER);
}

export function isZeroedSyncCommittee(syncCommittee: SyncCommittee): boolean {
// Fast return for when constructing full LightClientUpdate from partial updates
return syncCommittee === ZERO_SYNC_COMMITTEE || byteArrayEquals(syncCommittee.pubkeys[0], ZERO_PUBKEY);
// Spec requires the whole SyncCommittee to equal SyncCommittee() in the non-sync-committee-update case.
return syncCommittee === ZERO_SYNC_COMMITTEE || ssz.altair.SyncCommittee.equals(syncCommittee, ZERO_SYNC_COMMITTEE);
}

export function isValidMerkleBranch(
Expand Down
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