Sandbox Escape in Fiber with Jwt Tokens
Sandbox Escape in Fiber with Jwt Tokens — how this specific combination creates or exposes the vulnerability
A sandbox escape in the context of a Fiber application that uses JWT tokens occurs when an attacker who can influence or forge a token gains access to server-side resources that should remain restricted. This typically maps to BOLA/IDOR and Authentication checks in a scan, and can also intersect with Property Authorization and Unsafe Consumption findings when token claims are used to make authorization decisions without proper validation.
Fiber is a fast HTTP framework for Go. When JWT tokens are used for authentication, the security boundary depends on how the token is verified, what claims are trusted, and how those claims are enforced at the route or handler level. If the application trusts the JWT payload without re-verifying scopes, roles, or tenant context on each request, an attacker may be able to escalate privileges by altering the token payload or by leveraging a token issued for one purpose to access another.
Consider an example where a JWT is issued with a role claim such as "user", but the server route checks only the presence of a valid signature and not the required scope or role. An attacker who manages to obtain or forge a token with an altered role claim can then call administrative endpoints that should be restricted. This becomes a sandbox escape because the token, which should act as an authorization boundary, is instead used to bypass intended access controls.
In a real-world scenario, if the application decodes the token using a weak method (e.g., skipping signature verification for convenience during debugging and accidentally leaving it in production), it effectively removes the sandbox provided by the token. Similarly, if the token contains tenant or namespace information in a claim and the server does not enforce that claim against the requested resource, one user may operate on another user’s data, which is an IDOR-like sandbox escape facilitated by JWT trust assumptions.
middleBrick detects such patterns by correlating OpenAPI/Swagger spec definitions (including $ref resolution) with runtime behavior. For instance, if the spec marks an endpoint as requiring specific scopes or roles but the implementation does not enforce them, or if token validation settings are inconsistent across routes, the scanner can surface Authentication and Authorization findings with severity and remediation guidance. This helps teams identify where the JWT-based sandbox is incomplete or misaligned with the declared API contract.
Jwt Tokens-Specific Remediation in Fiber — concrete code fixes
Remediation focuses on strict token validation, claim verification, and ensuring that every request enforces authorization based on the token context, not just route-level assumptions. Below are concrete, working code examples for a Fiber application using JWT tokens securely.
1. Use a robust JWT middleware with proper validation
Instead of manually parsing and verifying tokens in each handler, use a well-maintained middleware that validates the signature, issuer, audience, and expiration on every request.
package main
import (
"github.com/gofiber/fiber/v2"
"github.com/gofiber/fiber/v2/middleware/jwt"
)
func main() {
app := fiber.New()
config := jwt.Config{
SigningKey: []byte("your-secure-secret-key"),
ContextKey: "user", // where the decoded claims will be accessible
SigningMethod: "HS256",
Claims: &CustomClaims{
StandardClaims: jwt.StandardClaims{},
Role: "user",
Scope: "read write",
},
TokenLookup: "header:Authorization",
}
app.Use(jwt.New(config))
app.Get("/public", func(c *fiber.Ctx) error {
return c.SendString("public endpoint")
})
app.Get("/admin", func(c *fiber.Ctx) error {
user := c.Locals("user").(*CustomClaims)
if user.Role != "admin" {
return c.Status(fiber.StatusForbidden).SendString("forbidden")
}
return c.SendString("admin endpoint")
})
app.Listen(":3000")
}
type CustomClaims struct {
Role string `json:"role"`
Scope string `json:"scope"`
jwt.StandardClaims
}
2. Enforce scope or role checks on sensitive endpoints
Even with a valid token, ensure that each handler validates the claims required for that operation. Do not rely solely on the route being protected by middleware; explicitly check the scope or role when the operation is sensitive.
// Example of explicit scope check inside a handler
app.Get("/data/:id", func(c *fiber.Ctx) error {
user := c.Locals("user").(*CustomClaims)
// Enforce scope-based authorization
if user.Scope != "read" {
return c.Status(fiber.StatusForbidden).SendString("insufficient scope")
}
// Further enforcement: ensure the user can access the specific resource
resourceOwnerID := c.Params("id")
if user.ID != resourceOwnerID && user.Role != "admin" {
return c.Status(fiber.StatusForbidden).SendString("access denied to this resource")
}
return c.SendString("data response")
})
3. Avoid accepting unsigned or "none" algorithm tokens
Ensure your JWT library is configured to reject tokens with the "none" algorithm and that the signing method is explicitly set. This prevents attackers from swapping a signed token for an unsigned one.
// Within the jwt.Config, set SigningMethod explicitly and avoid "none"
config := jwt.Config{
SigningKey: []byte("your-secure-secret-key"),
SigningMethod: "HS256",
// Reject tokens that do not use the expected signing method
}
4. Validate standard claims and use short expirations
Set reasonable expiration times and validate issuer/audience when applicable. This limits the window for token misuse and reduces the impact of a leaked token.
config := jwt.Config{
SigningKey: []byte("your-secure-secret-key"),
Expiration: 30, // short-lived tokens in minutes
Claims: &CustomClaims{
StandardClaims: jwt.StandardClaims{
Issuer: "myapp",
// Audience can be validated if your setup requires it
},
},
}
5. Use middleware to enforce tenant or namespace checks
If your API serves multiple tenants, decode the tenant claim and ensure it matches the resource being accessed before proceeding.
app.Get("/tenant/resource", func(c *fiber.Ctx) error {
user := c.Locals("user").(*CustomClaims)
// Assume tenantID is a claim in the token
if user.TenantID != c.Params("tenant") {
return c.Status(fiber.StatusForbidden).SendString("tenant mismatch")
}
return c.SendString("tenant-specific data")
})
By combining strong middleware configuration with explicit claim checks in handlers, you reduce the risk of a sandbox escape via JWT tokens. These practices align with Authentication and Authorization findings that middleBrick reports, and they help ensure that token-based boundaries are enforced consistently.