diff --git a/Projects/UOContent.Tests/Tests/Accounting/AccountPasswordTests.cs b/Projects/UOContent.Tests/Tests/Accounting/AccountPasswordTests.cs
index 42389cbef..5253d24de 100644
--- a/Projects/UOContent.Tests/Tests/Accounting/AccountPasswordTests.cs
+++ b/Projects/UOContent.Tests/Tests/Accounting/AccountPasswordTests.cs
@@ -73,4 +73,66 @@ public class AccountPasswordTests : IDisposable
         Assert.True(account.CheckPassword("hunter2"));
         Assert.Equal(afterFirst, account.Password);
     }
+
+    /// <summary>
+    /// Builds an account whose stored hash is what the pre-fix SetPassword produced when a SHA2
+    /// account was upgraded to Argon2: argon2(username + password), tagged Argon2, whose phrase
+    /// rule omits the username. Unrecoverable without the plaintext, hence the repair path.
+    /// </summary>
+    private static Account CreateMisMigratedAccount(string username)
+    {
+        AccountSecurity.CurrentAlgorithm = PasswordProtectionAlgorithm.Argon2;
+        var account = new Account(username, Password);
+        account.Password = AccountSecurity.CurrentPasswordProtection
+            .EncryptPassword($"{username}{Password}");
+
+        return account;
+    }
+
+    [Fact]
+    public void MisMigratedAccount_IsRejected_WhenRepairIsDisabled()
+    {
+        var account = CreateMisMigratedAccount("repair-off-user");
+        AccountSecurity.RepairMigratedPasswords = false;
+
+        Assert.False(account.CheckPassword(Password));
+    }
+
+    [Fact]
+    public void MisMigratedAccount_IsRepaired_WhenRepairIsEnabled()
+    {
+        var account = CreateMisMigratedAccount("repair-on-user");
+        AccountSecurity.RepairMigratedPasswords = true;
+
+        try
+        {
+            Assert.True(account.CheckPassword(Password));
+        }
+        finally
+        {
+            AccountSecurity.RepairMigratedPasswords = false;
+        }
+
+        // Repaired in place: it must now verify with the flag back off.
+        Assert.True(account.CheckPassword(Password));
+        Assert.False(account.CheckPassword("wrong-password"));
+    }
+
+    [Fact]
+    public void WrongPassword_IsStillRejected_WhenRepairIsEnabled()
+    {
+        AccountSecurity.CurrentAlgorithm = PasswordProtectionAlgorithm.Argon2;
+        var account = new Account("repair-wrong-pass-user", Password);
+        AccountSecurity.RepairMigratedPasswords = true;
+
+        try
+        {
+            Assert.False(account.CheckPassword("wrong-password"));
+            Assert.True(account.CheckPassword(Password));
+        }
+        finally
+        {
+            AccountSecurity.RepairMigratedPasswords = false;
+        }
+    }
 }
diff --git a/Projects/UOContent/Accounting/Account.cs b/Projects/UOContent/Accounting/Account.cs
index a4246784f..d60f387d0 100644
--- a/Projects/UOContent/Accounting/Account.cs
+++ b/Projects/UOContent/Accounting/Account.cs
@@ -403,18 +403,36 @@ public partial class Account : IAccount, IComparable<Account>
 
     public bool CheckPassword(string plainPassword)
     {
+        var protection = AccountSecurity.GetPasswordProtection(_passwordAlgorithm);
         var phrase = UsesUsernamePhrase(_passwordAlgorithm) ? $"{_username}{plainPassword}" : plainPassword;
+        var forceRehash = false;
 
-        var ok = AccountSecurity.GetPasswordProtection(_passwordAlgorithm).ValidatePassword(Password, phrase);
-        if (!ok)
+        if (!protection.ValidatePassword(Password, phrase))
         {
-            return false;
+            // A pre-fix SetPassword stored username + password under an algorithm whose phrase rule
+            // omits the username. Nothing in the hash distinguishes that from a forgotten password,
+            // so the only way to detect it is to try the other phrase -- which costs a second
+            // verify on every failed login. Off by default for exactly that reason.
+            if (!AccountSecurity.RepairMigratedPasswords || UsesUsernamePhrase(_passwordAlgorithm) ||
+                !protection.ValidatePassword(Password, $"{_username}{plainPassword}"))
+            {
+                return false;
+            }
+
+            logger.Warning(
+                "Account '{Username}' had a password mis-migrated by a pre-fix SetPassword; repairing it.",
+                _username
+            );
+
+            // The stored hash may already carry current parameters, so NeedsRehash would say no
+            // and the account would verify once and stay broken.
+            forceRehash = true;
         }
 
         // Rehash when either the algorithm or its cost parameters have moved on. The short-circuit
         // ordering is load-bearing: NeedsRehash must never be handed a hash produced by a different
-        // algorithm, and the first clause guarantees it is not.
-        if (_passwordAlgorithm != AccountSecurity.CurrentAlgorithm ||
+        // algorithm, and the second clause guarantees it is not.
+        if (forceRehash || _passwordAlgorithm != AccountSecurity.CurrentAlgorithm ||
             AccountSecurity.CurrentPasswordProtection.NeedsRehash(Password))
         {
             logger.Debug("Rehashing the password for account '{Username}'.", _username);
diff --git a/Projects/UOContent/Accounting/Security/AccountSecurity.cs b/Projects/UOContent/Accounting/Security/AccountSecurity.cs
index 9f7faeafc..c2c495c2f 100644
--- a/Projects/UOContent/Accounting/Security/AccountSecurity.cs
+++ b/Projects/UOContent/Accounting/Security/AccountSecurity.cs
@@ -35,6 +35,15 @@ public static class AccountSecurity
 {
     public static PasswordProtectionAlgorithm CurrentAlgorithm { get; set; }
 
+    /// <summary>
+    /// Enables a one-time repair for accounts whose password was corrupted by the pre-fix
+    /// SetPassword, which hashed username + password but tagged it with an algorithm whose phrase
+    /// rule omits the username. Off by default: the repair costs a second verify on every FAILED
+    /// login, and failed logins are the credential-stuffing surface. Turn it on for a migration
+    /// window, then off again.
+    /// </summary>
+    public static bool RepairMigratedPasswords { get; set; }
+
     public static IPasswordProtection CurrentPasswordProtection => GetPasswordProtection(CurrentAlgorithm);
 
     public static void Configure()
@@ -45,6 +54,9 @@ public static class AccountSecurity
                 PasswordProtectionAlgorithm.Argon2
             );
 
+        RepairMigratedPasswords =
+            ServerConfiguration.GetOrUpdateSetting("accountSecurity.repairMigratedPasswords", false);
+
         if (CurrentAlgorithm < PasswordProtectionAlgorithm.SHA2)
         {
             throw new Exception($"Security: {CurrentAlgorithm} is obsolete and not secure. Do not use it.");