fix(security): config PUT derives its preserved system-owned fields from the PATCH deny-list (BaseDeniedFields) so ProDowngradedUtc/OnboardedAt can no longer be written by a tenant admin — a client-supplied ProDowngradedUtc refreshed the retention-downgrade grace anchor indefinitely, keeping the Pro retention cap on a Community tenant

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/Backend/AutopilotMonitor.Functions.Tests/TenantConfigPatchServiceTests.cs

@@ -680,6 +680,51 @@ public class TenantConfigPatchServiceTests
         Assert.Equal(schema.Count, schema.Select(f => f.Name).Distinct().Count());
     }
 
+    /// <summary>
+    /// The full-model PUT (config/{tenantId}) must honour the same deny-list as the field
+    /// patch: a client body carrying ProDowngradedUtc (retention-downgrade grace anchor),
+    /// OnboardedAt, DomainName, plan/trial or provenance fields must not change the stored
+    /// values. Driven by the deny-list itself so a new denied field is covered automatically.
+    /// </summary>
+    [Fact]
+    public void PreserveDeniedFields_OverwritesEveryDeniedModelPropertyFromStored()
+    {
+        var props = typeof(TenantConfiguration)
+            .GetProperties(System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.Instance)
+            .Where(p => p.CanWrite && TenantConfigPatchService.BaseDeniedFields.Contains(p.Name))
+            .ToList();
+        Assert.Contains(props, p => p.Name == "ProDowngradedUtc");
+        Assert.Contains(props, p => p.Name == "OnboardedAt");
+        Assert.Contains(props, p => p.Name == "DomainName");
+
+        var stored = new TenantConfiguration { TenantId = "t1" };
+        var incoming = new TenantConfiguration { TenantId = "t1" };
+        foreach (var p in props)
+        {
+            p.SetValue(stored, Distinct(p.PropertyType, 1));
+            p.SetValue(incoming, Distinct(p.PropertyType, 2));
+            Assert.NotEqual(p.GetValue(stored), p.GetValue(incoming));
+        }
+
+        TenantConfigPatchService.PreserveDeniedFields(incoming, stored);
+
+        foreach (var p in props)
+            Assert.Equal(p.GetValue(stored), p.GetValue(incoming));
+    }
+
+    private static object Distinct(Type t, int seed)
+    {
+        var u = Nullable.GetUnderlyingType(t) ?? t;
+        if (u == typeof(string)) return $"value-{seed}";
+        if (u == typeof(bool)) return seed % 2 == 0;
+        if (u == typeof(int)) return seed;
+        if (u == typeof(long)) return (long)seed;
+        if (u == typeof(DateTime)) return new DateTime(2026, 1, seed, 0, 0, 0, DateTimeKind.Utc);
+        if (u == typeof(DateTimeOffset)) return new DateTimeOffset(2026, 1, seed, 0, 0, 0, TimeSpan.Zero);
+        if (u.IsEnum) return Enum.GetValues(u).GetValue(seed % Enum.GetValues(u).Length)!;
+        throw new NotSupportedException($"Add a Distinct() case for {u.Name}");
+    }
+
     [Fact]
     public void FieldsSchema_DeniedFields_MatchThePatchDenyList()
     {

src/Backend/AutopilotMonitor.Functions/Functions/Config/UpdateTenantConfigurationFunction.cs

@@ -110,11 +110,15 @@ namespace AutopilotMonitor.Functions.Functions.Config
                 // admins are served the FULL config so this is normally a no-op for them.)
                 config.RestoreRedactedSecretsFrom(existingConfig);
 
-                // DomainName is tenant identity: seeded once from the first login's UPN (AuthFunction)
-                // and never client-writable — for ANY caller, GA included — mirroring the patch flow
-                // ("identity — never writable"). Without this preserve the full-model PUT persisted an
-                // arbitrary string here, which is rendered into the welcome/farewell mails.
-                config.DomainName = existingConfig.DomainName;
+                // System-owned fields are never client-writable — for ANY caller, GA included:
+                // identity (DomainName, seeded once from the first login's UPN), onboarding/auth
+                // provenance (OnboardedAt/OnboardedBy/LastAuthClientId*), app-reg homing
+                // (HomedAppClientId — the 2026-07-31 stale-round-trip incident), and plan/trial/
+                // downgrade state (PlanTier/Trial*/ProDowngradedUtc — mutable only via the plan
+                // and trial endpoints; a client-supplied ProDowngradedUtc would refresh the
+                // retention-downgrade grace anchor indefinitely). The set is the PATCH endpoint's
+                // BaseDeniedFields, so the two write paths cannot drift.
+                TenantConfigPatchService.PreserveDeniedFields(config, existingConfig);
 
                 // Shared model validation (rate limits, contact address, webhook/Teams SSRF, custom
                 // headers, notification channels, diagnostics SAS, retention cap) — single source
@@ -164,26 +168,6 @@ namespace AutopilotMonitor.Functions.Functions.Config
                     config.DisabledUntil = existingConfig.DisabledUntil;
                 }
 
-                // App-reg homing is IMMUTABLE via the generic PUT — for ALL callers, GA included.
-                // The only writers are the consent-driven auto-flip and POST config/{tenantId}/
-                // app-homing (AppHomingFunction). Prod incident 2026-07-31 18:03Z: the consent
-                // auto-flip landed, and 600ms later the frontend's routine "persist validation
-                // toggle" PUT — round-tripping a config loaded BEFORE the flip — silently reverted
-                // the homing to legacy, because a GA caller's stale null was indistinguishable
-                // from an intentional revert. A full-model PUT can never carry that intent.
-                config.HomedAppClientId = existingConfig.HomedAppClientId;
-
-                // Last-seen auth provenance is system-written (AuthFunction) — never via PUT,
-                // for ANY caller: a round-tripped stale view must not rewind the observability.
-                config.LastAuthClientId = existingConfig.LastAuthClientId;
-                config.LastAuthClientIdSince = existingConfig.LastAuthClientIdSince;
-
-                // OnboardedBy is system-written once at first login (AuthFunction) and immutable
-                // after that — never via PUT, for ANY caller: a revoke → re-approve cycle
-                // auto-promotes whatever UPN is stored here, so a client-supplied value could
-                // smuggle in an arbitrary promotable user (or null out the audit provenance).
-                config.OnboardedBy = existingConfig.OnboardedBy;
-
                 // Safety: if GA gate is off, force UnrestrictedMode to false
                 if (!config.UnrestrictedModeEnabled)
                 {
@@ -193,16 +177,6 @@ namespace AutopilotMonitor.Functions.Functions.Config
                 // MaxNdjsonPayloadSizeMB is table-only — always preserve existing value
                 config.MaxNdjsonPayloadSizeMB = existingConfig.MaxNdjsonPayloadSizeMB;
 
-                // Plan/trial fields are mutable ONLY via the dedicated plan/trial endpoints
-                // (PATCH config/{tenantId}/plan, POST config/{tenantId}/trial). The generic PUT
-                // deserializes the full model, so without this preserve a round-tripped stale
-                // view would silently reset the tenant's edition/trial — for ALL callers, GA included.
-                config.PlanTier = existingConfig.PlanTier;
-                config.TrialExpiresUtc = existingConfig.TrialExpiresUtc;
-                config.TrialStartedUtc = existingConfig.TrialStartedUtc;
-                config.TrialConsumed = existingConfig.TrialConsumed;
-                config.TrialGrantedBy = existingConfig.TrialGrantedBy;
-
                 // Save configuration (retention cap already enforced by ValidateModel above)
                 var writeSource = ResolveWriteSource(req.Query["intent"]);
                 await _configService.SaveConfigurationAsync(config, writeSource, null);

src/Backend/AutopilotMonitor.Functions/Services/TenantConfigPatchService.cs

@@ -127,6 +127,25 @@ namespace AutopilotMonitor.Functions.Services
             "ProDowngradedUtc",
         };
 
+        /// <summary>
+        /// Full-model PUT counterpart of <see cref="BaseDeniedFields"/>: copies every denied model
+        /// property from the stored config onto the incoming one so a client body can never write
+        /// them — for ANY caller, GA included. The PUT deserializes the whole model, so a hand-kept
+        /// preserve list drifts (ProDowngradedUtc/OnboardedAt were missing, letting a tenant admin
+        /// refresh the retention-downgrade grace anchor forever). Deriving it from the single
+        /// deny-list keeps both write paths in lock-step. Server-stamped fields (TenantId,
+        /// UpdatedBy, LastUpdated) are copied too; the PUT overwrites them afterwards.
+        /// </summary>
+        internal static void PreserveDeniedFields(TenantConfiguration target, TenantConfiguration stored)
+        {
+            foreach (var name in BaseDeniedFields)
+            {
+                // PartitionKey/RowKey/Timestamp/ETag are table plumbing, not model properties.
+                if (ModelProperties.TryGetValue(name, out var prop))
+                    prop.SetValue(target, prop.GetValue(stored));
+            }
+        }
+
         /// <summary>
         /// Additional deny-list for non-GA tiers (phase 2): the GA-only toggles the PUT
         /// endpoint silently reverts for tenant admins are an explicit 400 here.