Server Side Template Injection in Actix with Api Keys

Server Side Template Injection (SSTI) occurs when an attacker can inject template expressions that are evaluated on the server. In Actix web applications that render dynamic content using templating engines such as Askama or Tera, SSTI can arise if user-controlled data is passed into the template context without proper validation or escaping. When API keys are used for authentication and are also exposed to the template rendering layer—whether through debug variables, error messages, or contextual data—they can become an unintended data source for template logic, increasing the impact of injection.

Consider an endpoint that accepts an API key as a header and passes it to a template for display or conditional rendering. If the application constructs the template context by directly including the API key value (e.g., as a string variable) and the template engine evaluates expressions, an attacker may craft input that manipulates template syntax to read or modify variables. For example, a debug mode that dumps context variables can expose the API key in rendered output, and a vulnerable template can be coerced into invoking methods or accessing attributes that reveal internal state or configuration. The combination of SSTI and exposed API keys can lead to sensitive data disclosure, and in frameworks like Actix where templates are rendered per request, the risk surface expands if the API key is treated as trusted input rather than a secret.

middleBrick detects this class of issue by analyzing OpenAPI specs and runtime behavior: it checks whether authentication parameters such as API keys appear in template contexts, and whether user input flows into template rendering without sanitization. The scanner’s OWASP API Top 10 mapping flags SSTI under Injection and highlights occurrences where secrets are unnecessarily exposed to the rendering pipeline. Because Actix applications often integrate multiple crates for templating and authentication, inconsistent handling of API keys across layers can create subtle paths for template logic abuse.

In practice, an attacker might send a request with a malformed API key containing template syntax (e.g., {{7*7}} for Tera or {@inject} for certain engines) and observe whether the response contains evaluated results or stack traces. The presence of API keys in error pages or debug output further aids an attacker in refining injection payloads. middleBrick’s LLM/AI Security checks are not relevant to SSTI, but its inventory and input validation modules correlate authentication usage patterns with template rendering code to prioritize findings with severity and remediation guidance.

Remediation centers on strict separation of secrets from template contexts and robust input validation. API keys should be confined to server-side authentication flows and never propagated to templates. When templates must display metadata, use minimal, sanitized representations and enforce allowlists for dynamic values. The following code examples demonstrate secure handling of API keys in Actix applications.

To mitigate SSTI risks related to API keys in Actix, ensure API keys are treated as server-side credentials and are never injected into template variables. Use typed request extractors for authentication and pass only necessary, non-sensitive data to templates. The following examples illustrate secure patterns.

Insecure pattern to avoid: Passing the API key directly into the template context.

use actix_web::{web, HttpResponse, Responder};
use askama::Template;

#[derive(Template)]
#[template(path = "debug.html")]
struct DebugTemplate {
    api_key: String, // Unsafe: exposes secret to template
}

async fn debug_handler(headers: web::Header<actix_web::http::header::Authorization<actix_web::http::header::Bearer>>) -> impl Responder {
    let api_key = headers.into_inner().to_string();
    let context = DebugTemplate { api_key };
    HttpResponse::Ok().content_type("text/html").body(context.render().unwrap_or_default())
}

This pattern risks SSTI if the template uses the api_key variable in expressions or if debug endpoints are accessible. An attacker who influences the header could manipulate template evaluation.

Secure pattern: Use authentication middleware to validate the API key and avoid exposing it to templates.

use actix_web::{web, HttpResponse, Responder, dev::ServiceRequest, Error};
use actix_web::http::header::Authorization;
use actix_web::middleware::Next;
use std::future::{ready, Ready};

// Simple guard that validates API key from headers and rejects invalid requests
async fn validate_api_key(req: ServiceRequest, next: Next<impl actix_web::body::MessageBody>) -> Result<actix_web::dev::ServiceResponse<impl actix_web::body::MessageBody>, Error> {
    const VALID_KEY: &str = "super-secret-key";
    let auth = req.headers().get("authorization")
        .and_then(|v| v.to_str().ok())
        .and_then(|s| s.strip_prefix("Bearer "));
    match auth {
        Some(key) if key == VALID_KEY => next.call(req).await,
        _ => Err(actix_web::error::ErrorUnauthorized("Invalid API key")),
    }
}

// Handler that does not pass the API key to the template
#[derive(Template)]
#[template(path = "status.html")]
struct StatusTemplate {
    ok: bool,
}

async fn status_handler() -> HttpResponse {
    let context = StatusTemplate { ok: true };
    HttpResponse::Ok().content_type("text/html").body(context.render().unwrap_or_default())
}

In the secure version, the API key is validated in middleware and never reaches the template context. The template receives only non-sensitive flags. This aligns with middleBrick’s recommendations: authentication parameters should be confined to security boundaries and not propagated to rendering logic.

Additionally, ensure templates are configured with autoescaping and that user-controlled inputs are sanitized before inclusion. Regularly review dependencies for template engine updates that address injection vectors. middleBrick’s continuous monitoring can help detect regressions by scanning updated endpoints and flagging insecure patterns in authentication and template integration.

• Can SSTI via API keys be detected by scanning authenticated endpoints only?
  middleBrick scans the unauthenticated attack surface by default, but you can use the CLI to include authenticated scans when relevant patterns are exposed. API keys should never be required for template rendering; if they are, the design should be reconsidered regardless of authentication coverage.
• How does middleBrick differentiate between low-risk debug templates and production risks?
  The scanner correlates findings with OpenAPI paths and tags, prioritizing contexts where API keys appear in template variables or error surfaces. Production endpoints with authentication parameters in templates are flagged with higher severity and specific remediation steps.