Nosql Injection in Aspnet with Firestore

NoSQL injection occurs when untrusted input is concatenated into a query structure without proper validation or parameterization, altering the intended logic. In an ASP.NET application that uses Google Cloud Firestore, this typically manifests through dynamic construction of queries using string concatenation or object building where user-controlled data influences field paths, collection names, or filter values.

Firestore queries in C# are commonly built using the Query object and its methods (WhereEqualTo, WhereGreaterThan, etc.). When application code interpolates user input directly into these calls—such as using a variable for a field path or operator—it can produce unsafe queries. For example, using Query.Where(userSuppliedFieldPath, QueryOperator.Equal, userSuppliedValue) with userSuppliedFieldPath derived from request data can allow an attacker to traverse fields or inject unintended filter logic.

The ASP.NET runtime does not inherently treat Firestore queries as parameterized statements. Unlike SQL with prepared statements, Firestore’s client libraries rely on developers to avoid passing untrusted strings as field paths or keys. If an attacker can supply a field path like __name__ or exists, they may probe document structure or bypass intended access patterns. In a web API endpoint that builds queries from JSON payload keys, an attacker could submit payloads such as { "field": "anyValue", "$where": "1==1" } if the server merges this into a query-building dictionary.

Another vector arises from collection or document ID usage. If an ASP.NET handler uses user input to reference a document ID directly—e.g., DocumentReference docRef = db.Collection("items").Document(userControlledId)—and then performs sensitive reads or writes without verifying ownership or access rights, this can lead to Insecure Direct Object References (IDOR) or data manipulation. When combined with missing server-side authorization checks, NoSQL injection in this context can expose or modify data across tenant boundaries.

Because Firestore rules operate at the database level and are not a substitute for application-layer validation, the ASP.NET service must treat all incoming data as potentially malicious. Attackers may exploit verbose error messages from the Firestore client library to infer schema details, such as which field paths exist or which indexes are configured. These side channels amplify the impact of injection attempts by enabling reconnaissance for further exploitation.

Securing ASP.NET integrations with Firestore requires strict input validation, canonicalization of field paths, and avoiding dynamic query assembly from untrusted sources. All user input must be treated as opaque values rather than query language components.

Use whitelisted field names and map user-friendly keys to known-safe field identifiers. Instead of passing raw input to Where, validate against an enumeration and map to the actual document field. Below is a secure pattern for filtering documents by a known set of fields:

var allowedFields = new Dictionary<string, string>
{
    { "createdAt", "created_at" },
    { "status", "status" },
    { "priority", "priority" }
};

if (!allowedFields.TryGetValue(userSuppliedKey, out var fieldPath))
{
    throw new ArgumentException("Invalid filter key");
}

Query query = db.Collection("items").WhereEqualTo(fieldPath, userSuppliedValue);

For document ID references, enforce strict format checks and avoid direct concatenation. Use regular expressions to validate identifiers and ensure they conform to expected patterns, preventing path traversal or injection via special characters:

string documentId = userSuppliedId;
if (!System.Text.RegularExpressions.Regex.IsMatch(documentId, @"^[a-zA-Z0-9_-]{1,100}$"))
{
    throw new ArgumentException("Invalid document ID");
}

DocumentReference docRef = db.Collection("items").Document(documentId);
DocumentSnapshot snapshot = await docRef.GetSnapshotAsync();
if (!snapshot.Exists)
{
    // Handle missing document safely
}

When constructing collection names, avoid any form of interpolation. Use hardcoded or configuration-driven references. If multi-tenancy is required, resolve the tenant identifier through authenticated context rather than request input:

string tenantId = GetTenantFromAuthContext(); // validated server-side
CollectionReference tenantCollection = db.Collection($"tenants_{tenantId}_items");
Query tenantQuery = tenantCollection.WhereEqualTo("active", true);

Additionally, normalize inputs before use. Trim whitespace, enforce case sensitivity consistently, and avoid treating numeric strings as integers when constructing queries. Firestore is strongly typed; mismatched types can lead to unexpected behavior or bypassed checks if coercion is applied on the client side.

Logging and monitoring should capture attempted anomalies without exposing sensitive data. Record invalid field paths or document ID patterns as security events, but never log raw user input in plaintext at scale. Combine these practices with runtime security tooling such as middleBrick to detect misconfigurations and verify that your ASP.NET endpoints do not exhibit NoSQL injection patterns during automated scans.

Using the middleBrick CLI, you can regularly scan your ASP.NET endpoints that interact with Firestore to validate remediation effectiveness. Run middlebrick scan <url> from your terminal to perform an unauthenticated black-box assessment focused on input validation and query manipulation risks. For teams integrating security into development workflows, the GitHub Action can fail builds when new endpoints introduce potential NoSQL injection vectors, while the MCP Server allows you to scan APIs directly from your AI coding assistant within the IDE.

On the Dashboard, track security scores over time and review per-category findings related to authentication, data exposure, and input validation. This continuous visibility complements secure coding practices by highlighting regressions introduced by dependency updates or new query patterns.