From 40c29bbfd66360c5c9e597aaeea809588456cdaa Mon Sep 17 00:00:00 2001
From: Vito Tumas <5780819+Tapanito@users.noreply.github.com>
Date: Tue, 16 Dec 2025 19:19:27 +0100
Subject: [PATCH] Fix LoanBrokerSet debtMaximum limits (#6116)

---
 src/test/app/LoanBroker_test.cpp          | 73 +++++++++++++++++++++++
 src/xrpld/app/tx/detail/LoanBrokerSet.cpp | 12 ++++
 2 files changed, 85 insertions(+)

diff --git a/src/test/app/LoanBroker_test.cpp b/src/test/app/LoanBroker_test.cpp
index 72a732d043..ef4804f217 100644
--- a/src/test/app/LoanBroker_test.cpp
+++ b/src/test/app/LoanBroker_test.cpp
@@ -1436,10 +1436,83 @@ class LoanBroker_test : public beast::unit_test::suite
         });
     }
 
+    void
+    testLoanBrokerSetDebtMaximum()
+    {
+        testcase("testLoanBrokerSetDebtMaximum");
+        using namespace jtx;
+        using namespace loanBroker;
+        Account const issuer{"issuer"};
+        Account const alice{"alice"};
+        Env env(*this);
+        Vault vault{env};
+
+        env.fund(XRP(100'000), issuer, alice);
+        env.close();
+
+        PrettyAsset const asset = [&]() {
+            env(trust(alice, issuer["IOU"](1'000'000)), THISLINE);
+            env.close();
+            return PrettyAsset(issuer["IOU"]);
+        }();
+
+        env(pay(issuer, alice, asset(100'000)), THISLINE);
+        env.close();
+
+        auto [tx, vaultKeylet] = vault.create({.owner = alice, .asset = asset});
+        env(tx, THISLINE);
+        env.close();
+        auto const le = env.le(vaultKeylet);
+        VaultInfo vaultInfo = [&]() {
+            if (BEAST_EXPECT(le))
+                return VaultInfo{asset, vaultKeylet.key, le->at(sfAccount)};
+            return VaultInfo{asset, {}, {}};
+        }();
+        if (vaultInfo.vaultID == uint256{})
+            return;
+
+        env(vault.deposit(
+                {.depositor = alice,
+                 .id = vaultKeylet.key,
+                 .amount = asset(50)}),
+            THISLINE);
+        env.close();
+
+        auto const brokerKeylet =
+            keylet::loanbroker(alice.id(), env.seq(alice));
+        env(set(alice, vaultInfo.vaultID), THISLINE);
+        env.close();
+
+        Account const borrower{"borrower"};
+        env.fund(XRP(1'000), borrower);
+        env(loan::set(borrower, brokerKeylet.key, asset(50).value()),
+            sig(sfCounterpartySignature, alice),
+            fee(env.current()->fees().base * 2),
+            THISLINE);
+        auto const broker = env.le(brokerKeylet);
+        if (!BEAST_EXPECT(broker))
+            return;
+
+        BEAST_EXPECT(broker->at(sfDebtTotal) == 50);
+        auto debtTotal = broker->at(sfDebtTotal);
+
+        auto tx2 = set(alice, vaultInfo.vaultID);
+        tx2[sfLoanBrokerID] = to_string(brokerKeylet.key);
+        tx2[sfDebtMaximum] = debtTotal - 1;
+        env(tx2, ter(tecLIMIT_EXCEEDED), THISLINE);
+
+        tx2[sfDebtMaximum] = debtTotal + 1;
+        env(tx2, ter(tesSUCCESS), THISLINE);
+
+        tx2[sfDebtMaximum] = 0;
+        env(tx2, ter(tesSUCCESS), THISLINE);
+    }
+
 public:
     void
     run() override
     {
+        testLoanBrokerSetDebtMaximum();
         testLoanBrokerCoverDepositNullVault();
 
         testDisabled();
diff --git a/src/xrpld/app/tx/detail/LoanBrokerSet.cpp b/src/xrpld/app/tx/detail/LoanBrokerSet.cpp
index 7b12a6cf39..a3ac785674 100644
--- a/src/xrpld/app/tx/detail/LoanBrokerSet.cpp
+++ b/src/xrpld/app/tx/detail/LoanBrokerSet.cpp
@@ -89,6 +89,18 @@ LoanBrokerSet::preclaim(PreclaimContext const& ctx)
             JLOG(ctx.j.warn()) << "Account is not the owner of the LoanBroker.";
             return tecNO_PERMISSION;
         }
+
+        if (auto const debtMax = tx[~sfDebtMaximum])
+        {
+            // Can't reduce the debt maximum below the current total debt
+            auto const currentDebtTotal = sleBroker->at(sfDebtTotal);
+            if (*debtMax != 0 && *debtMax < currentDebtTotal)
+            {
+                JLOG(ctx.j.warn())
+                    << "Cannot reduce DebtMaximum below current DebtTotal.";
+                return tecLIMIT_EXCEEDED;
+            }
+        }
     }
     else
     {