From 50c0b61a9d562240d5fe4bd79324b0c0e79caa5c Mon Sep 17 00:00:00 2001
From: dergoegge <n.goeggi@gmail.com>
Date: Tue, 6 Feb 2024 17:07:48 +0000
Subject: [validation] Merkle root malleation should be caught by
 IsBlockMutated

Github-Pull: #29412
Rebased-From: 2d8495e0800f5332748cd50eaad801ff77671bba
---
 src/test/validation_tests.cpp |  5 +++++
 src/validation.cpp            | 13 ++++++++++++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/src/test/validation_tests.cpp b/src/test/validation_tests.cpp
index 14440571eb..cb7d1a312f 100644
--- a/src/test/validation_tests.cpp
+++ b/src/test/validation_tests.cpp
@@ -4,12 +4,17 @@
 
 #include <chainparams.h>
 #include <consensus/amount.h>
+#include <consensus/merkle.h>
+#include <core_io.h>
+#include <hash.h>
 #include <net.h>
 #include <signet.h>
 #include <uint256.h>
 #include <util/chaintype.h>
 #include <validation.h>
 
+#include <string>
+
 #include <test/util/setup_common.h>
 
 #include <boost/test/unit_test.hpp>
diff --git a/src/validation.cpp b/src/validation.cpp
index 1ad4ebcdaa..a2128b231e 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -3815,7 +3815,18 @@ bool IsBlockMutated(const CBlock& block, bool check_witness_root)
     }
 
     if (block.vtx.empty() || !block.vtx[0]->IsCoinBase()) {
-        return false;
+        // Consider the block mutated if any transaction is 64 bytes in size (see 3.1
+        // in "Weaknesses in Bitcoin’s Merkle Root Construction":
+        // https://lists.linuxfoundation.org/pipermail/bitcoin-dev/attachments/20190225/a27d8837/attachment-0001.pdf).
+        //
+        // Note: This is not a consensus change as this only applies to blocks that
+        // don't have a coinbase transaction and would therefore already be invalid.
+        return std::any_of(block.vtx.begin(), block.vtx.end(),
+                           [](auto& tx) { return ::GetSerializeSize(tx, PROTOCOL_VERSION | SERIALIZE_TRANSACTION_NO_WITNESS) == 64; });
+    } else {
+        // Theoretically it is still possible for a block with a 64 byte
+        // coinbase transaction to be mutated but we neglect that possibility
+        // here as it requires at least 224 bits of work.
     }
 
     if (!CheckWitnessMalleation(block, check_witness_root, state)) {
-- 
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