Zone Transfer in Fastapi with Hmac Signatures

A DNS zone transfer is an operation where a secondary DNS server retrieves a full copy of DNS records from a primary server. When designing an API that configures or triggers zone transfers, developers sometimes use HMAC signatures to authenticate requests. The intent is to ensure integrity and origin authenticity: the server verifies the HMAC before performing the transfer. However, combining zone transfer endpoints with Hmac Signatures in Fastapi can still expose the unauthenticated attack surface if the implementation is incomplete.

In Fastapi, an endpoint might accept a DNS server address and a signature, then forward the request to an internal resolver. If the endpoint does not enforce authentication for the zone transfer action and only validates the Hmac Signatures in a way that can be bypassed (for example, accepting an empty or optional signature, or not tying the signature to the specific zone and operation), the unauthenticated attack surface remains. An attacker could probe the endpoint to see if zone transfer is allowed without valid credentials, and if the Hmac Signatures check is not strictly required, the server might initiate a transfer to an attacker-controlled secondary server, leaking internal DNS records.

middleBrick scans such endpoints using its DNS-related checks under Inventory Management and Property Authorization, testing whether zone transfer is possible without proper authentication even when Hmac Signatures are present. For example, a Fastapi route like the one below appears to use Hmac Signatures, but if the signature verification is not enforced for the transfer action, the endpoint remains vulnerable:

from fastapi import Fastapi, Header, HTTPException
import hmac
import hashlib

app = Fastapi()
SECRET = b"super-secret-key"

def verify_hmac(data: str, signature: str) -> bool:
    mac = hmac.new(SECRET, data.encode(), hashlib.sha256)
    return hmac.compare_digest(mac.hexdigest(), signature)

@app.post("/zone-transfer")
async def zone_transfer(
    server: str,
    zone: str,
    signature: str = Header(None)
):
    if not signature or not verify_hmac(f"{server}:{zone}", signature):
        raise HTTPException(status_code=401, detail="Invalid or missing signature")
    # Logic that may trigger a zone transfer to `server` for `zone`
    return {"status": "transfer initiated"}

If the signature header is optional in practice (e.g., due to a misconfiguration or fallback), middleBrick will flag this as a BOLA/IDOR or Property Authorization finding because the operation exposes internal DNS data without robust authentication. The presence of Hmac Signatures does not automatically remove the attack surface; the implementation must enforce strict validation and scope the signature to the exact operation and input.

Additionally, the LLM/AI Security checks in middleBrick do not apply here, as this scenario concerns traditional API authentication and authorization controls rather than prompt injection or system prompt leakage. The key takeaway is that Hmac Signatures must be mandatory, tied to the specific zone and server, and verified before any zone transfer logic is executed to reduce the risk of unauthorized data exposure.

To remediate zone transfer risks when using Hmac Signatures in Fastapi, ensure the signature is required for the transfer endpoint, is scoped to the exact parameters being transferred, and is verified using a constant-time comparison. Below is a concrete, secure implementation:

from fastapi import Fastapi, Header, HTTPException, Depends
import hmac
import hashlib

app = Fastapi()
SECRET = b"super-secret-key"

def verify_hmac(data: str, signature: str) -> bool:
    mac = hmac.new(SECRET, data.encode(), hashlib.sha256)
    return hmac.compare_digest(mac.hexdigest(), signature)

def get_signature_from_header(x_hmac_signature: str = Header(...)):
    if not x_hmac_signature:
        raise HTTPException(status_code=400, detail="Signature header is required")
    return x_hmac_signature

@app.post("/zone-transfer")
async def zone_transfer(
    server: str,
    zone: str,
    signature: str = Depends(get_signature_from_header)
):
    data = f"{server}:{zone}"
    if not verify_hmac(data, signature):
        raise HTTPException(status_code=403, detail="Invalid signature")
    # Proceed with zone transfer logic, ensuring server and zone are validated and rate-limited
    return {"status": "transfer initiated"}

Key fixes applied:

• The signature header is mandatory (Header(...) and enforced via a dependency), removing the possibility of optional or missing authentication.
• The HMAC is computed over a canonical string that includes both the server and the zone, preventing signature reuse across different targets.
• hmac.compare_digest is used for constant-time comparison to mitigate timing attacks.
• The endpoint returns a 403 on invalid signatures, clearly separating authorization failures from bad requests.

You can integrate this pattern into your CI/CD pipeline using the middleBrick GitHub Action to ensure future changes do not reintroduce the issue. The CLI tool (middlebrick scan <url>) can validate that the endpoint requires authentication and that Hmac Signatures are enforced. For ongoing protection, the Pro plan’s continuous monitoring can alert you if a new zone transfer endpoint appears or if an existing scan shows a drop in the security score.