Double Free in Fiber with Jwt Tokens

In the context of a Go API built with the Fiber web framework, a Double Free risk can emerge when JWT token handling interacts with request lifecycle management. Double Free occurs when the same memory is freed more than once, typically due to incorrect reference counting or improper cleanup after errors. In Fiber, this can surface when middleware processes JWT tokens and an early error path reuses or releases token-related resources without ensuring idempotent cleanup.

Consider a scenario where a Fiber route uses JWT authentication middleware. If token parsing fails after partial state mutation (for example, setting user context and then returning an error), a developer might manually release or reset token buffers in an error handler. If the same cleanup logic is also invoked at the end of the request, the token structure can be freed twice. This is especially risky when using C-based extensions or CGO wrappers around cryptographic libraries that manage their own memory, as double-free bugs in those libraries can lead to arbitrary code execution or crashes.

For JWT tokens specifically, the vulnerability can be triggered by malformed tokens or by attackers forcing repeated error conditions. For instance, an invalid signature or an expired token might cause the middleware to attempt cleanup multiple times across different error branches. Because Fiber allows easy chaining of middleware and error handlers, it is possible to introduce non-atomic cleanup routines that double-free native resources. The risk is compounded when token validation logic is coupled with context cancellation or timeout handling, where similar cleanup routines are invoked in overlapping scopes.

Another vector involves reflection-based token claims mapping. If a developer maps JWT claims into a struct and later performs manual memory operations on the claims container (for example, to reset it between retries), a double-free can occur if the map or buffer is released both by the mapping logic and by Fiber’s request context teardown. Real-world attack patterns seen in other frameworks show that memory safety issues in token processing can lead to security bypasses or denial of service, which aligns with the Double Free category in middleBrick’s security checks.

Because Fiber is often used with high-throughput APIs, a Double Free in JWT handling may not always cause an immediate crash; it can lead to subtle memory corruption that is exploitable under specific timing conditions. This makes static analysis and runtime security scanning essential. Tools like middleBrick test for such patterns by correlating OpenAPI specifications with runtime behavior, checking for inconsistent authorization flows and unsafe request handling that could expose memory safety issues tied to token processing.

To mitigate Double Free risks with JWT tokens in Fiber, ensure that token-related resources are managed with a single, deterministic cleanup point. Use Fiber’s context lifecycle correctly and avoid manual memory release in multiple branches. Below are concrete code examples that demonstrate safe patterns.

Example 1: Safe JWT validation with a single cleanup path

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-secret-key"),
		ContextKey:   "user",
		ErrorHandler: func(ctx *fiber.Ctx, err error) error {
			// Only send error response; do not manually free token data here
			return ctx.Status(fiber.StatusUnauthorized).JSON(fiber.Map{
				"message": "Invalid or expired token",
			})
		},
	}

	app.Get("/protected", jwt.New(config).LoginRequired, func(c *fiber.Ctx) error {
		// Token is validated once; user claims are available via context
		user := c.Locals("user").(*jwt.Token)
		return c.JSON(fiber.Map{"status": "ok", "claims": user.Claims})
	})

	app.Listen(":3000")
}

In this example, the JWT middleware handles parsing and validation. The error handler avoids any additional cleanup of token structures, ensuring that no double-free can occur. Claims are accessed via context without duplicating or freeing underlying buffers.

Example 2: Avoiding manual cleanup in error branches

package main

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

type Claims struct {
	Username string `json:"username"`
	jwt.RegisteredClaims
}

func safeHandler(c *fiber.Ctx) error {
	raw := c.Params("token")
	claims := &Claims{}
	token, err := jwt.ParseWithClaims(raw, claims, func(token *jwt.Token) (interface{}, error) {
		return []byte("your-secret-key"), nil
	})
	if err != nil || !token.Valid {
		// Return early without modifying any shared token structures
		return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid token"})
	}

	// Use claims safely; do not call free or reset on claims after this point
	return c.JSON(fiber.Map{"user": claims.Username})
}

This pattern avoids multiple cleanup calls by returning errors before any state mutation that would require later cleanup. Do not invoke functions that release memory associated with claims or token objects after successful validation, as Fiber’s context management does not expect such manual intervention.

Example 3: Context-aware token usage without cross-scope cleanup

package main

import (
	"context"
	"github.com/gofiber/fiber/v2"
)

func handler(ctx *fiber.Ctx) error {
	// Store token-derived data in context without retaining references to raw buffers
	userID := ctx.Locals("user_id")
	_ = context.WithValue(ctx.UserContext(), "uid", userID)

	// Process request and exit; Fiber handles context teardown once
	return ctx.SendString("processed")
}

By relying on Fiber’s context lifecycle and avoiding manual memory operations on JWT-related objects, developers prevent double-free conditions. MiddleBrick scans can verify that such patterns are in place by checking error handling paths and ensuring that token validation does not trigger redundant cleanup routines.