From 90ef662bc74620b5f73e52a58f167c07390e5527 Mon Sep 17 00:00:00 2001
From: Bart <11445373+bthomee@users.noreply.github.com>
Date: Fri, 21 Aug 2026 17:04:43 -0400
Subject: [PATCH] test: Cover the leaf key check in `verifyProofPath`

The check that the terminal leaf of a proof path actually holds the key being proved had no test
covering it, because the substitution it guards against cannot be built by swapping a leaf under a
genuine root: the hash comparison one level up rejects that first.

This test forges the root instead. It puts a genuine leaf from elsewhere in the map at the bottom of
a two-element path and wraps it in an inner node whose branch for the proved key carries that leaf's
hash, so the chain up to the root hash validates even though the leaf belongs to another key. A
control case runs the same forged root over the proved key's own leaf and expects acceptance, which
pins the leaf key comparison as the only reason the substituted path is rejected.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
---
 src/tests/libxrpl/shamap/SHAMap.cpp | 77 +++++++++++++++++++++++++++++
 1 file changed, 77 insertions(+)

diff --git a/src/tests/libxrpl/shamap/SHAMap.cpp b/src/tests/libxrpl/shamap/SHAMap.cpp
index 92492c514e..7f7d6ffba2 100644
--- a/src/tests/libxrpl/shamap/SHAMap.cpp
+++ b/src/tests/libxrpl/shamap/SHAMap.cpp
@@ -418,4 +418,81 @@ TEST_F(SHAMapPathProof, all_inner_path_at_leaf_depth_is_rejected)
     EXPECT_FALSE(SHAMap::verifyProofPath(childHash.asUInt256(), kTestKey, path));
 }
 
+/**
+ * Wrap a leaf blob in a forged root inner node whose branch for `key` carries that leaf's hash.
+ *
+ * The resulting two-element path hash-chains for `key` no matter which leaf sits at the bottom,
+ * which is exactly the substitution a peer could attempt.
+ *
+ * @param leafBlob the wire form of the leaf to place at the bottom of the path.
+ * @param key the key the forged path claims to prove.
+ * @return the path (deepest element first) and the forged root hash, or an empty path if the leaf
+ *         blob does not parse.
+ */
+static std::pair<std::vector<Blob>, uint256>
+forgeRootOverLeaf(Blob const& leafBlob, uint256 const& key)
+{
+    auto leaf = SHAMapTreeNode::makeFromWire(makeSlice(leafBlob));
+    if (!leaf || !leaf->isLeaf())
+        return {};
+    leaf->updateHash();
+
+    auto const branch = selectBranch(SHAMapNodeID::createID(0, key), key);
+    Serializer s;
+    for (auto i = 0u; i < SHAMap::kBranchFactor; ++i)
+        s.addBitString(i == branch ? leaf->getHash().asUInt256() : uint256{});
+    s.add8(kWireTypeInner);
+
+    auto root = SHAMapTreeNode::makeFromWire(makeSlice(s.peekData()));
+    if (!root)
+        return {};
+    root->updateHash();
+
+    return {std::vector<Blob>{leafBlob, s.getData()}, root->getHash().asUInt256()};
+}
+
+// The hash chain above a leaf proves nothing about which key that leaf holds, so a peer can graft a
+// genuine leaf from elsewhere in the map onto a path forged for another key. Comparing the terminal
+// leaf's own key against the key being proved is what rejects it.
+TEST_F(SHAMapPathProof, substituted_leaf_for_other_key_is_rejected)
+{
+    tests::TestNodeFamily f{j_};
+    SHAMap map{SHAMapType::FREE, f};
+    map.setUnbacked();
+
+    // Two arbitrary keys differing in their first nibble, so each leaf hangs off the root directly.
+    constexpr uint256 kKey("1c8cec8e5e9b0e5e0e0f5b3e2c9f7a1d6b4e8c2a0d7f3b9e5c1a8d4f2b6e0c93");
+    constexpr uint256 kOtherKey("e3f1a7d5b9c2e8f406a1d3b5c7e9f2a4d6b8c0e2f4a6d8b0c2e4f6a8d0b2c4e6");
+
+    for (auto const& k : {kKey, kOtherKey})
+    {
+        ASSERT_TRUE(map.addItem(
+            SHAMapNodeType::TnAccountState, makeShamapitem(k, Slice{k.data(), k.size()})));
+    }
+    map.invariants();
+
+    auto const ownPath = map.getProofPath(kKey);
+    auto const otherPath = map.getProofPath(kOtherKey);
+    ASSERT_TRUE(ownPath.has_value());
+    ASSERT_TRUE(otherPath.has_value());
+
+    // NOLINTBEGIN(bugprone-unchecked-optional-access) has_value() checked above
+    // The genuine leaf blobs, deepest element first.
+    auto const& ownLeaf = ownPath->front();
+    auto const& otherLeaf = otherPath->front();
+    // NOLINTEND(bugprone-unchecked-optional-access)
+
+    // Control: the forged root is accepted when the leaf below it really is kKey's leaf, so the
+    // rejection below can only come from the leaf key comparison.
+    auto const [goodPath, goodRoot] = forgeRootOverLeaf(ownLeaf, kKey);
+    ASSERT_EQ(goodPath.size(), 2u);
+    EXPECT_TRUE(SHAMap::verifyProofPath(goodRoot, kKey, goodPath));
+
+    // Same forged root, but kOtherKey's leaf substituted at the bottom: the hash chain still
+    // validates, yet the path does not prove anything about kKey.
+    auto const [badPath, badRoot] = forgeRootOverLeaf(otherLeaf, kKey);
+    ASSERT_EQ(badPath.size(), 2u);
+    EXPECT_FALSE(SHAMap::verifyProofPath(badRoot, kKey, badPath));
+}
+
 }  // namespace xrpl::tests