Stack Overflow in Aspnet with Firestore

A Stack Overflow in an Aspnet application that uses Cloud Firestore typically occurs through unbounded data accumulation in server-side objects, often involving recursive models or deeply nested object graphs. When Firestore documents are mapped to .NET entities without controlling reference depth and serialization behavior, an attacker can trigger excessive memory consumption during deserialization or recursive navigation of document references.

Consider a scenario where Firestore stores hierarchical data such as organizational units or threaded comments, and the Aspnet models represent these structures with navigation properties that reference parent or child documents. If the JSON serializer is configured to eagerly resolve references and the client can influence which documents are retrieved, a crafted set of references can form a cycle or a very deep tree. During deserialization in the Aspnet layer (for example, when using JsonConvert.DeserializeObject or System.Text.Json with default options), the runtime can exhaust the call stack or heap, resulting in an unhandled StackOverflowException or out-of-memory condition.

This combination is notable because Firestore does not inherently prevent deep nesting or reference cycles; it stores references as strings. The risk emerges in the Aspnet layer when the application layer reconstructs object graphs from those references without safeguards. For instance, serializing a document that contains a list of child document references, which in turn reference back to the parent, can lead to infinite recursion if the traversal logic does not track visited nodes.

Real-world triggers include unbounded recursion in service methods that follow document references, or misconfigured JSON serialization settings that do not limit depth or ignore reference handling. Attack patterns mirror classic memory-exhaustion vectors but are specific to Firestore’s document-reference model and Aspnet’s serialization pipelines. The impact is denial of service on the server process, potentially affecting availability for other requests.

Key contributing factors specific to this stack:

• Firestore data model allows arbitrary document references that can form graphs with cycles or extreme depth.
• Aspnet model binding and JSON serialization can recursively traverse these graphs without guardrails.
• Default serializer settings in many Aspnet templates do not constrain depth or handle reference loops.

Remediation focuses on controlling how Firestore documents are materialized into Aspnet models and how object graphs are traversed. You should avoid automatic reference resolution and enforce explicit, bounded loading of related documents.

First, configure JSON serialization to ignore reference loops and limit depth. For System.Text.Json, set ReferenceHandler to IgnoreCycles and use converters that bound recursion. For Newtonsoft.Json, use ReferenceLoopHandling.Ignore and set MaxDepth.

// System.Text.Json example in Aspnet with bounded depth and cycle ignoring
var options = new JsonSerializerOptions
{
    ReferenceHandler = ReferenceHandler.IgnoreCycles,
    MaxDepth = 32
};
string json = JsonSerializer.Serialize(firestoreDoc, options);
var document = JsonSerializer.Deserialize<DocumentModel>(json, options);

Second, explicitly model references as IDs rather than nested objects. When reading from Firestore, load only the necessary fields and resolve references on demand with explicit queries, rather than embedding full child objects.

// Firestore document model in Aspnet: store references as IDs
public class OrganizationNode
{
    public string DocumentId { get; set; }
    public string Name { get; set; }
    // Avoid public List<OrganizationNode> Children { get; set; }
    public List<string> ChildNodeIds { get; set; } // IDs only
}

// Service method that loads children with explicit depth control
public async Task<OrganizationNode> GetNodeWithBoundedChildren(string nodeId, int maxDepth = 5)
{
    if (maxDepth < 0) throw new InvalidOperationException("Max depth exceeded.");
    var doc = await _firestore.Document($"nodes/{nodeId}").GetSnapshotAsync();
    if (!doc.Exists) return null;
    var node = doc.ConvertTo<OrganizationNode>();
    // Explicitly load children one level at a time
    foreach (var childId in node.ChildNodeIds)
    {
        var child = await GetNodeWithBoundedChildren(childId, maxDepth - 1);
        // Process child as needed, but do not auto-recurse into a full graph
    }
    return node;
}

Third, validate and constrain incoming requests that specify document paths or query parameters. Reject requests that would traverse beyond a reasonable depth or breadth, and enforce server-side limits on how many documents can be fetched in a single operation.

// Example guard in an Aspnet controller
[ApiController]
[Route("api/[controller]")]
public class NodesController : ControllerBase
{
    private const int MaxAllowedDepth = 5;

    [HttpGet("{nodeId}")]
    public async Task<IActionResult> Get(int maxDepth = 0)
    {
        if (maxDepth < 0 || maxDepth > MaxAllowedDepth)
        {
            return BadRequest("maxDepth must be between 0 and " + MaxAllowedDepth);
        }
        var node = await _service.GetNodeWithBoundedChildren(Request.RouteValues["nodeId"]?.ToString(), maxDepth);
        return Ok(node);
    }
}

These measures ensure that Firestore references are handled safely within Aspnet, preventing uncontrolled recursion and memory exhaustion while preserving the ability to work with hierarchical data.