Bound GraphQL nested-filter recursion depth

src/Core/Models/GraphQLFilterParsers.cs

@@ -39,6 +39,40 @@ public GQLFilterParser(RuntimeConfigProvider runtimeConfigProvider, IMetadataPro
         _metadataProviderFactory = metadataProviderFactory;
     }
 
+    // Absolute ceiling on relationship-nesting depth within a single GraphQL filter argument.
+    // Applied when no stricter runtime.graphql.depth-limit is configured, to prevent nested-filter depth-bomb DoS
+    // (deeply nested filters amplify into correlated EXISTS subqueries that HotChocolate's execution-depth rule does not bound).
+    internal const int MAX_NESTED_FILTER_DEPTH = 20;
+
+    /// <summary>
+    /// Returns the maximum allowed relationship-nesting depth for GraphQL filters:
+    /// the configured runtime.graphql.depth-limit when set and stricter, otherwise the hardcoded safety ceiling.
+    /// </summary>
+    internal int GetMaxNestedFilterDepth()
+    {
+        int? configuredDepthLimit = _configProvider.GetConfig().Runtime?.GraphQL?.DepthLimit;
+        if (configuredDepthLimit is > 0 && configuredDepthLimit.Value < MAX_NESTED_FILTER_DEPTH)
+        {
+            return configuredDepthLimit.Value;
+        }
+
+        return MAX_NESTED_FILTER_DEPTH;
+    }
+
+    /// <summary>
+    /// Enforces the relationship-nesting depth limit for GraphQL filters, throwing a BadRequest when exceeded.
+    /// </summary>
+    internal static void EnsureWithinNestedFilterDepth(int nestingLevel, int maxNestedFilterDepth)
+    {
+        if (nestingLevel > maxNestedFilterDepth)
+        {
+            throw new DataApiBuilderException(
+                message: $"The provided GraphQL filter exceeds the maximum allowed nesting depth of {maxNestedFilterDepth}.",
+                statusCode: HttpStatusCode.BadRequest,
+                subStatusCode: DataApiBuilderException.SubStatusCodes.BadRequest);
+        }
+    }
+
     /// <summary>
     /// Parse a predicate for a *FilterInput input type
     /// </summary>
@@ -55,6 +89,21 @@ public Predicate Parse(
         List<ObjectFieldNode> fields,
         BaseQueryStructure queryStructure)
     {
+        return Parse(ctx, filterArgumentSchema, fields, queryStructure, nestingLevel: 0);
+    }
+
+    /// <summary>
+    /// Recursive overload that tracks and bounds the relationship-nesting depth of the filter being parsed.
+    /// </summary>
+    private Predicate Parse(
+        IMiddlewareContext ctx,
+        IInputValueDefinition filterArgumentSchema,
+        List<ObjectFieldNode> fields,
+        BaseQueryStructure queryStructure,
+        int nestingLevel)
+    {
+        EnsureWithinNestedFilterDepth(nestingLevel, GetMaxNestedFilterDepth());
+
         string schemaName = queryStructure.DatabaseObject.SchemaName;
         string sourceName = queryStructure.DatabaseObject.Name;
         string sourceAlias = queryStructure.SourceAlias;
@@ -113,7 +162,8 @@ public Predicate Parse(
                     filterArgumentSchema: filterArgumentSchema,
                     otherPredicates,
                     queryStructure,
-                    op)));
+                    op,
+                    nestingLevel)));
             }
             else
             {
@@ -185,7 +235,8 @@ public Predicate Parse(
                             subfields,
                             predicates,
                             queryStructure,
-                            metadataProvider);
+                            metadataProvider,
+                            nestingLevel);
                     }
                     else if (queryStructure is CosmosQueryStructure cosmosQueryStructure)
                     {
@@ -212,7 +263,8 @@ public Predicate Parse(
                                 nestedFieldTypeName,
                                 predicates,
                                 cosmosQueryStructure,
-                                metadataProvider);
+                                metadataProvider,
+                                nestingLevel);
                         }
                         else
                         {
@@ -223,7 +275,8 @@ public Predicate Parse(
                             predicates.Push(new PredicateOperand(Parse(ctx,
                                 filterArgumentObject.Fields[name],
                                 subfields,
-                                cosmosQueryStructure)));
+                                cosmosQueryStructure,
+                                nestingLevel + 1)));
 
                             cosmosQueryStructure.DatabaseObject.Name = sourceName;
                             cosmosQueryStructure.SourceAlias = sourceAlias;
@@ -292,7 +345,8 @@ private void HandleNestedFilterForCosmos(
         string entityType,
         List<PredicateOperand> predicates,
         CosmosQueryStructure queryStructure,
-        ISqlMetadataProvider metadataProvider)
+        ISqlMetadataProvider metadataProvider,
+        int nestingLevel)
     {
         // Validate that the field referenced in the nested input filter can be accessed.
         bool entityAccessPermitted = queryStructure.AuthorizationResolver.AreRoleAndOperationDefinedForEntity(
@@ -327,7 +381,8 @@ private void HandleNestedFilterForCosmos(
         Predicate existsQueryFilterPredicate = Parse(ctx,
                 filterField,
                 subfields,
-                existsQuery);
+                existsQuery,
+                nestingLevel + 1);
 
         predicatesForExistsQuery.Push(existsQueryFilterPredicate);
 
@@ -369,7 +424,8 @@ private void HandleNestedFilterForSql(
         List<ObjectFieldNode> subfields,
         List<PredicateOperand> predicates,
         BaseQueryStructure queryStructure,
-        ISqlMetadataProvider metadataProvider)
+        ISqlMetadataProvider metadataProvider,
+        int nestingLevel)
     {
         string? targetGraphQLTypeNameForFilter = RelationshipDirectiveType.GetTarget(filterField);
 
@@ -416,7 +472,8 @@ private void HandleNestedFilterForSql(
         Predicate existsQueryFilterPredicate = Parse(ctx,
                 filterField,
                 subfields,
-                existsQuery);
+                existsQuery,
+                nestingLevel + 1);
         predicatesForExistsQuery.Push(existsQueryFilterPredicate);
 
         // Add JoinPredicates to the subquery query structure so a predicate connecting
@@ -531,7 +588,8 @@ private Predicate ParseAndOr(
         IInputValueDefinition filterArgumentSchema,
         List<IValueNode> fields,
         BaseQueryStructure baseQuery,
-        PredicateOperation op)
+        PredicateOperation op,
+        int nestingLevel)
     {
         if (fields.Count == 0)
         {
@@ -573,7 +631,8 @@ private Predicate ParseAndOr(
                 Parse(ctx,
                     filterArgumentSchema,
                     subfields,
-                    baseQuery)));
+                    baseQuery,
+                    nestingLevel)));
         }
 
         return MakeChainPredicate(operands, op);

src/Service.Tests/UnitTests/GraphQLFilterParserUnitTests.cs

@@ -0,0 +1,82 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+
+using System.Collections.Generic;
+using Azure.DataApiBuilder.Config.ObjectModel;
+using Azure.DataApiBuilder.Core.Configurations;
+using Azure.DataApiBuilder.Core.Models;
+using Azure.DataApiBuilder.Core.Services.MetadataProviders;
+using Azure.DataApiBuilder.Service.Exceptions;
+using Microsoft.VisualStudio.TestTools.UnitTesting;
+using Moq;
+
+namespace Azure.DataApiBuilder.Service.Tests.UnitTests
+{
+    /// <summary>
+    /// Unit tests for the GraphQL nested-filter depth guard in <see cref="GQLFilterParser"/>.
+    /// The guard bounds relationship-nesting depth of filter arguments (e.g. filter:{rel:{rel:{...}}}),
+    /// which HotChocolate's execution-depth rule does not cover, to prevent nested-filter depth-bomb DoS.
+    /// </summary>
+    [TestClass]
+    public class GraphQLFilterParserUnitTests
+    {
+        /// <summary>
+        /// A nesting level beyond the maximum is rejected with a BadRequest.
+        /// </summary>
+        [TestMethod]
+        public void EnsureWithinNestedFilterDepth_ThrowsWhenExceeded()
+        {
+            DataApiBuilderException ex = Assert.ThrowsException<DataApiBuilderException>(
+                () => GQLFilterParser.EnsureWithinNestedFilterDepth(nestingLevel: 21, maxNestedFilterDepth: 20));
+
+            Assert.AreEqual(System.Net.HttpStatusCode.BadRequest, ex.StatusCode);
+            Assert.AreEqual(DataApiBuilderException.SubStatusCodes.BadRequest, ex.SubStatusCode);
+        }
+
+        /// <summary>
+        /// A nesting level at or below the maximum is allowed.
+        /// </summary>
+        [DataTestMethod]
+        [DataRow(0)]
+        [DataRow(1)]
+        [DataRow(20)]
+        public void EnsureWithinNestedFilterDepth_DoesNotThrowWithinLimit(int nestingLevel)
+        {
+            // Should not throw.
+            GQLFilterParser.EnsureWithinNestedFilterDepth(nestingLevel, maxNestedFilterDepth: 20);
+        }
+
+        /// <summary>
+        /// The effective nested-filter depth limit falls back to the hardcoded safety ceiling when
+        /// runtime.graphql.depth-limit is not configured, uses the depth-limit only when it is stricter,
+        /// and never exceeds the ceiling even when depth-limit is larger or set to -1 (unlimited).
+        /// </summary>
+        [DataTestMethod]
+        [DataRow(null, GQLFilterParser.MAX_NESTED_FILTER_DEPTH, DisplayName = "No depth-limit -> safety ceiling")]
+        [DataRow(5, 5, DisplayName = "Stricter depth-limit is used")]
+        [DataRow(50, GQLFilterParser.MAX_NESTED_FILTER_DEPTH, DisplayName = "Higher depth-limit is capped at ceiling")]
+        [DataRow(-1, GQLFilterParser.MAX_NESTED_FILTER_DEPTH, DisplayName = "Unlimited (-1) depth-limit still capped at ceiling")]
+        public void GetMaxNestedFilterDepth_ResolvesEffectiveLimit(int? depthLimit, int expected)
+        {
+            GQLFilterParser parser = CreateParserWithDepthLimit(depthLimit);
+            Assert.AreEqual(expected, parser.GetMaxNestedFilterDepth());
+        }
+
+        private static GQLFilterParser CreateParserWithDepthLimit(int? depthLimit)
+        {
+            RuntimeConfig config = new(
+                Schema: "",
+                DataSource: new(DatabaseType.MSSQL, "", new()),
+                Runtime: new(
+                    Rest: new(),
+                    GraphQL: new(DepthLimit: depthLimit) { UserProvidedDepthLimit = depthLimit is not null },
+                    Mcp: new(),
+                    Host: new(null, null)),
+                Entities: new(new Dictionary<string, Entity>()));
+
+            RuntimeConfigProvider provider = TestHelper.GenerateInMemoryRuntimeConfigProvider(config);
+            Mock<IMetadataProviderFactory> metadataProviderFactory = new();
+            return new GQLFilterParser(provider, metadataProviderFactory.Object);
+        }
+    }
+}