Symlink Attack in Fiber with Basic Auth

A symlink attack in the Fiber web framework combined with Basic Authentication involves an authenticated endpoint that dynamically references files based on user-controlled input, such as a filename or path parameter. When authentication is handled via HTTP Basic Auth, access control is enforced at the handler level, but the handler may still pass user input to filesystem operations without canonicalization or validation. An attacker who has obtained valid credentials (or uses a shared or compromised account) can supply a path that traverses directories and points to a symlink outside the intended directory. Because the application resolves the symlink at runtime, the request can read or overwrite files that the application process can access, bypassing logical access controls enforced by Basic Auth.

Consider a scenario where an authenticated endpoint serves files from a base directory but uses a raw query parameter to locate content. A handler written in Fiber might look like this:

// Insecure example: user input used directly in file resolution
app.Get("/files/:filename", func(c *fiber.Ctx) error {
    filename := c.Params("filename")
    filePath := path.Join("/var/data/", filename)
    content, err := os.ReadFile(filePath)
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).SendString("Unable to read file")
    }
    return c.Send(content)
})

An authenticated user could provide a filename such as ../../../etc/passwd. If a symlink exists at an intermediate location that points to a sensitive file, the resolved path may allow reading or overwriting that file. Basic Auth ensures that only users with credentials reach this handler, but it does not prevent the authenticated user from exploiting path traversal via symlink resolution. The attack combines valid credentials, insecure path handling, and filesystem indirection to violate the intended access boundaries.

OpenAPI specifications can inadvertently document these risky endpoints if path parameters are described as file identifiers without clarifying constraints. During a scan, middleware that resolves $ref documents and correlates them with runtime behavior may flag endpoints that accept path inputs used in filesystem operations while authentication is present but input validation is weak.

To mitigate symlink risks in Fiber when using Basic Auth, eliminate direct filesystem path construction from user input and enforce strict allowlists. Replace dynamic path joins with a controlled mapping from safe identifiers to filesystem locations. Validate and sanitize all inputs, and avoid passing raw user input to filesystem operations.

Below is a secure implementation example. It uses a predefined map to associate allowed identifiers with filesystem paths, preventing directory traversal and symlink abuse. Authentication via Basic Auth remains in place, but the handler no longer constructs paths from user input:

// Secure example: controlled mapping, no user-controlled paths
var files = map[string]string{
    "report": "/var/data/reports/report.pdf",
    "image":  "/var/data/assets/logo.png",
}

app.Get("/files/:name", func(c *fiber.Ctx) error {
    name := c.Params("name")
    filePath, ok := files[name]
    if !ok {
        return c.Status(fiber.StatusBadRequest).SendString("Invalid file identifier")
    }
    content, err := os.ReadFile(filePath)
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).SendString("Unable to read file")
    }
    return c.Send(content)
})

If you must derive paths, normalize and restrict them rigorously. Use filepath.Clean and verify that the resolved path remains within the intended base directory. Do not rely on Basic Auth alone to protect filesystem operations; treat authentication as one layer and enforce strict input validation and path canonicalization as another:
// Safer dynamic resolution with canonicalization and base directory check
base := "/var/data/uploads/"
app.Get("/uploads/:filename", func(c *fiber.Ctx) error {
    filename := c.Params("filename")
    // Clean the filename to remove ".." and other traversal elements
    clean := filepath.Clean(filename)
    // Ensure the clean path does not escape the base directory
    if !strings.HasPrefix(clean, base) {
        return c.Status(fiber.StatusBadRequest).SendString("Invalid path")
    }
    fullPath := filepath.Join(base, clean)
    // Double-check the resolved path
    resolved, err := filepath.EvalSymlinks(fullPath)
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).SendString("Unable to resolve path")
    }
    if !strings.HasPrefix(resolved, base) {
        return c.Status(fiber.StatusForbidden).SendString("Access denied")
    }
    content, err := os.ReadFile(resolved)
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).SendString("Unable to read file")
    }
    return c.Send(content)
})

Complement these coding practices with automated scanning in your workflow. Use the CLI tool to run regular checks: middlebrick scan <url>. Add the GitHub Action to your CI/CD pipeline to fail builds if security findings appear, and consider the Pro plan for continuous monitoring of APIs that handle sensitive file operations.