included feedback by Rusty to check the max_concurrent_htlc value for both peers of a channel

channeld/full_channel.c

@@ -367,6 +367,7 @@ static enum channel_add_err add_htlc(struct channel *channel,
 	enum side sender = htlc_state_owner(state), recipient = !sender;
 	const struct htlc **committed, **adding, **removing;
 	const struct channel_view *view;
+	u32 min_concurrent_htlcs;
 
 	htlc = tal(tmpctx, struct htlc);
 
@@ -443,8 +444,16 @@ static enum channel_add_err add_htlc(struct channel *channel,
 	 *     HTLCs to its local commitment transaction...
 	 *     - SHOULD fail the channel.
 	 */
+	/* Also we should not add more htlc's than sender or recipient
+	 * configured.  This mitigates attacks in which a peer can force the
+	 * funder of the channel to pay unnecessary onchain fees during a fee
+	 * spike with large commitment transactions.
+	 */
+	min_concurrent_htlcs = channel->config[recipient].max_accepted_htlcs;
+	if (min_concurrent_htlcs > channel->config[sender].max_accepted_htlcs)
+		min_concurrent_htlcs = channel->config[sender].max_accepted_htlcs;
 	if (tal_count(committed) - tal_count(removing) + tal_count(adding)
-	    > channel->config[recipient].max_accepted_htlcs) {
+	    > min_concurrent_htlcs) {
 		return CHANNEL_ERR_TOO_MANY_HTLCS;
 	}