Cache Poisoning in Buffalo with Jwt Tokens

Cache poisoning in the Buffalo web framework when JWT tokens are handled incorrectly can occur when responses that vary by authorization are cached in shared or public caches. If a server includes sensitive JWT claims in a response but fails to differentiate cache keys by authorization, an attacker may be able to retrieve another user’s token or force the cache to serve a poisoned response that exposes or manipulates authentication data.

Buffalo does not automatically add authorization-based cache variation. If you render a page or API response that embeds a JWT (for example, in a meta tag, a JavaScript variable, or a header) and you use the default cache key (often the request path), users with different credentials might receive the same cached response containing another user’s token. This can lead to token disclosure or to scenarios where an attacker tricks the application into caching a response that includes a manipulated or forged JWT.

For example, consider an endpoint that returns HTML with a JWT embedded in a script tag. If the response is cached without considering the Authorization header or the JWT’s claims (like sub or role), a low-privilege user might receive a cached response meant for an admin, exposing elevated-scoped tokens or session identifiers. Similarly, if the JWT is passed via a header and the response varies only by host or query parameters, an attacker may exploit shared caches to perform cache poisoning by injecting malicious headers or parameters that alter the cached response without invalidating the token’s validity.

Because JWTs often carry authorization information, cached responses that include them can violate the principle of least privilege and enable horizontal or vertical privilege escalation if the cache serves the wrong token to a different user. This is especially relevant when responses include custom headers or cookies derived from JWT payloads and those responses are cached by proxies or CDNs without considering the authenticated context.

To assess this risk using middleBrick, you can run a scan with the CLI: middlebrick scan https://your-buffalo-app.example.com. The scan will identify whether responses that contain or are influenced by JWT tokens are improperly cached and will provide findings mapped to the Authentication and Data Exposure checks, including remediation guidance and references to the OWASP API Top 10 and relevant compliance frameworks.

Remediation focuses on ensuring that cached responses are unique per authorization context and that JWTs are never embedded in cacheable output unless strictly necessary. In Buffalo, you can vary caching by adding the Authorization header or specific JWT claims (such as sub or role) to the cache key, and you should avoid placing JWTs in parts of the response that may be stored by shared caches.

Use Vary headers to indicate that the response depends on Authorization. For JSON APIs, set Vary: Authorization so that caches store separate versions per bearer token. For HTML responses, vary by a canonical user identifier derived from the token claims, and avoid including raw JWTs in HTML or JavaScript where they might be cached.

Example: customizing cache keys in a Buffalo action to include a user identifier extracted from the JWT:

import "github.com/gobuffalo/buffalo"
import "github.com/gobuffalo/packr/v2"
import "github.com/dgrijalva/jwt-go"

func MyProtectedAction(c buffalo.Context) error {
    tokenString := c.Request().Header.Get("Authorization")
    if tokenString == "" {
        return c.Render(401, r.JSON(map[string]string{"error": "unauthorized"}))
    }
    token, err := jwt.Parse(tokenString, func(token *jwt.Token) (interface{}, error) {
        // provide your key and signing method validation here
        return myKey, nil
    })
    if err != nil || !token.Valid {
        return c.Render(401, r.JSON(map[string]string{"error": "invalid token"}))
    }
    claims, ok := token.Claims.(jwt.MapClaims)
    if !ok {
        return c.Render(400, r.JSON(map[string]string{"error": "invalid claims"}))
    }
    userID := claims["sub"]
    // Vary cache by user ID extracted from the JWT to prevent cross-user cache poisoning
    c.Response().Header().Set("Vary", "Authorization")
    // Use userID in cache key if you implement custom caching
    _ = userID
    return c.Render(200, r.JSON(map[string]string{"hello": "secure"}))
}

Example: ensuring JWTs are not embedded in cacheable HTML by using template partials that are never cached and by setting appropriate cache-control headers:

import "github.com/gobuffalo/buffalo"

func PublicAction(c buffalo.Context) error {
    // Do not embed JWTs in HTML; if you must, ensure the response is not cached
    c.Response().Header().Set("Cache-Control", "no-store, no-cache, must-revalidate, private")
    c.Response().Header().Set("Vary", "Authorization")
    return c.Render(200, r.HTML("public/index.html"))
}

For API tokens used by clients, prefer storing JWTs in HttpOnly, Secure cookies with SameSite attributes or in Authorization headers, and ensure that any caching layer respects these headers and does not collapse distinct authorization contexts into the same cache entry. middleBrick’s scans can validate that your headers and cache controls are effective; you can add the GitHub Action to fail builds if a scan detects missing Vary on responses that vary by authorization.