fix: bump numFeatures to 116 for nestedmultisign + ioulockedbalanceinvariant

# Conflicts:
#	include/xrpl/protocol/detail/features.macro

include/xrpl/protocol/Feature.h

@@ -80,7 +80,7 @@ namespace detail {
 // Feature.cpp. Because it's only used to reserve storage, and determine how
 // large to make the FeatureBitset, it MAY be larger. It MUST NOT be less than
 // the actual number of amendments. A LogicError on startup will verify this.
-static constexpr std::size_t numFeatures = 115;
+static constexpr std::size_t numFeatures = 116;
 
 /** Amendments that this server supports and the default voting behavior.
    Whether they are enabled depends on the Rules defined in the validated

include/xrpl/protocol/STTx.h

@@ -195,6 +195,71 @@ STTx::getTransactionID() const
     return tid_;
 }
 
+//------------------------------------------------------------------------------
+// Multi-sign depth and leaf limits
+//------------------------------------------------------------------------------
+
+/** Maximum nesting depth for nested multi-signing (featureNestedMultiSign). */
+constexpr int nestedMultiSignMaxDepth = 4;
+
+/** Maximum nesting depth when nested multi-signing is disabled (flat only). */
+constexpr int legacyMultiSignMaxDepth = 1;
+
+/** Maximum total leaf signers across the entire nested tree.
+    Bounds worst-case signature verification cost. Only enforced when
+    featureNestedMultiSign is enabled; flat signing is already capped by the
+    per-array multisign limit.
+*/
+constexpr std::size_t nestedMultiSignMaxLeafSigners = 64;
+
+//------------------------------------------------------------------------------
+// Multi-sign signer entry helpers
+//------------------------------------------------------------------------------
+
+/** Count populated fields in an STObject.
+    STObject::getCount() includes template slots for optional fields that are
+    not present. Signer shape validation needs only populated fields.
+*/
+inline std::size_t
+countPresentFields(STObject const& obj)
+{
+    std::size_t count = 0;
+    for (auto const& field : obj)
+    {
+        if (field.getSType() != STI_NOTPRESENT)
+            ++count;
+    }
+    return count;
+}
+
+/** A leaf signer has Account + SigningPubKey + TxnSignature only. */
+inline bool
+isLeafSigner(STObject const& signer)
+{
+    return signer.isFieldPresent(sfAccount) &&
+        signer.isFieldPresent(sfSigningPubKey) &&
+        signer.isFieldPresent(sfTxnSignature) &&
+        !signer.isFieldPresent(sfSigners) && countPresentFields(signer) == 3;
+}
+
+/** A nested signer has Account + Signers only. */
+inline bool
+isNestedSigner(STObject const& signer)
+{
+    return signer.isFieldPresent(sfAccount) &&
+        signer.isFieldPresent(sfSigners) &&
+        !signer.isFieldPresent(sfSigningPubKey) &&
+        !signer.isFieldPresent(sfTxnSignature) &&
+        countPresentFields(signer) == 2;
+}
+
+/** True when a signer entry is either a valid leaf or a valid nested signer. */
+inline bool
+isValidSignerEntry(STObject const& signer)
+{
+    return isLeafSigner(signer) || isNestedSigner(signer);
+}
+
 }  // namespace ripple
 
 #endif

include/xrpl/protocol/detail/features.macro

@@ -31,6 +31,7 @@
 // If you add an amendment here, then do not forget to increment `numFeatures`
 // in include/xrpl/protocol/Feature.h.
 
+XRPL_FEATURE(NestedMultiSign,            Supported::yes, VoteBehavior::DefaultNo)
 XRPL_FIX    (IOULockedBalanceInvariant,  Supported::yes, VoteBehavior::DefaultNo)
 XRPL_FIX    (ImportIssuer,               Supported::yes, VoteBehavior::DefaultYes)
 XRPL_FEATURE(HookAPISerializedType240,   Supported::yes, VoteBehavior::DefaultNo)

src/libxrpl/protocol/InnerObjectFormats.cpp

@@ -49,8 +49,9 @@ InnerObjectFormats::InnerObjectFormats()
         sfSigner.getCode(),
         {
             {sfAccount, soeREQUIRED},
-            {sfSigningPubKey, soeREQUIRED},
-            {sfTxnSignature, soeREQUIRED},
+            {sfSigningPubKey, soeOPTIONAL},
+            {sfTxnSignature, soeOPTIONAL},
+            {sfSigners, soeOPTIONAL},
         });
 
     add(sfMajority.jsonName,

src/libxrpl/protocol/STTx.cpp

@@ -370,11 +370,6 @@ STTx::checkMultiSign(
 
     STArray const& signers{getFieldArray(sfSigners)};
 
-    // There are well known bounds that the number of signers must be within.
-    if (signers.size() < minMultiSigners ||
-        signers.size() > maxMultiSigners(&rules))
-        return Unexpected("Invalid Signers array size.");
-
     // We can ease the computational load inside the loop a bit by
     // pre-constructing part of the data that we hash.  Fill a Serializer
     // with the stuff that stays constant from signature to signature.
@@ -387,64 +382,117 @@ STTx::checkMultiSign(
     bool const fullyCanonical = (getFlags() & tfFullyCanonicalSig) ||
         (requireCanonicalSig == RequireFullyCanonicalSig::yes);
 
-    // Signers must be in sorted order by AccountID.
-    AccountID lastAccountID(beast::zero);
-
     bool const isWildcardNetwork =
         isFieldPresent(sfNetworkID) && getFieldU32(sfNetworkID) == 65535;
 
-    for (auto const& signer : signers)
-    {
-        auto const accountID = signer.getAccountID(sfAccount);
+    // Set max depth and leaf cap based on feature flag.
+    bool const nested = rules.enabled(featureNestedMultiSign);
+    int const maxDepth =
+        nested ? nestedMultiSignMaxDepth : legacyMultiSignMaxDepth;
+    std::size_t const maxLeafSigners =
+        nested ? nestedMultiSignMaxLeafSigners : maxMultiSigners(&rules);
+    std::size_t totalLeafSigners = 0;
 
-        // The account owner may not multisign for themselves.
-        if (accountID == txnAccountID)
-            return Unexpected("Invalid multisigner.");
+    // Define recursive lambda for checking signatures at any depth.
+    std::function<Expected<void, std::string>(
+        STArray const&, AccountID const&, int)>
+        checkSignersArray;
 
-        // No duplicate signers allowed.
-        if (lastAccountID == accountID)
-            return Unexpected("Duplicate Signers not allowed.");
+    checkSignersArray = [&](STArray const& signersArray,
+                            [[maybe_unused]] AccountID const& parentAccountID,
+                            int depth) -> Expected<void, std::string> {
+        // Check depth limit.
+        if (depth > maxDepth)
+            return Unexpected("Multi-signing depth limit exceeded.");
 
-        // Accounts must be in order by account ID.  No duplicates allowed.
-        if (lastAccountID > accountID)
-            return Unexpected("Unsorted Signers array.");
+        // There are well known bounds that the number of signers must be
+        // within.
+        if (signersArray.size() < minMultiSigners ||
+            signersArray.size() > maxMultiSigners(&rules))
+            return Unexpected("Invalid Signers array size.");
 
-        // The next signature must be greater than this one.
-        lastAccountID = accountID;
+        // Signers must be in sorted order by AccountID.
+        AccountID lastAccountID(beast::zero);
 
-        // Verify the signature.
-        bool validSig = false;
-        try
+        for (auto const& signer : signersArray)
         {
-            Serializer s = dataStart;
-            finishMultiSigningData(accountID, s);
+            auto const accountID = signer.getAccountID(sfAccount);
 
-            auto spk = signer.getFieldVL(sfSigningPubKey);
+            // The account owner may not multisign for themselves.
+            if (accountID == txnAccountID)
+                return Unexpected("Invalid multisigner.");
 
-            if (publicKeyType(makeSlice(spk)))
+            // No duplicate signers allowed.
+            if (lastAccountID == accountID)
+                return Unexpected("Duplicate Signers not allowed.");
+
+            // Accounts must be in order by account ID. No duplicates allowed.
+            if (lastAccountID > accountID)
+                return Unexpected("Unsorted Signers array.");
+
+            // The next signature must be greater than this one.
+            lastAccountID = accountID;
+
+            if (isNestedSigner(signer))
             {
-                Blob const signature = signer.getFieldVL(sfTxnSignature);
+                if (!nested)
+                    return Unexpected("FeatureNestedMultiSign is disabled");
 
-                // wildcard network gets a free pass
-                validSig = isWildcardNetwork ||
-                    verify(PublicKey(makeSlice(spk)),
-                           s.slice(),
-                           makeSlice(signature),
-                           fullyCanonical);
+                STArray const& nestedSigners = signer.getFieldArray(sfSigners);
+                auto result =
+                    checkSignersArray(nestedSigners, accountID, depth + 1);
+                if (!result)
+                    return result;
+            }
+            else if (isLeafSigner(signer))
+            {
+                if (++totalLeafSigners > maxLeafSigners)
+                    return Unexpected(std::string("Too many leaf signers."));
+
+                bool validSig = false;
+                try
+                {
+                    Serializer s = dataStart;
+                    finishMultiSigningData(accountID, s);
+
+                    auto spk = signer.getFieldVL(sfSigningPubKey);
+
+                    if (publicKeyType(makeSlice(spk)))
+                    {
+                        Blob const signature =
+                            signer.getFieldVL(sfTxnSignature);
+
+                        // wildcard network gets a free pass
+                        validSig = isWildcardNetwork ||
+                            verify(PublicKey(makeSlice(spk)),
+                                   s.slice(),
+                                   makeSlice(signature),
+                                   fullyCanonical);
+                    }
+                }
+                catch (std::exception const&)
+                {
+                    // We assume any problem lies with the signature.
+                    validSig = false;
+                }
+                if (!validSig)
+                    return Unexpected(
+                        std::string("Invalid signature on account ") +
+                        toBase58(accountID) + ".");
+            }
+            else
+            {
+                return Unexpected(
+                    std::string("Malformed signer entry for account ") +
+                    toBase58(accountID) + ".");
             }
         }
-        catch (std::exception const&)
-        {
-            // We assume any problem lies with the signature.
-            validSig = false;
-        }
-        if (!validSig)
-            return Unexpected(
-                std::string("Invalid signature on account ") +
-                toBase58(accountID) + ".");
-    }
-    // All signatures verified.
-    return {};
+
+        return {};
+    };
+
+    // Start the recursive check at depth 1.
+    return checkSignersArray(signers, txnAccountID, 1);
 }
 
 //------------------------------------------------------------------------------

src/test/jtx/impl/multisign.cpp

@@ -66,15 +66,45 @@ signers(Account const& account, none_t)
 
 //------------------------------------------------------------------------------
 
-msig::msig(std::vector<msig::Reg> signers_) : signers(std::move(signers_))
+// Helper function to recursively sort nested signers
+void
+sortSignersRecursive(std::vector<msig::SignerPtr>& signers)
 {
-    // Signatures must be applied in sorted order.
+    // Sort current level by account ID
     std::sort(
         signers.begin(),
         signers.end(),
-        [](msig::Reg const& lhs, msig::Reg const& rhs) {
-            return lhs.acct.id() < rhs.acct.id();
+        [](msig::SignerPtr const& lhs, msig::SignerPtr const& rhs) {
+            return lhs->id() < rhs->id();
         });
+
+    // Recursively sort nested signers for each signer at this level
+    for (auto& signer : signers)
+    {
+        if (signer->isNested() && !signer->nested.empty())
+        {
+            sortSignersRecursive(signer->nested);
+        }
+    }
+}
+
+msig::msig(std::vector<msig::SignerPtr> signers_) : signers(std::move(signers_))
+{
+    // Recursively sort all signers at all nesting levels
+    // This ensures account IDs are in strictly ascending order at each level
+    sortSignersRecursive(signers);
+}
+
+msig::msig(std::vector<msig::Reg> signers_)
+{
+    // Convert Reg vector to SignerPtr vector for backward compatibility
+    signers.reserve(signers_.size());
+    for (auto const& s : signers_)
+        signers.push_back(s.toSigner());
+
+    // Recursively sort all signers at all nesting levels
+    // This ensures account IDs are in strictly ascending order at each level
+    sortSignersRecursive(signers);
 }
 
 void
@@ -93,19 +123,47 @@ msig::operator()(Env& env, JTx& jt) const
             env.test.log << pretty(jtx.jv) << std::endl;
             Rethrow();
         }
+
+        // Recursive function to build signer JSON
+        std::function<Json::Value(SignerPtr const&)> buildSignerJson;
+        buildSignerJson = [&](SignerPtr const& signer) -> Json::Value {
+            Json::Value jo;
+            jo[jss::Account] = signer->acct.human();
+
+            if (signer->isNested())
+            {
+                // For nested signers, we use the already-sorted nested vector
+                // (sorted during construction via sortSignersRecursive)
+                // This ensures account IDs are in strictly ascending order
+                auto& subJs = jo[sfSigners.getJsonName()];
+                for (std::size_t i = 0; i < signer->nested.size(); ++i)
+                {
+                    auto& subJo = subJs[i][sfSigner.getJsonName()];
+                    subJo = buildSignerJson(signer->nested[i]);
+                }
+            }
+            else
+            {
+                // This is a leaf signer - add signature
+                jo[jss::SigningPubKey] = strHex(signer->sig.pk().slice());
+
+                Serializer ss{buildMultiSigningData(*st, signer->acct.id())};
+                auto const sig = ripple::sign(
+                    *publicKeyType(signer->sig.pk().slice()),
+                    signer->sig.sk(),
+                    ss.slice());
+                jo[sfTxnSignature.getJsonName()] =
+                    strHex(Slice{sig.data(), sig.size()});
+            }
+
+            return jo;
+        };
+
         auto& js = jtx[sfSigners.getJsonName()];
         for (std::size_t i = 0; i < mySigners.size(); ++i)
         {
-            auto const& e = mySigners[i];
             auto& jo = js[i][sfSigner.getJsonName()];
-            jo[jss::Account] = e.acct.human();
-            jo[jss::SigningPubKey] = strHex(e.sig.pk().slice());
-
-            Serializer ss{buildMultiSigningData(*st, e.acct.id())};
-            auto const sig = ripple::sign(
-                *publicKeyType(e.sig.pk().slice()), e.sig.sk(), ss.slice());
-            jo[sfTxnSignature.getJsonName()] =
-                strHex(Slice{sig.data(), sig.size()});
+            jo = buildSignerJson(mySigners[i]);
         }
     };
 }

src/test/jtx/multisign.h

@@ -26,6 +26,7 @@
 #include <test/jtx/tags.h>
 #include <concepts>
 #include <cstdint>
+#include <memory>
 #include <optional>
 
 namespace ripple {
@@ -66,6 +67,48 @@ signers(Account const& account, none_t);
 class msig
 {
 public:
+    // Recursive signer structure
+    struct Signer
+    {
+        Account acct;
+        Account sig;  // For leaf signers (same as acct for master key)
+        std::vector<std::shared_ptr<Signer>> nested;  // For nested signers
+
+        // Leaf signer constructor (regular signing)
+        Signer(Account const& masterSig) : acct(masterSig), sig(masterSig)
+        {
+        }
+
+        // Leaf signer constructor (with different signing key)
+        Signer(Account const& acct_, Account const& regularSig)
+            : acct(acct_), sig(regularSig)
+        {
+        }
+
+        // Nested signer constructor
+        Signer(
+            Account const& acct_,
+            std::vector<std::shared_ptr<Signer>> nested_)
+            : acct(acct_), sig(acct_), nested(std::move(nested_))
+        {
+        }
+
+        bool
+        isNested() const
+        {
+            return !nested.empty();
+        }
+
+        AccountID
+        id() const
+        {
+            return acct.id();
+        }
+    };
+
+    using SignerPtr = std::shared_ptr<Signer>;
+
+    // For backward compatibility
     struct Reg
     {
         Account acct;
@@ -74,16 +117,13 @@ public:
         Reg(Account const& masterSig) : acct(masterSig), sig(masterSig)
         {
         }
-
         Reg(Account const& acct_, Account const& regularSig)
             : acct(acct_), sig(regularSig)
         {
         }
-
         Reg(char const* masterSig) : acct(masterSig), sig(masterSig)
         {
         }
-
         Reg(char const* acct_, char const* regularSig)
             : acct(acct_), sig(regularSig)
         {
@@ -94,13 +134,32 @@ public:
         {
             return acct < rhs.acct;
         }
+
+        // Convert to Signer
+        SignerPtr
+        toSigner() const
+        {
+            return std::make_shared<Signer>(acct, sig);
+        }
     };
 
-    std::vector<Reg> signers;
+    std::vector<SignerPtr> signers;
 
 public:
+    // Initializer list constructor - resolves brace-init ambiguity
+    msig(std::initializer_list<SignerPtr> signers_)
+        : msig(std::vector<SignerPtr>(signers_))
+    {
+        // handled by :
+    }
+
+    // Direct constructor with SignerPtr vector
+    explicit msig(std::vector<SignerPtr> signers_);
+
+    // Backward compatibility constructor
     msig(std::vector<Reg> signers_);
 
+    // Variadic constructor for backward compatibility
     template <class AccountType, class... Accounts>
         requires std::convertible_to<AccountType, Reg>
     explicit msig(AccountType&& a0, Accounts&&... aN)
@@ -114,6 +173,30 @@ public:
     operator()(Env&, JTx& jt) const;
 };
 
+// Helper functions to create signers - renamed to avoid conflict with sig()
+// transaction modifier
+inline msig::SignerPtr
+msigner(Account const& acct)
+{
+    return std::make_shared<msig::Signer>(acct);
+}
+
+inline msig::SignerPtr
+msigner(Account const& acct, Account const& signingKey)
+{
+    return std::make_shared<msig::Signer>(acct, signingKey);
+}
+
+// Create nested signer with initializer list
+template <typename... Args>
+inline msig::SignerPtr
+msigner(Account const& acct, Args&&... args)
+{
+    std::vector<msig::SignerPtr> nested;
+    (nested.push_back(std::forward<Args>(args)), ...);
+    return std::make_shared<msig::Signer>(acct, std::move(nested));
+}
+
 //------------------------------------------------------------------------------
 
 /** The number of signer lists matches. */

src/test/protocol/STTx_test.cpp

@@ -1798,30 +1798,32 @@ public:
 
         // This lambda contains the bulk of the test code.
         auto testMalformedSigningAccount =
-            [this, &txn](STObject const& signer, bool expectPass) {
-                // Create SigningAccounts array.
-                STArray signers(sfSigners, 1);
-                signers.push_back(signer);
+            [this, &txn](
+                STObject const& signer, bool expectPass) -> bool /* passed */ {
+            // Create SigningAccounts array.
+            STArray signers(sfSigners, 1);
+            signers.push_back(signer);
 
-                // Insert signers into transaction.
-                STTx tempTxn(txn);
-                tempTxn.setFieldArray(sfSigners, signers);
+            // Insert signers into transaction.
+            STTx tempTxn(txn);
+            tempTxn.setFieldArray(sfSigners, signers);
 
-                Serializer rawTxn;
-                tempTxn.add(rawTxn);
-                SerialIter sit(rawTxn.slice());
-                bool serialized = false;
-                try
-                {
-                    STTx copy(sit);
-                    serialized = true;
-                }
-                catch (std::exception const&)
-                {
-                    ;  // If it threw then serialization failed.
-                }
-                BEAST_EXPECT(serialized == expectPass);
-            };
+            Serializer rawTxn;
+            tempTxn.add(rawTxn);
+            SerialIter sit(rawTxn.slice());
+            bool serialized = false;
+            try
+            {
+                STTx copy(sit);
+                serialized = true;
+            }
+            catch (std::exception const&)
+            {
+                ;  // If it threw then serialization failed.
+            }
+            BEAST_EXPECT(serialized == expectPass);
+            return serialized == expectPass;
+        };
 
         {
             // Test case 1.  Make a valid Signer object.
@@ -1831,12 +1833,21 @@ public:
             soTest1.setFieldVL(sfTxnSignature, saMultiSignature);
             testMalformedSigningAccount(soTest1, true);
         }
+
         {
-            // Test case 2.  Omit sfSigningPubKey from SigningAccount.
+            // Test case 2. Omit sfSigningPubKey from SigningAccount.
+            // With nested multi-sign, sfSigningPubKey is optional in the
+            // template, so serialization succeeds. The signer-shape helpers
+            // still reject {Account + TxnSignature} as neither leaf nor nested.
             STObject soTest2(sfSigner);
             soTest2.setAccountID(sfAccount, id2);
             soTest2.setFieldVL(sfTxnSignature, saMultiSignature);
-            testMalformedSigningAccount(soTest2, false);
+            testMalformedSigningAccount(soTest2, true);
+
+            soTest2.applyTemplateFromSField(sfSigner);
+            BEAST_EXPECT(!isLeafSigner(soTest2));
+            BEAST_EXPECT(!isNestedSigner(soTest2));
+            BEAST_EXPECT(!isValidSignerEntry(soTest2));
         }
         {
             // Test case 3.  Extra sfAmount in SigningAccount.

src/test/unit_test/multi_runner.h

@@ -332,6 +332,7 @@ multi_runner_child::run_multi(Pred pred)
     {
         if (!pred(*t))
             continue;
+
         try
         {
             failed = run(*t) || failed;

src/xrpld/app/tx/detail/Transactor.cpp

@@ -38,8 +38,12 @@
 #include <xrpl/protocol/Indexes.h>
 #include <xrpl/protocol/Protocol.h>
 #include <xrpl/protocol/STAccount.h>
+#include <xrpl/protocol/STTx.h>
 #include <xrpl/protocol/UintTypes.h>
+#include <functional>
 #include <limits>
+#include <map>
+#include <optional>
 #include <set>
 
 namespace ripple {
@@ -317,8 +321,36 @@ Transactor::calculateBaseFee(ReadView const& view, STTx const& tx)
 
     // Each signer adds one more baseFee to the minimum required fee
     // for the transaction.
-    std::size_t const signerCount =
-        tx.isFieldPresent(sfSigners) ? tx.getFieldArray(sfSigners).size() : 0;
+    std::size_t signerCount = 0;
+    if (tx.isFieldPresent(sfSigners))
+    {
+        // Depth guard to prevent stack overflow from malicious deep nesting.
+        int const maxDepth = view.rules().enabled(featureNestedMultiSign)
+            ? nestedMultiSignMaxDepth
+            : legacyMultiSignMaxDepth;
+
+        std::function<std::size_t(STArray const&, int)> countSigners;
+
+        countSigners = [&](STArray const& signers, int depth) -> std::size_t {
+            if (depth > maxDepth)
+                return 0;
+
+            std::size_t count = 0;
+
+            for (auto const& signer : signers)
+            {
+                if (isNestedSigner(signer))
+                    count += countSigners(
+                        signer.getFieldArray(sfSigners), depth + 1);
+                else if (isLeafSigner(signer))
+                    count += 1;
+            }
+
+            return count;
+        };
+
+        signerCount = countSigners(tx.getFieldArray(sfSigners), 1);
+    }
 
     XRPAmount hookExecutionFee{0};
     uint64_t burden{1};
@@ -972,167 +1004,273 @@ NotTEC
 Transactor::checkMultiSign(PreclaimContext const& ctx)
 {
     auto const id = ctx.tx.getAccountID(sfAccount);
-    // Get mTxnAccountID's SignerList and Quorum.
-    std::shared_ptr<STLedgerEntry const> sleAccountSigners =
-        ctx.view.read(keylet::signers(id));
-    // If the signer list doesn't exist the account is not multi-signing.
-    if (!sleAccountSigners)
-    {
-        JLOG(ctx.j.trace())
-            << "applyTransaction: Invalid: Not a multi-signing account.";
-        return tefNOT_MULTI_SIGNING;
-    }
 
-    // We have plans to support multiple SignerLists in the future.  The
-    // presence and defaulted value of the SignerListID field will enable that.
-    XRPL_ASSERT(
-        sleAccountSigners->isFieldPresent(sfSignerListID),
-        "ripple::Transactor::checkMultiSign : has signer list ID");
-    XRPL_ASSERT(
-        sleAccountSigners->getFieldU32(sfSignerListID) == 0,
-        "ripple::Transactor::checkMultiSign : signer list ID is 0");
+    bool const allowNested = ctx.view.rules().enabled(featureNestedMultiSign);
+    int const maxDepth =
+        allowNested ? nestedMultiSignMaxDepth : legacyMultiSignMaxDepth;
 
-    auto accountSigners =
-        SignerEntries::deserialize(*sleAccountSigners, ctx.j, "ledger");
-    if (!accountSigners)
-        return accountSigners.error();
+    // Nested multi-sign is dynamic account delegation: a parent SignerList
+    // authorizes signer accounts, not a frozen set of leaf keys. A nested
+    // signer contributes when that signer account's current on-ledger
+    // SignerList is satisfied by the transaction evidence.
+    //
+    // ancestors tracks the current proof path. An authorized signer entry that
+    // points back to an ancestor is unavailable on that path and may cause the
+    // local quorum to be cycle-adjusted below.
+    std::function<NotTEC(
+        AccountID const&, STArray const&, int, std::set<AccountID>)>
+        validateSigners;
 
-    // Get the array of transaction signers.
-    STArray const& txSigners(ctx.tx.getFieldArray(sfSigners));
+    validateSigners = [&](AccountID const& acc,
+                          STArray const& signers,
+                          int depth,
+                          std::set<AccountID> ancestors) -> NotTEC {
+        // Cycle detection is handled per authorized edge in the loop below,
+        // rather than by failing the delegated account outright.
 
-    // Walk the accountSigners performing a variety of checks and see if
-    // the quorum is met.
-
-    // Both the multiSigners and accountSigners are sorted by account.  So
-    // matching multi-signers to account signers should be a simple
-    // linear walk.  *All* signers must be valid or the transaction fails.
-    std::uint32_t weightSum = 0;
-    auto iter = accountSigners->begin();
-    for (auto const& txSigner : txSigners)
-    {
-        AccountID const txSignerAcctID = txSigner.getAccountID(sfAccount);
-
-        // Attempt to match the SignerEntry with a Signer;
-        while (iter->account < txSignerAcctID)
+        if (depth > maxDepth)
         {
-            if (++iter == accountSigners->end())
+            JLOG(ctx.j.trace())
+                << "checkMultiSign: Multi-signing depth limit exceeded at "
+                << depth << " (max=" << maxDepth << ")";
+            return temMALFORMED;
+        }
+
+        ancestors.insert(acc);
+
+        // Get the SignerList for the account we're validating signers for.
+        std::shared_ptr<STLedgerEntry const> sleAllowedSigners =
+            ctx.view.read(keylet::signers(acc));
+
+        if (!sleAllowedSigners)
+        {
+            JLOG(ctx.j.trace()) << "checkMultiSign: Account " << acc
+                                << " not set up for multi-signing.";
+            return tefNOT_MULTI_SIGNING;
+        }
+
+        // We have plans to support multiple SignerLists in the future. The
+        // presence and defaulted value of the SignerListID field will enable
+        // that.
+        XRPL_ASSERT(
+            sleAllowedSigners->isFieldPresent(sfSignerListID),
+            "ripple::Transactor::checkMultiSign : has signer list ID");
+        XRPL_ASSERT(
+            sleAllowedSigners->getFieldU32(sfSignerListID) == 0,
+            "ripple::Transactor::checkMultiSign : signer list ID is 0");
+
+        uint32_t const quorum = sleAllowedSigners->getFieldU32(sfSignerQuorum);
+        uint32_t sum{0};
+
+        auto allowedSigners =
+            SignerEntries::deserialize(*sleAllowedSigners, ctx.j, "ledger");
+        if (!allowedSigners)
+            return allowedSigners.error();
+
+        // Build lookup map for signer validation and weight retrieval.
+        std::map<AccountID, uint16_t> signerWeights;
+        uint32_t totalWeight{0}, cyclicWeight{0};
+        for (auto const& entry : *allowedSigners)
+        {
+            signerWeights[entry.account] = entry.weight;
+            totalWeight += entry.weight;
+            if (ancestors.count(entry.account))
+                cyclicWeight += entry.weight;
+        }
+
+        // Walk the signers array, validating each signer. Signers must be in
+        // strict ascending order for consensus.
+        std::optional<AccountID> prevSigner;
+
+        for (auto const& signerEntry : signers)
+        {
+            AccountID const signer = signerEntry.getAccountID(sfAccount);
+
+            if (prevSigner && signer <= *prevSigner)
             {
                 JLOG(ctx.j.trace())
-                    << "applyTransaction: Invalid SigningAccount.Account.";
+                    << "checkMultiSign: Signers not in strict ascending order: "
+                    << signer << " <= " << *prevSigner;
+                return temMALFORMED;
+            }
+            prevSigner = signer;
+
+            // Parent-edge authorization is checked before cycle skipping so an
+            // unauthorized cyclic entry is rejected, not silently ignored.
+            auto const weightIt = signerWeights.find(signer);
+            if (weightIt == signerWeights.end())
+            {
+                JLOG(ctx.j.trace())
+                    << "checkMultiSign: Invalid signer " << signer
+                    << " not in signer list for " << acc;
                 return tefBAD_SIGNATURE;
             }
-        }
-        if (iter->account != txSignerAcctID)
-        {
-            // The SigningAccount is not in the SignerEntries.
-            JLOG(ctx.j.trace())
-                << "applyTransaction: Invalid SigningAccount.Account.";
-            return tefBAD_SIGNATURE;
-        }
 
-        // We found the SigningAccount in the list of valid signers.  Now we
-        // need to compute the accountID that is associated with the signer's
-        // public key.
-        auto const spk = txSigner.getFieldVL(sfSigningPubKey);
-
-        if (!(ctx.flags & tapDRY_RUN) && !publicKeyType(makeSlice(spk)))
-        {
-            JLOG(ctx.j.trace())
-                << "checkMultiSign: signing public key type is unknown";
-            return tefBAD_SIGNATURE;
-        }
-
-        // This ternary is only needed to handle `simulate`
-        XRPL_ASSERT(
-            (ctx.flags & tapDRY_RUN) || !spk.empty(),
-            "ripple::Transactor::checkMultiSign : non-empty signer or "
-            "simulation");
-        AccountID const signingAcctIDFromPubKey = spk.empty()
-            ? txSignerAcctID
-            : calcAccountID(PublicKey(makeSlice(spk)));
-
-        // Verify that the signingAcctID and the signingAcctIDFromPubKey
-        // belong together.  Here are the rules:
-        //
-        //   1. "Phantom account": an account that is not in the ledger
-        //      A. If signingAcctID == signingAcctIDFromPubKey and the
-        //         signingAcctID is not in the ledger then we have a phantom
-        //         account.
-        //      B. Phantom accounts are always allowed as multi-signers.
-        //
-        //   2. "Master Key"
-        //      A. signingAcctID == signingAcctIDFromPubKey, and signingAcctID
-        //         is in the ledger.
-        //      B. If the signingAcctID in the ledger does not have the
-        //         asfDisableMaster flag set, then the signature is allowed.
-        //
-        //   3. "Regular Key"
-        //      A. signingAcctID != signingAcctIDFromPubKey, and signingAcctID
-        //         is in the ledger.
-        //      B. If signingAcctIDFromPubKey == signingAcctID.RegularKey (from
-        //         ledger) then the signature is allowed.
-        //
-        // No other signatures are allowed.  (January 2015)
-
-        // In any of these cases we need to know whether the account is in
-        // the ledger.  Determine that now.
-        auto sleTxSignerRoot = ctx.view.read(keylet::account(txSignerAcctID));
-
-        if (signingAcctIDFromPubKey == txSignerAcctID)
-        {
-            // Either Phantom or Master.  Phantoms automatically pass.
-            if (sleTxSignerRoot)
+            // The signer is authorized by acc, but is already in the current
+            // proof path. Treat that cyclic ancestor edge as unavailable for
+            // this path; do not recurse into it and do not count its weight.
+            if (ancestors.count(signer))
             {
-                // Master Key.  Account may not have asfDisableMaster set.
-                std::uint32_t const signerAccountFlags =
-                    sleTxSignerRoot->getFieldU32(sfFlags);
+                JLOG(ctx.j.trace())
+                    << "checkMultiSign: Skipping cyclic signer: " << signer;
+                continue;
+            }
+            uint16_t const weight = weightIt->second;
 
-                if (signerAccountFlags & lsfDisableMaster)
+            if (isNestedSigner(signerEntry))
+            {
+                STArray const& nestedSigners =
+                    signerEntry.getFieldArray(sfSigners);
+                NotTEC result = validateSigners(
+                    signer, nestedSigners, depth + 1, ancestors);
+                if (!isTesSuccess(result))
+                    return result;
+
+                sum += weight;
+
+                JLOG(ctx.j.trace())
+                    << "checkMultiSign: Nested signer " << signer
+                    << " validated, weight=" << weight << ", depth=" << depth
+                    << ", sum=" << sum << "/" << quorum;
+            }
+            else if (isLeafSigner(signerEntry))
+            {
+                auto const spk = signerEntry.getFieldVL(sfSigningPubKey);
+
+                // spk being non-empty in non-simulate is checked in
+                // STTx::checkMultiSign.
+                if (!spk.empty() && !publicKeyType(makeSlice(spk)))
                 {
                     JLOG(ctx.j.trace())
-                        << "applyTransaction: Signer:Account lsfDisableMaster.";
-                    return tefMASTER_DISABLED;
+                        << "checkMultiSign: Unknown public key type for signer "
+                        << signer;
+                    return tefBAD_SIGNATURE;
                 }
+
+                XRPL_ASSERT(
+                    (ctx.flags & tapDRY_RUN) || !spk.empty(),
+                    "ripple::Transactor::checkMultiSign : non-empty signer or "
+                    "simulation");
+                AccountID const signingAcctIDFromPubKey = spk.empty()
+                    ? signer
+                    : calcAccountID(PublicKey(makeSlice(spk)));
+
+                auto sleTxSignerRoot = ctx.view.read(keylet::account(signer));
+
+                if (signingAcctIDFromPubKey == signer)
+                {
+                    // Either Phantom or Master. Phantoms automatically pass.
+                    if (sleTxSignerRoot)
+                    {
+                        // Master Key. Account may not have asfDisableMaster
+                        // set.
+                        std::uint32_t const signerAccountFlags =
+                            sleTxSignerRoot->getFieldU32(sfFlags);
+
+                        if (signerAccountFlags & lsfDisableMaster)
+                        {
+                            JLOG(ctx.j.trace())
+                                << "checkMultiSign: Signer " << signer
+                                << " has lsfDisableMaster set.";
+                            return tefMASTER_DISABLED;
+                        }
+                    }
+                }
+                else
+                {
+                    // May be a Regular Key. Let's find out.
+                    if (!sleTxSignerRoot)
+                    {
+                        JLOG(ctx.j.trace())
+                            << "checkMultiSign: Non-phantom signer " << signer
+                            << " lacks account root.";
+                        return tefBAD_SIGNATURE;
+                    }
+
+                    if (!sleTxSignerRoot->isFieldPresent(sfRegularKey))
+                    {
+                        JLOG(ctx.j.trace()) << "checkMultiSign: Signer "
+                                            << signer << " lacks RegularKey.";
+                        return tefBAD_SIGNATURE;
+                    }
+                    if (signingAcctIDFromPubKey !=
+                        sleTxSignerRoot->getAccountID(sfRegularKey))
+                    {
+                        JLOG(ctx.j.trace())
+                            << "checkMultiSign: Signer " << signer
+                            << " pubkey doesn't match RegularKey.";
+                        return tefBAD_SIGNATURE;
+                    }
+                }
+
+                sum += weight;
+
+                JLOG(ctx.j.trace())
+                    << "checkMultiSign: Leaf signer " << signer
+                    << " validated, weight=" << weight << ", depth=" << depth
+                    << ", sum=" << sum << "/" << quorum;
+            }
+            else
+            {
+                JLOG(ctx.j.trace())
+                    << "checkMultiSign: Malformed signer entry for " << signer;
+                return tefBAD_SIGNATURE;
             }
         }
-        else
+
+        if (cyclicWeight > totalWeight)
         {
-            // May be a Regular Key.  Let's find out.
-            // Public key must hash to the account's regular key.
-            if (!sleTxSignerRoot)
-            {
-                JLOG(ctx.j.trace()) << "applyTransaction: Non-phantom signer "
-                                       "lacks account root.";
-                return tefBAD_SIGNATURE;
-            }
-
-            if (!sleTxSignerRoot->isFieldPresent(sfRegularKey))
-            {
-                JLOG(ctx.j.trace())
-                    << "applyTransaction: Account lacks RegularKey.";
-                return tefBAD_SIGNATURE;
-            }
-            if (signingAcctIDFromPubKey !=
-                sleTxSignerRoot->getAccountID(sfRegularKey))
-            {
-                JLOG(ctx.j.trace())
-                    << "applyTransaction: Account doesn't match RegularKey.";
-                return tefBAD_SIGNATURE;
-            }
+            JLOG(ctx.j.error()) << "checkMultiSign: Invariant violation for "
+                                << acc << ": cyclicWeight (" << cyclicWeight
+                                << ") > totalWeight (" << totalWeight << ")";
+            return tefINTERNAL;
         }
-        // The signer is legitimate.  Add their weight toward the quorum.
-        weightSum += iter->weight;
-    }
 
-    // Cannot perform transaction if quorum is not met.
-    if (weightSum < sleAccountSigners->getFieldU32(sfSignerQuorum))
+        // Dynamic delegation still requires the delegated account's own policy
+        // to be satisfied. The only adjustment is for authorized ancestor edges
+        // that cannot be used without circular proof. If those cyclic edges
+        // make the configured quorum unreachable, require all remaining
+        // non-cyclic weight.
+        uint32_t cycleAdjustedQuorum = quorum;
+        uint32_t const maxAchievable = totalWeight - cyclicWeight;
+
+        if (cyclicWeight > 0 && maxAchievable < quorum)
+        {
+            JLOG(ctx.j.warn()) << "checkMultiSign: Cycle-adjusted quorum for "
+                               << acc << ": " << quorum << " -> "
+                               << maxAchievable << " (total=" << totalWeight
+                               << ", cyclic=" << cyclicWeight << ")";
+            cycleAdjustedQuorum = maxAchievable;
+        }
+
+        if (cycleAdjustedQuorum == 0)
+        {
+            JLOG(ctx.j.warn()) << "checkMultiSign: All signers for " << acc
+                               << " are cyclic - no valid signing path exists.";
+            return tefBAD_QUORUM;
+        }
+
+        if (sum < cycleAdjustedQuorum)
+        {
+            JLOG(ctx.j.trace()) << "checkMultiSign: Quorum not met for " << acc
+                                << " at depth " << depth << " (sum=" << sum
+                                << ", required=" << cycleAdjustedQuorum << ")";
+            return tefBAD_QUORUM;
+        }
+
+        return tesSUCCESS;
+    };
+
+    STArray const& entries(ctx.tx.getFieldArray(sfSigners));
+
+    NotTEC result = validateSigners(id, entries, 1, {});
+    if (!isTesSuccess(result))
     {
         JLOG(ctx.j.trace())
-            << "applyTransaction: Signers failed to meet quorum.";
-        return tefBAD_QUORUM;
+            << "checkMultiSign: Validation failed with " << transToken(result);
+        return result;
     }
 
-    // Met the quorum.  Continue.
     return tesSUCCESS;
 }
 

src/xrpld/rpc/detail/TransactionSign.cpp

@@ -37,9 +37,11 @@
 #include <xrpl/protocol/RPCErr.h>
 #include <xrpl/protocol/STAccount.h>
 #include <xrpl/protocol/STParsedJSON.h>
+#include <xrpl/protocol/STTx.h>
 #include <xrpl/protocol/Sign.h>
 #include <xrpl/protocol/TxFlags.h>
 #include <algorithm>
+#include <functional>
 #include <iterator>
 
 namespace ripple {
@@ -1318,19 +1320,31 @@ transactionSubmitMultiSigned(
     if (signers.empty())
         return RPC::make_param_error("tx_json.Signers array may not be empty.");
 
-    // The Signers array may only contain Signer objects.
-    if (std::find_if_not(
-            signers.begin(), signers.end(), [](STObject const& obj) {
-                return (
-                    // A Signer object always contains these fields and no
-                    // others.
-                    obj.isFieldPresent(sfAccount) &&
-                    obj.isFieldPresent(sfSigningPubKey) &&
-                    obj.isFieldPresent(sfTxnSignature) && obj.getCount() == 3);
-            }) != signers.end())
+    // Recursively validate signer entry structure (including nested ones).
+    // Feature enablement is enforced later in preflight; RPC only validates
+    // shape here.
+    std::function<bool(STArray const&, int)> validateSignersRecursive;
+    validateSignersRecursive = [&](STArray const& arr, int depth) -> bool {
+        if (depth > nestedMultiSignMaxDepth)
+            return false;
+
+        for (auto const& signer : arr)
+        {
+            if (!isValidSignerEntry(signer))
+                return false;
+
+            if (isNestedSigner(signer) &&
+                !validateSignersRecursive(
+                    signer.getFieldArray(sfSigners), depth + 1))
+                return false;
+        }
+        return true;
+    };
+
+    if (!validateSignersRecursive(signers, 1))
     {
         return RPC::make_param_error(
-            "Signers array may only contain Signer entries.");
+            "Signers array may only contain valid Signer entries.");
     }
 
     // The array must be sorted and validated.