fix(beacon): verify fetched beacon state hashes to the block state root

[qu0b]

Problem

downloadAndStoreBeaconState fetches the finalized state by slot from a single upstream and stores whatever bytes come back under the block's state_root, without ever verifying them. The state is then served verbatim at /eth/v2/debug/beacon/states/finalized.

A beacon node that has fallen behind can answer a by-slot state request with a state from a different chain view instead of erroring — lodestar (and lighthouse, in a different way) serve their stale head state dialed forward through empty slots for slots beyond their head. Checkpointz caches that fabricated state under the real block.state_root and serves it; checkpoint-syncing clients then fail their hash_tree_root(state) == block.state_root anchor check.

Observed on glamsterdam-devnet-7

• lodestar-geth-1's head froze at slot 56351 while checkpointz still listed it healthy: true.
• Asked for the state at the network's finalized checkpoint slot 71391, it returned a state with latest_block_header.slot = 56351 dialed forward ~15k empty slots, HTR 0xcb39fc23… — matching nothing on the canonical chain (expected 0x2d414744…).
• The serving path's PastFinalizedCheckpoint node filter usually keeps such nodes out, but it filters on claimed finality, not head — finality stalls, optimistic finality claims, or a node freezing near a serving flip let a fabricating upstream through, and nothing downstream catches it.

Fix

Compute the hash tree root of the fetched state (existing sszEncoder.GetStateRoot) and reject the bundle if it doesn't match the state root committed to in the block. The bad upstream's bundle fails, and the serving loop retries with another node.

Cost: one state HTR per bundle download (once per serving-checkpoint flip / genesis fetch) — ~1s for a 73 MB gloas state.

Testing

• go build ./... / go test ./... green.
• Patched binary run against three glamsterdam-devnet-7 upstreams: downloaded, verified and served the epoch-2236 bundle (no false positive on a real 73 MB / 503k-validator gloas state), served bytes byte-identical to the upstream's by-root copy.

The same code exists on master; happy to open a port there too.

🤖 Generated with Claude Code

https://claude.ai/code/session_017K6pyDaSgX2FcLtQ1qQ8Vu