Validate Esplora merkle proof against the block header's merkle root

`EsploraSyncClient::get_confirmed_tx` parsed the SPV proof returned by
the Esplora server but threw away the security check: the merkle root
computed by `PartialMerkleTree::extract_matches` was discarded
(`let _ = …`), and only the leaf-equality check (`matches[0] == txid`)
remained. Anyone can construct a single-leaf partial tree advertising
an arbitrary txid via `PartialMerkleTree::from_txids(&[txid], &[true])`,
so this gate was vacuous.

A malicious or compromised Esplora server could therefore convince
`EsploraSyncClient` that any transaction was confirmed in any block by
returning `MerkleBlock { header: real_header, txn: forged_partial_tree }`,
causing LDK to feed a synthesized `ConfirmedTx` into `Confirm`
implementations such as `ChannelManager` / `ChainMonitor`. From there,
the channel-funding / closing / HTLC flows would treat the transaction
as confirmed at an attacker-chosen height, with consequences ranging
from premature state transitions to force-close races.

Capture the merkle root returned by `extract_matches` and require it
to equal `block_header.merkle_root`, matching the validation the
Electrum sibling already performs via `validate_merkle_proof`.

Co-Authored-By: HAL 9000

Author: tnull

lightning-transaction-sync/src/esplora.rs

@@ -361,8 +361,13 @@ impl<L: Logger> EsploraSyncClient<L> {
 
 			let mut matches = Vec::new();
 			let mut indexes = Vec::new();
-			let _ = merkle_block.txn.extract_matches(&mut matches, &mut indexes);
-			if indexes.len() != 1 || matches.len() != 1 || matches[0] != txid {
+			let computed_merkle_root =
+				merkle_block.txn.extract_matches(&mut matches, &mut indexes).ok();
+			if computed_merkle_root != Some(block_header.merkle_root)
+				|| indexes.len() != 1
+				|| matches.len() != 1
+				|| matches[0] != txid
+			{
 				log_error!(self.logger, "Retrieved Merkle block for txid {} doesn't match expectations. This should not happen. Please verify server integrity.", txid);
 				return Err(InternalError::Failed);
 			}