Insecure Deserialization in Aspnet with Mongodb

Insecure deserialization occurs when an application processes untrusted data into an object without proper validation. In an Aspnet application that uses Mongodb as a data store, this typically manifests when incoming JSON, BSON, or form values are deserialized into concrete .NET types and then written to or read from Mongodb without strict schema enforcement or type checks.

Mongodb’s document model is schemaless by default, which means an Aspnet app often maps loosely-typed BSON documents directly to C# objects using a library such as the official MongoDB.Driver. If the application uses permissive deserialization settings—for example, BsonClassMap with SetIsRequired omitted, or polymorphic type resolution without a strict discriminator—an attacker can craft payloads that instantiate unexpected types during deserialization. This can lead to gadget chains that execute code, bypass authorization, or manipulate server-side logic when the object is later used (e.g., in business rules or queries).

Because Mongodb often stores complex object graphs, an Aspnet endpoint that accepts a document to be saved may deserialize attacker-controlled data into nested objects. If the application later rehydrates these objects or uses reflection-based operations (such as dynamic filters or update definitions), malicious properties can influence behavior. Common patterns that increase risk include using BsonDocument directly without validation, relying on default JSON serializer settings that allow type hints (e.g., $type), and constructing update definitions from user input without whitelisting fields.

Real-world impact scenarios enabled by this combination include remote code execution via gadget chains in .NET object graphs, privilege escalation through tampered role claims stored in documents, and data exfiltration by injecting malicious payloads that are later deserialized in a higher-privilege context. Because the scan tests unauthenticated attack surfaces, middleBrick checks for indicators of insecure deserialization such as missing input validation on document-bound models, absence of schema constraints, and usage of reflection or dynamic object creation that can be abused during deserialization.

To detect these risks, middleBrick runs checks aligned with OWASP API Top 10 (e.g., A05:2021 Security Misconfiguration and A03:2021 Injection) and maps findings to compliance frameworks. Note that middleBrick detects and reports these issues; it provides remediation guidance but does not patch or block execution.

Remediation focuses on strict deserialization rules, explicit schema validation, and avoiding reflection-based object construction when interacting with Mongodb in Aspnet.

• Use strongly typed models with required member validation. Define POCO classes with data annotations and configure the MongoDB driver to require members, which prevents extra fields from being silently accepted. Example model and mapping:

public class UserProfile
{
    [BsonRequired]
    public string Id { get; set; }
    [BsonRequired]
    [StringLength(100)]
    public string Username { get; set; }
    [BsonRequired]
    [EmailAddress]
    public string Email { get; set; }
    [BsonRequired]
    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
}

// In Startup or Program.cs
BsonClassMap.RegisterClassMap<UserProfile>(cm =>
{
    cm.AutoMap();
    cm.SetIsRequired<UserProfile>(x => x.Id, true);
    cm.SetIsRequired<UserProfile>(x => x.Username, true);
});

• Configure the JSON/BSON serializer to reject type hints. Avoid polymorphic deserialization unless strictly necessary, and use a known discriminator value. Example with limited type resolution:

var client = new MongoClient("mongodb://localhost:27017");
var database = client.GetDatabase("appdb");

// Use a contract resolver that excludes $type and other type hints
var settings = new JsonSerializerSettings
{
    TypeNameHandling = TypeNameHandling.None,
    ContractResolver = new CamelCasePropertyNamesContractResolver()
};

// When storing as JSON in a field, serialize explicitly
var user = new UserProfile { Id = "u1", Username = "alice", Email = "[email protected]" };
var json = JsonConvert.SerializeObject(user, settings);
var doc = BsonDocument.Parse(json);
database.GetCollection<BsonDocument>("profiles").InsertOne(doc);

• Validate and whitelist fields for updates. Do not build update definitions directly from user input. Use a whitelist of allowed fields and strongly typed update definitions:

public static UpdateDefinition<UserProfile> BuildSafeUpdate(Dictionary<string, object> updates)
{
    var allowed = new HashSet<string>(StringComparer.OrdinalIgnoreCase) { "username", "email" };
    var update = new UpdateDefinitionBuilder<UserProfile>();
    var updatesList = new List<UpdateDefinition<UserProfile>>();

    if (updates != null)
    {
        foreach (var kvp in updates)
        {
            if (allowed.Contains(kvp.Key, StringComparer.OrdinalIgnoreCase))
            {
                if (kvp.Key.Equals("username", StringComparison.OrdinalIgnoreCase))
                    updatesList.Add(update.Set(x => x.Username, kvp.Value.ToString()));
                else if (kvp.Key.Equals("email", StringComparison.OrdinalIgnoreCase))
                    updatesList.Add(update.Set(x => x.Email, kvp.Value.ToString()));
            }
        }
    }
    return updatesList.Count == 1 ? updatesList[0] : update.Combine(updatesList);
}

• Use schema validation at the database level. Define JSON schema rules in Mongodb to restrict document structure and prevent unexpected fields that could enable gadget chains:

var validator = new BsonDocument
{
    { "$jsonSchema", new BsonDocument
        {
            { "bsonType", "object" },
            { "required", new BsonArray { "username", "email" } },
            { "properties", new BsonDocument
                {
                    { "username", new BsonDocument("bsonType", "string") },
                    { "email", new BsonDocument("bsonType", "string") },
                    { "createdAt", new BsonDocument("bsonType", "date") }
                }
            }
        }
    }
};

database.CreateCollection(
    "profiles",
    new CreateCollectionOptions
    {
        Validator = validator
    });

By combining strict class mapping, disabled type hints, field whitelisting for updates, and schema validation, you reduce the attack surface for deserialization-based attacks when using Aspnet with Mongodb. middleBrick scans can verify that such controls are present and flag missing constraints or unsafe deserialization practices.