Cache Poisoning in Buffalo with Basic Auth

Cache poisoning occurs when an attacker causes a cache (e.g., CDN, reverse proxy, or in-memory cache) to store malicious responses that are then served to other users. In the Buffalo framework, when Basic Auth is used but not properly enforced for cache-sensitive routes, this risk becomes more nuanced.

Buffalo does not automatically strip or vary cache keys based on authentication headers. If a route is cached at the infrastructure layer and the response depends on per-user authorization (e.g., user-specific data rendered via a current_user helper), an attacker who can influence the request path or headers might cause a cached response for one user to be reused for another. With Basic Auth, credentials are sent in the Authorization header on every request. If the caching layer does not include the Authorization header (or a derived scope) in the cache key, the same cached response can be served regardless of the requesting user, effectively leaking data across users.

Additionally, if the application caches representation URLs or links that include user-specific identifiers (such as an embedded user ID in a JSON API response), and those links are later served to unauthorized users via cache poisoning, attackers can trick clients into accessing unintended resources. Because Buffalo applications often render HTML or JSON that may include sensitive user context, failing to align cache behavior with authentication state exposes the application to information disclosure via cache poisoning. This is especially relevant when using shared caches that do not differentiate based on the Authorization header, which is common in many CDNs and load balancers.

Another angle involves query parameters that affect authentication or user context. If query parameters are not considered in cache variations and Basic Auth credentials are used to gate access, an attacker might try manipulating parameters to retrieve cached responses intended for different authorization contexts. Since Buffalo applications may rely on middleware or external caching for performance, developers must ensure cache rules account for authentication headers and user-specific scopes to prevent cross-user data exposure.

To mitigate cache poisoning when using Basic Auth in Buffalo, ensure that authentication state is explicitly considered in cache behavior and that sensitive responses are not cached in shared caches. Below are concrete remediation patterns with code examples.

1. Avoid caching authenticated responses in shared caches

For routes that require authentication, set cache-control headers to prevent shared caches from storing responses. In Buffalo, you can set headers in the action.

// app/controllers/users_controller.go
package controllers
type UsersController struct {
	*Buffalo.Controller
}

func (v UsersController) Show(c buffalo.Context) error {
    // Ensure no shared cache stores this response
    c.Response().Header().Set("Cache-Control", "no-store, no-cache, must-revalidate, private")
    c.Response().Header().Set("Pragma", "no-cache")
    c.Response().Header().Set("Expires", "0")

    user := &models.User{}
    if err := c.CurrentUser(user); err != nil {
        return c.Render(401, r.JSON(Error{Message: "Unauthorized"}))
    }
    return c.Render(200, r.JSON(user))
}

2. Use Vary headers to signal cache differentiation by Authorization

When you must allow caching, use the Vary header to indicate that the response varies based on the Authorization header. This helps caches differentiate responses per authorization context.

// app/controllers/api_controller.go
package controllers
type APIController struct {
	*Buffalo.Controller
}

func (v APIController) Index(c buffalo.Context) error {
    c.Response().Header().Set("Vary", "Authorization")
    // Proceed with logic, knowing caches should treat different Authorization values separately
    return c.Render(200, r.JSON(ResourceList{Items: []string{"item1", "item2"}}))
}

3. Scope cache keys to user or tenant identifiers in application-level caching

If you implement application-level caching (e.g., with redis or memcached), include user ID or tenant ID in the cache key to avoid cross-user contamination.

// app/models/user.go
package models
type User struct {
	ID   int
	Name string
}

// cache key example: user:123:profile
func (u User) CacheKey(action string) string {
    return fmt.Sprintf("user:%d:%s", u.ID, action)
}

// In your controller or service
func (v UsersController) Profile(c buffalo.Context) error {
    userID := c.CurrentUser().(*models.User).ID
    key := fmt.Sprintf("user:%d:profile", userID)
    var profile Profile
    if err := cache.Get(key, &profile); err != nil {
        // build profile
        cache.Set(key, profile, 5*time.Minute)
    }
    return c.JSON(200, profile)
}

4. Do not rely on Basic Auth for caching decisions at the edge

Edge caches may not forward Authorization headers by default. If you rely on them for cache variation, explicitly configure the cache to forward the header or avoid caching authenticated responses altogether.

5. Validate and sanitize inputs that affect cache keys

Ensure that any user-controlled input used in cache keys is validated and normalized to prevent cache key pollution or injection that could lead to poisoning.