ex_aws_sns: Trusted-attacker `SigningCertURL` permits complete SNS signature bypass

Summary

ExAws.SNS.verify_message/1 fetches the signing certificate from the SigningCertURL field of the incoming SNS message without validating that the URL uses HTTPS or that its host is an AWS-owned SNS certificate domain. An unauthenticated attacker who can POST to any endpoint that calls verify_message/1 can supply an attacker-controlled SigningCertURL, sign a forged SNS message with their own RSA key, and cause the function to return :ok, completely bypassing SNS signature verification.

Details

In lib/ex_aws/sns.ex (lines 475–483), verify_message/1 performs three checks: validate_message_params/1 (confirms required fields are present), validate_signature_version/1 (confirms SignatureVersion == "1"), then signature verification. The signature step calls ExAws.SNS.PublicKeyCache.get(message["SigningCertURL"]) and passes the result to :public_key.verify/4.

Neither validate_message_params/1 nor any other step checks that SigningCertURL is an HTTPS URL or that the hostname matches the expected pattern (e.g. sns.<region>.amazonaws.com). PublicKeyCache.get/1 in lib/ex_aws/sns/public_key_cache.ex fetches whatever URL is provided and caches the certificate. The RSA signature then verifies against the attacker's own public key, and verify_message/1 returns :ok.

PoC

1. Generate an RSA keypair and host the DER/PEM public certificate at any URL reachable from the target server (e.g. http://attacker.example/cert.pem).
2. Build a forged Notification payload with an arbitrary TopicArn and Message, compute the canonical string-to-sign per the SNS spec, and sign it with the attacker private key.
3. Set SigningCertURL to the attacker URL and Signature to the base64-encoded signature.
4. POST the forged payload to any SNS webhook endpoint that calls ExAws.SNS.verify_message/1.