Auth Bypass in Chi with Basic Auth

Chi is a lightweight, idiomatic router for Go that enables flexible route definition and middleware composition. When Basic Auth is used in Chi without additional safeguards, it can create conditions that lead to authentication bypass or exposure of protected endpoints. middleBrick detects this specific combination during unauthenticated black-box scans and flags it as an authentication risk finding.

Basic Auth in Chi is commonly implemented by adding a middleware function that checks the Authorization header. If the middleware is applied inconsistently—for example, only to a subset of routes or mounted subrouters—or if it fails to reject requests with missing or malformed credentials, an attacker may be able to reach sensitive handlers by omitting or manipulating the header. This becomes more likely when developers use optional authentication patterns, such as conditionally skipping middleware based on environment flags or route prefixes.

The vulnerability is not in Basic Auth itself, which is a well-defined RFC 7617 challenge-response mechanism, but in how it is integrated with Chi’s routing tree. Because Chi allows route-specific middleware stacks, it is possible to accidentally exclude the auth handler for certain paths, or to misorder middleware so that public routes shadow protected ones. In scans, middleBrick checks whether authentication enforcement is applied at the route level and whether responses for protected endpoints differ reliably between authenticated and unauthenticated requests, helping to surface inconsistent application of Basic Auth.

During a scan, middleBrick’s Authentication check sends requests without credentials and, where applicable, with malformed credentials to verify that protected routes reject access. If the endpoint returns a successful response (2xx) or exposes data that should require authentication, this is reported as a potential auth bypass with severity and remediation guidance. The scan also cross-references any OpenAPI specification if provided, validating that security schemes are declared and that $ref resolution includes the security requirement for affected paths.

Attack patterns such as path traversal via route prefix overlap can exacerbate the issue. For example, if a protected route is registered under /api/v1/admin and a public route exists at /api/v1, incorrect middleware attachment may allow unauthenticated access to admin functionality. middleBrick tests for these inconsistencies by probing routes defined in the spec and comparing runtime behavior, highlighting where route-specific authentication fails to align with intended access controls.

To prevent authentication bypass in Chi, apply Basic Auth consistently and explicitly across all protected routes, and ensure that middleware ordering and route definitions do not introduce unintended public access.

Consistent middleware attachment

Create a reusable middleware wrapper that validates the Authorization header against a known credential set. Attach this middleware to every route or group that requires protection, avoiding conditional skipping based on flags or environment.

func BasicAuthMiddleware(expectedUser, expectedPass string) func(http.Handler) http.Handler {
    return func(next http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            user, pass, ok := r.BasicAuth()
            if !ok || user != expectedUser || pass != expectedPass {
                w.Header().Set("WWW-Authenticate", `Basic realm="restricted", charset="UTF-8"`)
                http.Error(w, "Unauthorized", http.StatusUnauthorized)
                return
            }
            next.ServeHTTP(w, r)
        })
    }
}

func main() {
    r := chi.NewRouter()

    // Apply to all routes under /api
    r.Use(BasicAuthMiddleware("admin", "s3cr3t"))

    r.Get("/health", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("ok"))
    })

    r.Get("/admin/users", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("admin users list"))
    })

    http.ListenAndServe(":8080", r)
}

Scoped protection with route groups

If different route groups require different authentication requirements, define explicit groups and attach middleware at the group level. Avoid applying middleware at the root router and then selectively disabling it, as this pattern is error-prone.

func PublicHandler(w http.ResponseWriter, r *http.Request) {
    w.Write([]byte("public data"))
}

func main() {
    r := chi.NewRouter()

    // Public routes
    r.Get("/", PublicHandler)
    r.Get("/status", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("healthy"))
    })

    // Protected group
    authGroup := r.Group(func(next http.Handler) http.Handler {
        return BasicAuthMiddleware("admin", "s3cr3t")
    })
    authGroup.Get("/admin/dashboard", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("admin dashboard"))
    })

    http.ListenAndServe(":8080", r)
}

Reject ambiguous credentials and enforce HTTPS

Always reject requests where BasicAuth returns false, and ensure that credentials are compared using a constant-time comparison to mitigate timing attacks. While Chi does not include transport-layer controls, configure your deployment to terminate TLS before requests reach Chi, and never transmit credentials over cleartext HTTP.

func SafeBasicAuthMiddleware(expectedUser, expectedPass string) func(http.Handler) http.Handler {
    return func(next http.Handler) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
            user, pass, ok := r.BasicAuth()
            if !ok {
                http.Error(w, "Unauthorized", http.StatusUnauthorized)
                return
            }
            // Constant-time comparison pattern
            if subtle.ConstantTimeCompare([]byte(user), []byte(expectedUser)) != 1 ||
               subtle.ConstantTimeCompare([]byte(pass), []byte(expectedPass)) != 1 {
                http.Error(w, "Unauthorized", http.StatusUnauthorized)
                return
            }
            next.ServeHTTP(w, r)
        })
    }
}

After applying these fixes, rerun the scan to confirm that protected routes consistently reject unauthenticated requests and that no routes inadvertently expose sensitive functionality. middleBrick’s Authentication and BOLA/IDOR checks will verify that enforcement is reliable across the API surface.