Double Free in Fiber with Basic Auth

Double Free is a memory safety vulnerability that occurs when a program attempts to free the same dynamically allocated memory twice. In the context of a Fiber-based API using HTTP Basic Authentication, this risk arises when request-scoped objects are deallocated incorrectly across concurrent requests or when middleware logic reuses or releases the same buffer or structure. Fiber is a high-performance HTTP framework for Go that uses a per-request context and object pooling to reduce allocations; however, improper handling of Basic Auth credentials in middleware can introduce use-after-free or double-free conditions when the same underlying memory is returned to the pool or freed explicitly by custom logic.

When Basic Authentication is enforced in Fiber, the framework typically extracts credentials from the Authorization header, decodes the base64-encoded username:password pair, and attaches the parsed values to the request context. If the application or custom middleware manually manages memory for these credential objects—such as by copying pointers into a pool and freeing them after validation—a race condition or incorrect lifecycle management can cause the same memory block to be freed twice: once by the application and once by the pool’s cleanup routine. This is more likely when developers implement custom authentication logic outside of Fiber’s built-in mechanisms, inadvertently bypassing its managed lifecycle.

Moreover, because Fiber allows middleware stacking, a misconfigured chain that both authenticates and conditionally skips or aborts requests can leave dangling references. For example, if a developer writes middleware that frees a credential struct after a failed auth check and then another middleware or the route handler also attempts to release the same struct (or a reference derived from it), the runtime may detect a double-free during testing or, worse, allow memory corruption that leads to crashes or exploitable behavior. The vulnerability is not in Fiber itself but in how the developer integrates Basic Auth parsing and memory management alongside Fiber’s request handling flow.

In practice, this risk is detected during black-box scanning when tests send concurrent or malformed Basic Auth headers, causing inconsistent memory states or crashes in the runtime environment. middleBrick’s checks for Input Validation and Unsafe Consumption help surface these issues by probing how the service handles malformed or repeated authentication inputs. Because the scanner evaluates unauthenticated attack surfaces, it can observe whether the API responds with crashes, inconsistent status codes, or memory-related anomalies when subjected to repeated or overlapping Basic Auth payloads.

To understand the scope, consider that Double Free in this scenario does not require authentication bypass—it only requires that the same memory region be freed more than once during request processing. This can be triggered by malformed base64 input, repeated requests with the same credentials, or middleware that incorrectly reuses buffers. Because Basic Auth credentials are often copied into temporary structures for validation, any manual memory handling in that path must align precisely with Fiber’s request lifecycle to avoid such errors.

middleBrick’s LLM/AI Security checks are not directly relevant here, but its parallel security checks—particularly Authentication, Input Validation, and Unsafe Consumption—help identify conditions that could lead to memory safety issues. By correlating findings across these checks, the scanner can highlight risky patterns in how authentication data is handled, even in unauthenticated scans that probe endpoint resilience.

Remediation focuses on avoiding manual memory management for authentication data and relying on Fiber’s built-in context lifecycle. Developers should parse and validate Basic Auth credentials within the request scope without retaining or freeing pointers beyond the request’s lifetime. The following examples demonstrate safe approaches using the standard net/http library utilities available in Fiber-compatible Go code.

Safe Basic Auth Parsing in Fiber

package main

import (
    "encoding/base64"
    "strings"

    "github.com/gofiber/fiber/v2"
)

func BasicAuthMiddleware(c *fiber.Ctx) error {
    auth := c.Get("Authorization")
    if auth == "" {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "authorization header required"})
    }

    const prefix = "Basic "
    if !strings.HasPrefix(auth, prefix) {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid authorization header format"})
    }

    payload, err := base64.StdEncoding.DecodeString(auth[len(prefix):])
    if err != nil {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid authorization header"})
    }

    pair := string(payload)
    if i := strings.Index(pair, ":"); i < 0 {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid credentials"})
    }

    // At this point, use only immutable values (strings) from the request.
    // Do not store pointers to payload or c.Body() beyond this function.
    // Proceed to validation logic, e.g., lookup user credentials securely.
    // ...

    return c.Next()
}

In this example, the decoded credentials are converted to strings and used only for immediate validation. No structs are allocated on the heap in a way that would require explicit freeing, and no references to the raw base64 data are retained. This aligns with Fiber’s design and avoids double-free risks by letting Go’s garbage collector manage memory within the request context.

Avoiding Manual Memory Management

Do not attempt to pool or reuse buffers for Basic Auth credentials. For example, the following pattern is unsafe:

var credPool = sync.Pool{
    New: func() interface{} {
        return new([2]string) // [username, password]
    },
}

func UnsafeAuthMiddleware(c *fiber.Ctx) error {
    auth := c.Get("Authorization")
    // ... decode ...
    cred := credPool.Get().(*[2]string)
    cred[0], cred[1] = username, password
    // If another middleware or the framework frees this incorrectly,
    // a double-free may occur when the pool reuses or releases it.
    credPool.Put(cred)
    return c.Next()
}

Such manual pooling introduces the exact conditions that can lead to double-free errors, especially under concurrency or when the runtime’s internal management overlaps with user code. Instead, rely on short-lived variables and let Fiber manage request context lifecycle.

Finally, ensure that any custom authentication logic validates input strictly (e.g., rejecting malformed base64 or oversized headers) to reduce the attack surface covered by middleBrick’s Input Validation and Rate Limiting checks. This minimizes the likelihood of triggering memory safety bugs through unexpected or malicious authentication requests.