Zone Transfer in Fiber with Jwt Tokens

A DNS zone transfer is a mechanism that replicates DNS records between servers. When improperly restricted, it can expose internal hostnames, IP addresses, and network topology. In a Fiber application, if a DNS server or an internal endpoint that performs zone transfers does not adequately validate caller permissions, an unauthenticated or insufficiently authenticated request may trigger a zone transfer. JWT tokens are commonly used in Fiber to carry identity claims, but if token validation is misconfigured or bypassed, an attacker can obtain or forge a token that grants access to a zone-transfer endpoint.

Specifically, this combination becomes a vulnerability when:

• The zone-transfer route in Fiber relies only on JWT presence for authorization, without verifying scope, issuer, or audience strictly.
• JWT tokens are accepted from unauthenticated origins (e.g., public endpoints) and the token claims are trusted without signature verification or with a weak algorithm (such as none or HS256 with a leaked secret).
• The application exposes a handler like /dns/zone that performs an AXFR-style zone transfer and does not enforce additional authorization checks beyond token validation, allowing an attacker with a valid or forged token to request a full zone dump.

Consider a Fiber route that uses JWT middleware but does not enforce fine-grained permissions:

const jwtware = middleware.JWT(config.JWTSecret)
app.Get('/dns/zone', jwtware, func(c *fiber.C) error {
    // Dangerous: only checks JWT presence, not scope or specific claims
    zoneData, err := performZoneTransfer(c)
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": "transfer failed"})
    }
    return c.JSON(zoneData)
})

An attacker who obtains or guesses a valid JWT (through leakage, insecure storage, or algorithm confusion) can call this route and trigger a zone transfer that reveals internal hostnames and network structure. Additionally, if the JWT is unsigned (algorithm "none") or uses a weak secret, an attacker can craft a token with elevated or fabricated permissions and abuse the same route. This exposes sensitive DNS infrastructure data that can be leveraged for further attacks, such as internal reconnaissance or targeted spoofing.

Because middleBrick tests unauthenticated attack surfaces and validates JWT handling, it can identify whether a zone-transfer endpoint relies solely on JWT tokens without proper claim validation, scope checks, or rate controls. Findings include evidence of excessive data exposure and guidance on tightening authorization around sensitive operations.

Remediation focuses on strict JWT validation, claim verification, and scoping so that zone-transfer operations are permitted only to authorized identities. Do not rely on token presence alone; enforce algorithm, issuer, audience, and scope checks. Use strong secrets or asymmetric keys and avoid the "none" algorithm.

Secure JWT setup in Fiber example using the github.com/gofiber/contrib/jwt package:

config := jwt.Config{
    SigningKey: jwt.SigningKey{Key: []byte(config.JWTSecret)},
    SigningMethod: &jwt.SigningMethodHS256{},
    TokenLookup: &{Source: &{From: "Authorization", Part: "Bearer"}},
    ContextKey: "jwt",
}
app.Get("/dns/zone", middleware.JWT(config), func(c *fiber.C) error {
    claims := c.Locals("jwt").(*jwt.Token).Claims
    // Verify required claims: scope, issuer, audience
    if !claims.VerifyIssuer("https://auth.example.com", true) {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid issuer"})
    }
    if !claims.VerifyAudience("dns-service", true) {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid audience"})
    }
    if !claims.VerifyScopes("zone:transfer") {
        return c.Status(fiber.StatusForbidden).JSON(fiber.Map{"error": "insufficient scope"})
    }
    zoneData, err := performZoneTransfer(c)
    if err != nil {
        return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": "transfer failed"})
    }
    return c.JSON(zoneData)
})

Key practices:

• Always set SigningMethod explicitly and prefer RS256/Ed25519 over HS256 where key rotation is manageable.
• Validate iss, aud, and exp strictly; require a custom scope claim for sensitive operations like zone transfers.
• Use short token lifetimes and refresh mechanisms to reduce the impact of token leakage.
• Apply rate limiting to the zone-transfer endpoint to mitigate brute-force or abuse attempts, even when a valid JWT is presented.
• Never accept tokens with the "none" algorithm; reject unsigned tokens at the middleware level.

By combining strict JWT validation with endpoint-level authorization, you reduce the risk of unauthorized zone transfers. middleBrick can verify these controls by checking whether JWT validation includes issuer, audience, scope, and algorithm checks, and by testing whether the endpoint exposes zone-transfer functionality without proper authorization.