Recommendations

paymail lives on the public web. As such, it is recommended that standard defensive mechanisms are deployed. These include, but are not limited to:

maintain access logs
prevent information leakage, including through error messages
do not keep sensitive information on internet-connected servers
place limits on everything, even if they are high
- rate limits
- maximum message size limits
- bad request limits (leading to a ban)
  - malformed requests
  - suspicious requests
secure the host operating system and environment
operate under the principle of least privilege
do not trust any input to any API endpoint
- assume all API calls are hostile until proven otherwise
- sanitize everything
keep software/library/dependency versions up to date

There are additional concerns that are specific to the paymail service:

consider the meanings of request bodies
- whilst it might be normal for an online retail store to receive a high volume of payment destination requests and pay to email posts, consider the amounts being paid. a valid transaction might not count towards some sort of ban limit, but maybe it should if the service receives thousands of dust transactions a second
- infinite-loop payment destination requests might be valid but can lead to resource exhaustion, both CPU (during key derivation) and address space (individual Type 2 HD Wallet derivation paths, for example, are not infinite)
- consider if the request is really probing for leaked information, rather than performing its intended function
  - see the 401 Unauthorised response section of the Pay-to-Email specification for an example
consider risks to funds
- use xpub or nChain public key derivation for payment destination discovery. do not keep wallet seeds or private keys in paymail services, as these are a gold mine waiting to be stolen by a malicious adversary

Paymail