Dns Cache Poisoning in Echo Go with Hmac Signatures

DNS cache poisoning attacks aim to insert malicious entries into a resolver’s cache so that subsequent lookups return attacker-controlled IPs. In Echo Go, using HMAC signatures on DNS responses can mitigate tampering only if the verification logic is strict and tied to the request context. A common vulnerability pattern is signing the response data without including the query name, type, and unique transaction identifier, which allows an attacker to swap a valid signed record for a different domain or record type while the signature remains valid.

Consider an Echo Go handler that generates a DNS response and attaches an HMAC signature over a subset of the payload. If the server reuses a nonce or key across queries, or if the signature is computed only over the answer section, an attacker can perform a cache poisoning attack by injecting a crafted record that passes signature verification but maps a benign domain to a malicious IP. The risk is compounded when the implementation does not enforce minimum key lengths or does not protect the signing key in memory, enabling an attacker to recover or brute-force the HMAC key through side channels or prior data exposure.

In the context of the 12 security checks run by middleBrick, DNS cache poisoning maps into Input Validation and Property Authorization: the scanner evaluates whether the application properly validates and scopes data origin and integrity checks. If the Echo Go service exposes an unauthenticated endpoint that returns DNS responses and signs them with weak or predictable HMAC constructs, middleBrick can detect anomalies in the integrity verification flow and surface findings related to data manipulation risks.

To remediate DNS cache poisoning risks when using HMAC signatures in Echo Go, you must bind the signature to the full DNS query context and enforce strict verification before using cached responses. Below are concrete code examples that demonstrate a safer approach.

Example 1: Signing and verifying the full DNS question and answer

Ensure the HMAC covers the query name, query type, class, and the answer data. Use a strong key and a per-request nonce or timestamp to prevent replay and substitution attacks.

// Sign DNS response data
dataToSign := fmt.Sprintf("%s|%d|%d|%s", question.Name, question.Qtype, question.Qclass, answer.Rdata)
signature := computeHMAC([]byte(dataToSign), key)
response.Header.Extra = map[string]string{"x-dns-signature": hex.EncodeToString(signature)}

On verification, reconstruct the same string from the received question and answer and compare the HMAC using a constant-time function.

// Verify DNS response signature
expected := computeHMAC([]byte(fmt.Sprintf("%s|%d|%d|%s", req.Name, req.Qtype, req.Qclass, rdata)), key)
if !hmac.Equal(expected, receivedSignature) {
    // reject response
    return errors.New("invalid signature")
}

Example 2: Using per-request nonces and key rotation

Incorporate a request-bound nonce and rotate keys periodically to reduce the impact of key exposure. Store nonces server-side with a short TTL to prevent reuse.

nonce := generateNonce() // cryptographically random, e.g., 16 bytes
payload := fmt.Sprintf("%s|%d|%d|%s|%s", question.Name, question.Qtype, question.Qclass, answer.Rdata, nonce)
signature := computeHMAC([]byte(payload), key)
cache.Set(cacheKey, answer, cacheEntry{Signature: signature, Nonce: nonce, Ts: time.Now()})

Verify nonce freshness and signature together:

cached, found := cache.Get(cacheKey)
if !found || cached.Nonce != receivedNonce || time.Since(cached.Ts) > cacheTTL {
    // reject
    return errors.New("stale or missing nonce")
}
if !hmac.Equal(computeHMAC([]byte(fmt.Sprintf("%s|%d|%d|%s|%s", question.Name, question.Qtype, question.Qclass, answer.Rdata, receivedNonce)), key), receivedSignature) {
    return errors.New("invalid signature")
}

Operational and integration guidance

• Use a cryptographically secure random generator for nonces and keys; avoid predictable values.
• Prefer HMAC-SHA256 or stronger; avoid truncated or custom MAC constructions.
• Ensure the signing key is rotated and stored with restricted access; do not embed keys in source code.
• Validate and canonicalize inputs to prevent normalization bypasses that could evade verification.
• Combine HMAC checks with other controls such as rate limiting, source IP validation, and strict TTL policies to reduce poisoning impact.

middleBrick can help identify whether your Echo Go implementation correctly scopes HMAC verification and whether any endpoints expose unsigned or weakly signed DNS handling. By correlating runtime behavior with spec definitions, the scanner highlights gaps where integrity checks are incomplete or where inputs are accepted without proper validation.