Server Side Template Injection in Fiber with Jwt Tokens

Server Side Template Injection (SSTI) in the Fiber web framework becomes particularly nuanced when JWT tokens are used for authentication and data transport. The risk emerges not from a flaw in JWT itself, but from unsafe handling of token claims within server-side templates. When a developer deserializes a JWT and directly injects payload fields—such as user roles, display names, or custom claims—into a template engine like html/template, the application may inadvertently treat trusted data as executable template logic.

Consider a scenario where a Fiber handler decodes a JWT and passes the claims map to a template for rendering personalized content. If the template includes dynamic actions based on claim values, an attacker who can influence the token’s payload (for example, via a compromised client-side generation step or a misconfigured public key allowing algorithm confusion) may inject template syntax. Because the server evaluates this input as code, attackers can execute arbitrary template commands, leading to unauthorized data access or local file inclusion depending on the template engine’s capabilities.

In a black-box scan, middleBrick tests this attack surface by submitting API requests with manipulated JWT tokens containing template syntax in claim values. The tool examines whether these injected patterns are reflected in responses or cause execution artifacts, correlating findings with the framework’s typical usage patterns. This testing methodology aligns with the OWASP API Top 10 category of Injection, specifically highlighting how authentication tokens can become a vector when untrusted data enters the rendering layer. The scan also checks whether the API exposes verbose errors that could reveal template execution paths, which would amplify the impact of an SSTI attempt.

Proper API security practices require treating JWT tokens as opaque identifiers rather than trusted data sources when rendering server-side content. middleBrick verifies whether applications adhere to this principle by analyzing endpoint behavior with and without valid authentication, ensuring that template logic does not depend on decoded token contents. This approach helps identify misconfigurations where developers assume JWT integrity guarantees safety in presentation contexts, a dangerous assumption when templates are not rigorously isolated from input data.

Remediation focuses on strict separation between authentication data and presentation logic. Developers should avoid passing JWT claims directly to templates. Instead, extract only necessary, sanitized values and construct a dedicated view model that contains no executable logic or user-controlled template syntax.

Below is a secure Fiber handler example demonstrating proper handling:

// Secure Fiber handler with JWT validation and safe rendering
package main

import (
	"html/template"
	"log"
	"net/http"

	"github.com/gofiber/fiber/v2"
	"github.com/golang-jwt/jwt/v5"
)

type SafeViewModel struct {
	Username template.HTML // escaped by default
	Role     string
}

func main() {
	app := fiber.New()

	app.Get("/profile", func(c *fiber.Ctx) error {
		authHeader := c.Get("Authorization")
		if len(authHeader) < 7 || authHeader[:7] != "Bearer " {
			return c.Status(fiber.StatusUnauthorized).SendString("Unauthorized")
		}

		tokenString := authHeader[7:]
		claims := jwt.MapClaims{}
		token, err := jwt.ParseWithClaims(tokenString, claims, func(token *jwt.Token) (interface{}, error) {
			// Replace with your key retrieval logic
			return []byte("your-secret-key"), nil
		})
		if err != nil || !token.Valid {
			return c.Status(fiber.StatusUnauthorized).SendString("Invalid token")
		}

		// Build a safe view model, avoiding direct injection of claims
		viewModel := SafeViewModel{
			Username: template.HTML(claims["username"]), // properly escaped
			Role:     claims["role"].(string),
		}

		tmpl := template.Must(template.New("profile").Parse(`
			<h1>Profile</h1>
			<div>User: {{ .Username }}</div>
			<div>Role: {{ .Role }}</div>
		`))
		return tmpl.Execute(c.BodyWriter(), viewModel)
	})

	log.Fatal(app.Listen(":3000"))
}

This pattern ensures that JWT-derived data is treated as plain text, with HTML escaping applied automatically by template.HTML where appropriate. It eliminates the risk of injecting template actions through malicious claim values.

Additionally, validate and restrict token claims on the server side, and avoid using tokens to carry logic that could influence template behavior. middleBrick’s scans verify that endpoints following this approach do not reflect untrusted input in rendered output, confirming that SSTI vectors are mitigated.