Server Side Template Injection in Buffalo with Firestore

Buffalo is a web framework for Go that encourages rapid development by providing helpers for routing, rendering, and data binding. When developers use server-side templates (e.g., html/template) with user-controlled data and integrate Firestore as the backend datastore, a Server Side Template Injection (SSTI) vector can emerge if dynamic values are merged into templates without proper sanitization or isolation. Firestore documents often contain fields such as display names, configuration flags, or user-supplied content that may be passed into templates directly.

In this context, SSTI occurs when an attacker can influence the data used during template execution in a way that allows template code injection. For example, if a handler reads a Firestore document field like doc.Data()["welcome_message"] and passes it into a template that is executed with the .Execute method, an attacker who can control that Firestore field may be able to inject template actions. Because Buffalo typically binds URL parameters or JSON payloads to structs and then fetches related Firestore data, the risk arises when those Firestore fields are used in template contexts without strict context-aware escaping.

The exposure is compounded by Firestore’s schemaless nature: fields can contain unexpected strings that, when rendered in a template, may execute unintended logic. A common pattern in Buffalo apps is to render dynamic navigation or status badges using template conditionals; if a Firestore string contains template syntax such as {{ .SomeInternal }}, and the template engine re-evaluates it, it may attempt to access the current template context, leading to information disclosure or unauthorized operations. This is not a vulnerability in Firestore itself, but a consequence of how developer code bridges Firestore data and template execution in Buffalo.

Real-world exploit patterns mirror classic SSTI impacts: an attacker might attempt to read sensitive template context variables, invoke functions, or chain further logic if the template environment permits. While the Go html/template package is designed to be safe by default, unsafe practices—such as using template.New("custom").Parse(strings.Replace(...)) with raw input—can weaken isolation. MiddleBrick’s checks for SSTI align with OWASP API Top 10 A05:2023, emphasizing the need to validate and sanitize data flows from databases like Firestore before presentation layers consume them.

Additionally, because Buffalo apps often integrate authentication and authorization helpers, a compromised Firestore field used in templates might escalate impact if rendered in admin-only views. Developers should treat all data from Firestore as untrusted when destined for templates, applying context-sensitive escaping and strict allowlists for dynamic content. Regular scanning with tools that understand framework-database-template interactions—such as MiddleBrick’s 12 parallel security checks—can surface these integration risks before attackers do.

Remediation focuses on ensuring Firestore data is never directly interpolated into executable template logic. In Buffalo, use the standard library’s html/template package, which automatically escapes variables based on context. Always pass Firestore document fields as plain data values, not as template fragments. Avoid dynamic template parsing or re-execution of user-influenced strings.

Safe Data Fetching and Rendering

Retrieve Firestore documents and extract fields into strongly typed structs. This enforces a clear boundary between data and execution logic.

type PageData struct {
    Title       string
    WelcomeMsg  string // Firestore field mapped here
    UserName    string
}

func ShowHandler(c buffalo.Context) error {
    client, err := firestore.NewClient(context.Background(), "my-project")
    if err != nil {
        return c.Error(500, errors.New("failed to create client"))
    }
    defer client.Close()

    doc, err := client.Collection("pages").Doc("home").Get(context.Background())
    if err != nil {
        return c.Error(500, errors.New("failed to fetch document"))
    }

    data := PageData{
        Title:      "Home",
        WelcomeMsg: doc.Data()["welcome_message"].(string), // explicit type assertion
        UserName:   doc.Data()["user_name"].(string),
    }
    return c.Render(200, r.HTML("home.html", data))
}

Template Design: No Logic with Dynamic Content

In your home.html, treat WelcomeMsg as plain text. Do not allow template actions to be stored in Firestore fields. If you need conditional UI based on Firestore flags, handle those conditions in Go code before rendering.

{{/* home.html */}}
<h1>{{ .Title }}</h1>
<p>{{ .WelcomeMsg | html }}</p> <!-- html auto-escaped by html/template --}
<span class="status-{{ .StatusClass }}">{{ .StatusLabel }}</span>

Allowlist Approach for Dynamic Includes

If your app supports dynamic partials (e.g., switching templates based on Firestore configuration), use an allowlist map instead of direct name resolution from Firestore strings.

var templates = map[string]*template.Template{
    "summary": template.Must(template.New("summary").Parse("Summary: {{ .Content }}")),
    "detail":  template.Must(template.New("detail").Parse("Detail: {{ .Content }}")),
}

func RenderDynamic(c buffalo.Context) error {
    requested := c.Param("template") // e.g., from URL /render/summary
    t, ok := templates[requested]
    if !ok {
        return c.Error(400, errors.New("invalid template"))
    }
    // Fetch Firestore data safely
    doc, _ := client.Collection("templates").Doc(requested).Get(context.Background())
    return c.Render(200, t.Data("content", doc.Data()["body"]))
}

Input Validation and Contextual Escaping

Always validate and sanitize Firestore string fields that may be reused across contexts (HTML, JS, URLs). Use packages like bluemonday for HTML sanitization when displaying rich text from Firestore, and never mark content as template.JS unless absolutely necessary and rigorously controlled.

Finally, integrate MiddleBrick scans to continuously verify that Firestore-driven templates remain free of injection risks. Its findings map to OWASP API Top 10 and compliance frameworks, helping you prioritize fixes without disrupting development velocity.