diff --git a/rskj-core/src/main/java/org/ethereum/crypto/HashUtil.java b/rskj-core/src/main/java/org/ethereum/crypto/HashUtil.java
index 59bed4e8cb6..aad79a2df3c 100644
--- a/rskj-core/src/main/java/org/ethereum/crypto/HashUtil.java
+++ b/rskj-core/src/main/java/org/ethereum/crypto/HashUtil.java
@@ -53,7 +53,9 @@ public static MessageDigest makeMessageDigest() {
      * @return - sha256 hash of the data
      */
     public static byte[] sha256(byte[] input) {
-        return sha256digest.digest(input);
+        synchronized (sha256digest) {
+            return sha256digest.digest(input);
+        }
     }
 
     public static byte[] keccak256(byte[] input) {