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
• 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