Server Side Template Injection in Buffalo with Api Keys

Server Side Template Injection (SSTI) in the Buffalo framework can occur when user-controlled data is interpolated into templates without proper escaping or validation. Buffalo uses the packr asset pipeline and the pop ORM, but template rendering is typically handled by plush (a Go-based templating engine). If API keys are accepted as input (for example via query parameters, headers, or JSON payloads) and then passed into plush templates for dynamic content generation, an attacker may be able to inject template code that executes on the server.

Consider a scenario where an API key is used to personalize a response or to select a configuration template. If the application does not treat the API key as untrusted, an attacker could provide a payload such as {{ `test` | printf "%s" }} or more complex expressions that read filesystem paths or environment variables. Because Buffalo applications often serve both web and API routes, an unauthenticated endpoint that accepts API keys and renders them in templates can become an SSTI vector. The risk is compounded when OpenAPI/Swagger specs describe the API key as a simple string parameter, while the backend inadvertently uses it in a template context without sanitization.

During a middleBrick scan, which runs unauthenticated black-box tests including checks for Input Validation and SSRF, such patterns are flagged. The scanner does not assume trust in HTTP headers or query parameters labeled as API keys; it probes endpoints that reflect user input into template rendering. If the API key influences template selection or variable interpolation, findings related to SSTI, Data Exposure, and Unsafe Consumption may be reported. Real-world CVEs in similar web frameworks demonstrate that template injection can lead to local file inclusion or information disclosure, aligning with OWASP API Top 10 categories such as Broken Object Level Authorization and Improper Neutralization of Special Elements.

An example of unsafe usage in a Buffalo handler might involve reading an API key from a header and passing it directly to a plush template context:

// Unsafe example: using API key in template context
func ApiKeyHandler(c buffalo.Context) error {
    apiKey := c.Request().Header.Get("X-API-Key")
    c.Set("apiKey", apiKey)
    return c.Render(200, r.HTML("debug.tpl"))
}

If debug.tpl contains {{ .apiKey }}, the raw key is reflected. If the template is user-influenced or selected dynamically, an attacker could inject template syntax. middleBrick’s LLM/AI Security checks are not applicable here because this scenario does not involve LLM endpoints; the concern is strictly traditional SSTI in API-driven Buffalo applications.

Remediation focuses on ensuring API keys are never treated as template-renderable data and are strictly confined to authentication or cryptographic operations. In Buffalo, avoid setting API key values into the HTML or plush template context. Instead, use them only for middleware-level authentication or to sign requests.

1. Do not pass API keys to templates. Validate the key in middleware and abort if invalid, without exposing it in the rendering pipeline:

// Secure middleware approach
func ValidateAPIKey(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        apiKey := r.Header.Get("X-API-Key")
        if !isValidKey(apiKey) {
            http.Error(w, "Forbidden", http.StatusForbidden)
            return
        }
        next.ServeHTTP(w, r)
    })
}

func isValidKey(key string) bool {
    // Compare against environment or secure store; avoid timing attacks
    expected := os.Getenv("EXPECTED_API_KEY")
    return subtle.ConstantTimeCompare([]byte(key), []byte(expected)) == 1
}

2. If you must pass metadata to templates, pass only derived, non-sensitive values. For example, use a boolean flag instead of the key itself:

// Safe: passing a flag, not the key
func ApiKeyHandler(c buffalo.Context) error {
    apiKey := c.Request().Header.Get("X-API-Key")
    isValid := subtle.ConstantTimeCompare([]byte(apiKey), []byte(os.Getenv("EXPECTED_API_KEY"))) == 1
    if !isValid {
        return c.Error(403, errors.New("invalid key"))
    }
    c.Set("authenticated", true)
    return c.Render(200, r.HTML("page.tpl"))
}

3. For API-only endpoints, skip HTML rendering entirely and use JSON responses with strict content-type headers. Ensure the API key is validated before any business logic:

// JSON API endpoint with key validation
func ApiKeyJSONHandler(c buffalo.Context) error {
    apiKey := c.Request().Header.Get("X-API-Key")
    if !isValidKey(apiKey) {
        return c.Render(401, r.JSON(map[string]string{"error": "unauthorized"}))
    }
    return c.Render(200, r.JSON(map[string]string{"status": "ok"}))
}

These changes align with remediation guidance that would appear in a middleBrick Pro or Enterprise report, where continuous monitoring and compliance mapping (e.g., to OWASP API Top 10) help prioritize fixes. The scanner can detect whether endpoints reflect sensitive values and will flag missing validation or unsafe template usage.