fix: add pow limit in the check_target

btc-types/src/header.rs

@@ -12,6 +12,10 @@ pub const BLOCKS_PER_ADJUSTMENT: u64 = 2016;
 pub const TARGET_BLOCK_TIME_SECS: u64 = 10 * 60;
 pub const EXPECTED_TIME: u64 = BLOCKS_PER_ADJUSTMENT as u64 * TARGET_BLOCK_TIME_SECS;
 pub const MAX_ADJUSTMENT_FACTOR: u64 = 4;
+pub const POW_LIMIT: U256 = U256::new(
+    0x0000_0000_ffff_ffff_ffff_ffff_ffff_ffff,
+    0xffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff,
+);
 
 #[cfg(feature = "testnet")]
 pub mod testnet {

btc-types/src/u256.rs

@@ -29,6 +29,10 @@ impl U256 {
 
     pub const ONE: U256 = U256(0, 1);
 
+    pub const fn new(a: u128, b: u128) -> Self {
+        U256(a, b)
+    }
+
     /// Creates `U256` from a big-endian array of `u8`s.
     #[must_use]
     pub fn from_be_bytes(a: &[u8; 32]) -> U256 {

contract/src/lib.rs

@@ -1,7 +1,7 @@
 use btc_types::contract_args::{InitArgs, ProofArgs};
 use btc_types::hash::H256;
 use btc_types::header::{
-    ExtendedHeader, Header, BLOCKS_PER_ADJUSTMENT, EXPECTED_TIME, MAX_ADJUSTMENT_FACTOR,
+    ExtendedHeader, Header, BLOCKS_PER_ADJUSTMENT, EXPECTED_TIME, MAX_ADJUSTMENT_FACTOR, POW_LIMIT,
 };
 use btc_types::u256::U256;
 use near_plugins::{
@@ -500,6 +500,10 @@ impl BtcLightClient {
         require!(!new_target_overflow, "new target overflow");
         new_target = new_target / U256::from(EXPECTED_TIME);
 
+        if new_target > POW_LIMIT {
+            new_target = POW_LIMIT;
+        }
+
         let expected_bits = new_target.target_to_bits();
 
         require!(