Xss Cross Site Scripting in Buffalo with Bearer Tokens

Cross-site scripting (XSS) in a Buffalo application becomes particularly risky when Bearer Tokens are used for authentication. Bearer Tokens are typically stored in HTTP headers (e.g., Authorization: Bearer <token>), and if the application reflects or stores token-derived data in HTML contexts without proper escaping, an attacker can inject scripts that execute in the victim’s browser while possessing the token’s privileges.

Buffalo does not automatically escape data placed into templates, so developers must explicitly escape output. When a Bearer Token is exposed through an endpoint—either via logging, error messages, or reflected headers—and later rendered in a template (e.g., a user profile or debug page), an attacker can supply a crafted payload such as <script>steal(document.cookie, document.location)</script>. If the page is visited by a user whose browser includes the Bearer Token in requests, the injected script can read or exfiltrate the token, leading to account compromise.

Another scenario involves JSON APIs served by Buffalo that include sensitive fields derived from Bearer Token scopes or user metadata. If these JSON responses are consumed by a client-side JavaScript framework and inserted into the DOM without sanitization, XSS can occur. For example, an endpoint returning {"message": "Hello, <script>alert(1)</script>"} can execute arbitrary code in the browser of any authenticated user who views the page, assuming their session includes a valid Bearer Token.

The combination of Buffalo’s flexible template rendering and Bearer Token usage highlights the importance of context-aware output encoding. Tokens should never be embedded in HTML, JavaScript, or CSS. Security checks such as those in middleBrick can detect these risky patterns by correlating authentication mechanisms with output handling, helping teams identify places where a Bearer Token could be inadvertently exposed to script injection.

Remediation focuses on preventing token leakage and ensuring proper escaping in templates. Never write Bearer Token values into HTML attributes, JavaScript blocks, or error messages. In Buffalo, use context.Response.Header().Set("Authorization", "Bearer ***") only for logging masked values, and avoid including the real token in any response body.

When rendering user-controlled data in templates, always use Buffalo’s HTML escaping utilities. For example, in a template you should write:

<%== h(user.Content) %>

This ensures that characters like <, >, ", and ' are escaped before being inserted into HTML, neutralizing script injection attempts that could otherwise hijack a Bearer Token.

For APIs that return sensitive authorization metadata, apply strict field filtering and encoding. Instead of returning raw token scopes, map them to safe display strings:

// Go handler example
func (v VerifiedResource) Get() error {
    // Do NOT include the actual Bearer Token in the response
    safeScopes := []string{"read:posts", "write:comments"}
    return v.Render(200, r.JSON(map[string]interface{}{
        "scopes": safeScopes,
        "user":   v.User.Email,
    }))
}

In client-side JavaScript, avoid inserting untrusted data directly using innerHTML. Use textContent or a DOM-safe library. If your Buffalo app serves a frontend, ensure Content Security Policy (CSP) headers are configured to restrict script sources, reducing the impact of any potential XSS vectors involving token leakage.

Using middleBrick with the Pro plan enables continuous monitoring and GitHub Action integration, so any endpoint that reflects Bearer Token context in risky ways can fail a build before deployment, helping teams enforce secure handling patterns automatically.