diff --git a/src/rest.cpp b/src/rest.cpp
--- a/src/rest.cpp
+++ b/src/rest.cpp
@@ -194,6 +194,7 @@
         }
         case RetFormat::JSON: {
             UniValue jsonHeaders(UniValue::VARR);
+
             for (const CBlockIndex *pindex : headers) {
                 jsonHeaders.push_back(blockheaderToJSON(tip, pindex));
             }
diff --git a/src/rpc/blockchain.h b/src/rpc/blockchain.h
--- a/src/rpc/blockchain.h
+++ b/src/rpc/blockchain.h
@@ -5,8 +5,15 @@
 #ifndef BITCOIN_RPCBLOCKCHAIN_H
 #define BITCOIN_RPCBLOCKCHAIN_H
 
+#include <amount.h>
+#include <sync.h>
 #include <univalue.h>
 
+#include <stdint.h>
+#include <vector>
+
+extern CCriticalSection cs_main;
+
 class CBlock;
 class CBlockIndex;
 class Config;
@@ -28,7 +35,8 @@
 
 /** Block description to JSON */
 UniValue blockToJSON(const CBlock &block, const CBlockIndex *tip,
-                     const CBlockIndex *blockindex, bool txDetails = false);
+                     const CBlockIndex *blockindex, bool txDetails = false)
+    LOCKS_EXCLUDED(cs_main);
 
 /** Mempool information to JSON */
 UniValue mempoolInfoToJSON();
@@ -38,6 +46,7 @@
 
 /** Block header to JSON */
 UniValue blockheaderToJSON(const CBlockIndex *tip,
-                           const CBlockIndex *blockindex);
+                           const CBlockIndex *blockindex)
+    LOCKS_EXCLUDED(cs_main);
 
 #endif // BITCOIN_RPCBLOCKCHAIN_H
diff --git a/src/rpc/blockchain.cpp b/src/rpc/blockchain.cpp
--- a/src/rpc/blockchain.cpp
+++ b/src/rpc/blockchain.cpp
@@ -78,6 +78,9 @@
 
 UniValue blockheaderToJSON(const CBlockIndex *tip,
                            const CBlockIndex *blockindex) {
+    // Serialize passed information without accessing chain state of the active
+    // chain!
+    AssertLockNotHeld(cs_main); // For performance reasons
     UniValue result(UniValue::VOBJ);
     result.pushKV("hash", blockindex->GetBlockHash().GetHex());
     const CBlockIndex *pnext;
@@ -106,6 +109,9 @@
 
 UniValue blockToJSON(const CBlock &block, const CBlockIndex *tip,
                      const CBlockIndex *blockindex, bool txDetails) {
+    // Serialize passed information without accessing chain state of the active
+    // chain!
+    AssertLockNotHeld(cs_main); // For performance reasons
     UniValue result(UniValue::VOBJ);
     result.pushKV("hash", blockindex->GetBlockHash().GetHex());
     const CBlockIndex *pnext;
@@ -868,8 +874,6 @@
                                        "214adbda81d7e2a3dd146f6ed09\""));
     }
 
-    LOCK(cs_main);
-
     std::string strHash = request.params[0].get_str();
     uint256 hash(uint256S(strHash));
 
@@ -882,23 +886,32 @@
         }
     }
 
-    const CBlockIndex *pblockindex = LookupBlockIndex(hash);
-    if (!pblockindex) {
-        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
-    }
-
     CBlock block;
-    if (fHavePruned && !pblockindex->nStatus.hasData() &&
-        pblockindex->nTx > 0) {
-        throw JSONRPCError(RPC_MISC_ERROR, "Block not available (pruned data)");
-    }
+    const CBlockIndex *pblockindex;
+    const CBlockIndex *tip;
+    {
+        LOCK(cs_main);
+        pblockindex = LookupBlockIndex(hash);
+        tip = chainActive.Tip();
 
-    if (!ReadBlockFromDisk(block, pblockindex, config)) {
-        // Block not found on disk. This could be because we have the block
-        // header in our index but don't have the block (for example if a
-        // non-whitelisted node sends us an unrequested long chain of valid
-        // blocks, we add the headers to our index, but don't accept the block).
-        throw JSONRPCError(RPC_MISC_ERROR, "Block not found on disk");
+        if (!pblockindex) {
+            throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Block not found");
+        }
+
+        if (fHavePruned && !pblockindex->nStatus.hasData() &&
+            pblockindex->nTx > 0) {
+            throw JSONRPCError(RPC_MISC_ERROR,
+                               "Block not available (pruned data)");
+        }
+
+        if (!ReadBlockFromDisk(block, pblockindex, config)) {
+            // Block not found on disk. This could be because we have the block
+            // header in our index but don't have the block (for example if a
+            // non-whitelisted node sends us an unrequested long chain of valid
+            // blocks, we add the headers to our index, but don't accept the
+            // block).
+            throw JSONRPCError(RPC_MISC_ERROR, "Block not found on disk");
+        }
     }
 
     if (verbosity <= 0) {
@@ -909,7 +922,7 @@
         return strHex;
     }
 
-    return blockToJSON(block, chainActive.Tip(), pblockindex, verbosity >= 2);
+    return blockToJSON(block, tip, pblockindex, verbosity >= 2);
 }
 
 struct CCoinsStats {