Merge pull request #11265

### src/wallet/wallet2.cpp
@@ -14937,6 +14937,11 @@ size_t wallet2::import_multisig(std::vector<cryptonote::blobdata> blobs, bool re
   // parse and validate locally so failures preserve any pending rescan state
   std::vector<std::vector<tools::wallet2::multisig_info>> info;
   std::unordered_set<crypto::public_key> seen;
+
+  const size_t expected_n_partial_key_images = get_account().get_multisig_keys().size();
+  const size_t expected_n_lr = tools::combinations_count(m_multisig_signers.size() - m_multisig_threshold, m_multisig_signers.size() - 1)
+    * multisig::signing::kAlphaComponents;
+
   for (cryptonote::blobdata &data: blobs)
   {
     const size_t magiclen = strlen(MULTISIG_EXPORT_FILE_MAGIC);
@@ -14983,6 +14988,13 @@ size_t wallet2::import_multisig(std::vector<cryptonote::blobdata> blobs, bool re
 
     for (const auto &e: i)
     {
+      CHECK_AND_ASSERT_THROW_MES(e.m_signer == signer, "Multisig info body signer does not match header signer");
+
+      CHECK_AND_ASSERT_THROW_MES(e.m_partial_key_images.size() == expected_n_partial_key_images,
+        "Multisig info has an unexpected number of partial key images");
+      CHECK_AND_ASSERT_THROW_MES(e.m_LR.size() == expected_n_lr,
+        "Multisig info has an unexpected number of signing nonces");
+
       for (const auto &lr: e.m_LR)
       {
         CHECK_AND_ASSERT_THROW_MES(rct::isInMainSubgroup(lr.m_L), "Multisig value is not in the main subgroup");

### tests/unit_tests/multisig.cpp
@@ -27,14 +27,19 @@
 // THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include "crypto/crypto.h"
+#include "multisig/multisig.h"
 #include "multisig/multisig_account.h"
 #include "multisig/multisig_kex_msg.h"
 #include "ringct/rctOps.h"
+#include "serialization/binary_utils.h"
 #include "wallet/wallet2.h"
+#include "wallet/wallet2_basic/wallet2_boost_serialization.h"
+#include "wallet/wallet2_basic/wallet2_serialization.h"
 
 #include "gtest/gtest.h"
 
 #include <cstdint>
+#include <sstream>
 
 static const struct
 {
@@ -497,3 +502,106 @@ TEST(multisig, multisig_kex_msg)
   EXPECT_EQ(msg_rnd2.get_msg_privkey(), crypto::null_skey);
   EXPECT_EQ(msg_rnd2.get_msg_privkey(), msg_rnd2_reverse.get_msg_privkey());
 }
+
+TEST(multisig, import_multisig_validation)
+{
+  using namespace multisig;
+
+  // a 2-of-2 wallet pair is enough: no funded outputs are needed for this test, since the checks under test
+  // run on the deserialized contents of an import blob before anything touches m_transfers
+  const std::uint32_t M = 2, N = 2;
+  std::vector<tools::wallet2> wallets(N);
+
+  std::vector<std::string> initial_infos(wallets.size());
+  for (size_t i = 0; i < wallets.size(); ++i)
+  {
+    make_wallet(i, wallets[i]);
+    wallets[i].decrypt_keys("");
+    initial_infos[i] = wallets[i].get_multisig_first_kex_msg();
+    wallets[i].encrypt_keys("");
+  }
+
+  std::vector<std::string> intermediate_infos(wallets.size());
+  for (size_t i = 0; i < wallets.size(); ++i)
+    intermediate_infos[i] = wallets[i].make_multisig("", initial_infos, M);
+
+  multisig_account_status ms_status{wallets[0].get_multisig_status()};
+  while (!ms_status.is_ready)
+  {
+    intermediate_infos = exchange_round(wallets, intermediate_infos);
+    ms_status = wallets[0].get_multisig_status();
+  }
+
+  wallets[0].decrypt_keys("");
+  wallets[1].decrypt_keys("");
+
+  // 2-of-2 => each signer holds exactly 1 multisig private key, and each output needs exactly
+  // combinations_count(N-M, N-1) * kAlphaComponents = combinations_count(0, 1) * 2 = 2 LR nonce pairs
+  ASSERT_EQ(1u, wallets[1].get_account().get_multisig_keys().size());
+  const crypto::public_key other_signer = wallets[1].get_multisig_signer_public_key();
+
+  // a fake output pubkey: any valid curve point works here, since it's only used to derive LR pairs/key
+  // images with the right shape (main-subgroup curve points); it doesn't need to correspond to a real output
+  crypto::public_key fake_out_key;
+  ASSERT_TRUE(crypto::secret_key_to_public_key(rct::rct2sk(rct::skGen()), fake_out_key));
+
+  crypto::key_image valid_pki;
+  ASSERT_TRUE(generate_multisig_key_image(wallets[1].get_account().get_keys(), 0, fake_out_key, valid_pki));
+
+  tools::wallet2::multisig_info::LR valid_lr;
+  {
+    const crypto::secret_key k = rct::rct2sk(rct::skGen());
+    crypto::public_key L, R;
+    generate_multisig_LR(fake_out_key, k, L, R);
+    valid_lr.m_L = rct::pk2rct(L);
+    valid_lr.m_R = rct::pk2rct(R);
+  }
+
+  tools::wallet2::multisig_info valid_entry;
+  valid_entry.m_signer = other_signer;
+  valid_entry.m_LR = {valid_lr, valid_lr};
+  valid_entry.m_partial_key_images = {valid_pki};
+
+  // builds a raw multisig-info import blob (same wire format as wallet2::export_multisig()) for a single
+  // fake output, so wallet2::import_multisig()'s per-output validation can be exercised directly
+  const auto build_blob =
+    [&](const crypto::public_key &header_signer, const tools::wallet2::multisig_info &entry) -> cryptonote::blobdata
+    {
+      std::vector<tools::wallet2::multisig_info> info{entry};
+      std::stringstream oss;
+      binary_archive<true> ar(oss);
+      EXPECT_TRUE(::serialization::serialize(ar, info));
+
+      const cryptonote::account_public_address &addr = wallets[0].get_account().get_keys().m_account_address;
+      std::string header;
+      header.append((const char *)&addr.m_spend_public_key, sizeof(crypto::public_key));
+      header.append((const char *)&addr.m_view_public_key, sizeof(crypto::public_key));
+      header.append((const char *)&header_signer, sizeof(crypto::public_key));
+
+      return std::string("Monero multisig export\001") + wallets[0].encrypt_with_view_secret_key(header + oss.str());
+    };
+
+  // well-formed info is accepted
+  EXPECT_NO_THROW(wallets[0].import_multisig({build_blob(other_signer, valid_entry)}, false));
+
+  // wrong number of LR nonce pairs is rejected
+  {
+    tools::wallet2::multisig_info bad_entry = valid_entry;
+    bad_entry.m_LR = {valid_lr};  // should be 2
+    EXPECT_ANY_THROW(wallets[0].import_multisig({build_blob(other_signer, bad_entry)}, false));
+  }
+
+  // wrong number of partial key images is rejected
+  {
+    tools::wallet2::multisig_info bad_entry = valid_entry;
+    bad_entry.m_partial_key_images.clear();  // should be 1
+    EXPECT_ANY_THROW(wallets[0].import_multisig({build_blob(other_signer, bad_entry)}, false));
+  }
+
+  // a body-declared signer that doesn't match the header signer it was filed under is rejected
+  {
+    tools::wallet2::multisig_info bad_entry = valid_entry;
+    bad_entry.m_signer = wallets[0].get_multisig_signer_public_key();
+    EXPECT_ANY_THROW(wallets[0].import_multisig({build_blob(other_signer, bad_entry)}, false));
+  }
+}