Server Side Template Injection in Aspnet with Firestore

Server Side Template Injection (SSTI) in an ASP.NET application that interacts with Cloud Firestore occurs when attacker-controlled data is merged into a template before being sent to the Firestore client or rendered in a response. SSTI is an OWASP API Top 10 finding often surfaced by middleBrick during the Property Authorization and Input Validation checks. In this context, the vulnerability is not about Firestore itself executing templates, but about the ASP.NET layer constructing strings or expressions that are later interpreted by a server-side rendering engine or by downstream processing of Firestore document data.

Consider an endpoint that retrieves a document from Firestore and passes field values into a Razor view or a string format template. If the document contains user-provided content (for example, a displayName or a configuration template stored in Firestore) and that content is not validated or encoded, an attacker may supply payloads such as {{7*7}} in a field that later gets interpolated by a vulnerable template engine. In Blazor Server or older WebForms scenarios, this can lead to unintended code execution paths on the server because the template engine evaluates expressions embedded in the data.

middleBrick detects this class of issue by correlating OpenAPI/Swagger spec definitions with runtime findings. For example, if the spec describes a userProfile object with a string field bio, but runtime tests detect template-like input (e.g., containing ${ or {{) and observe unexpected behavior in responses, a high-severity finding is raised. The risk is especially relevant when Firestore documents are used to drive dynamic content in dashboards or admin panels, where an attacker who can write a document can indirectly influence the template evaluation context.

Because middleBrick scans the unauthenticated attack surface, it can identify endpoints that accept input used to query Firestore and then render results with insufficient encoding. Findings include concrete evidence such as reflected template syntax in responses, helping developers understand how data flows from Firestore reads to presentation layers. Remediation focuses on strict input validation, context-aware output encoding, and avoiding dynamic evaluation of user-controlled data within server-side templates.

To remediate SSTI when using Firestore in ASP.NET, ensure that data retrieved from Firestore is treated as untrusted and is never directly concatenated into templates or evaluated as code. Use context-aware encoding for the output context (HTML, JavaScript, URL) and validate inputs before using them in queries. The following examples assume you are using the Google Cloud Firestore client for .NET.

First, always retrieve documents by trusted identifiers and avoid building queries by string concatenation. Prefer parameterized approaches and validate inputs:

// Good: using a validated document ID and retrieving data safely
string userId = GetUserIdFromRequest(); // validated, e.g., Guid or integer parsed with TryParse
if (Guid.TryParse(userId, out Guid parsedUserId))
{
    DocumentReference docRef = db.Collection("users").Document(parsedUserId.ToString());
    DocumentSnapshot snapshot = await docRef.GetSnapshotAsync();
    if (snapshot.Exists)
    {
        // Treat snapshot data as untrusted; encode when rendering
        string displayName = snapshot.GetValue("displayName");
        // Encode for the target context (example: HTML encoded in Razor)
        ViewData["DisplayName"] = System.Net.WebUtility.HtmlEncode(displayName);
    }
}

Second, when rendering data in Razor views, use the built-in HTML encoder and avoid @Html.Raw unless you have fully trusted content. For JSON serialization to the client, configure options to prevent script injection:

// In Startup or Program.cs
services.AddControllersWithViews()
    .AddJsonOptions(options =>
    {
        options.JsonSerializerOptions.PropertyNamingPolicy = JsonNamingPolicy.CamelCase;
        // Ensure default encoding for strings; do not embed JSON directly in HTML without encoding
    });

Third, if your application stores templates in Firestore for rendering, do not allow user input to influence which template is selected or to inject variables without strict schema validation. Use a dedicated templating engine with sandboxed evaluation and whitelist allowed variables:

// Example: using a strict template schema; do not embed user input as template text
var allowedTemplates = new Dictionary<string, string>
{
    ["welcome"] = "Hello, {{name}}" // static template stored safely in Firestore
};
string templateName = GetTrustedTemplateName(); // validated lookup
if (allowedTemplates.TryGetValue(templateName, out string template))
{
    // Use a safe templating library, never eval
    var result = SimpleTemplate.Render(template, new { name = HtmlEncode(userName) });
    // output is encoded appropriately for the response context
}

Finally, apply middleware to enforce input constraints on query parameters used to reference Firestore documents and to set security headers that reduce the impact of any residual injection. middleBrick’s dashboard and CLI can validate these mitigations by re-scanning endpoints after changes and tracking findings over time through the Web Dashboard or via the GitHub Action for CI/CD integration.