Hatchet: SSRF via Unsigned UnsubscribeURL in SNS UnsubscribeConfirmation Handler

Summary

The SNS UnsubscribeConfirmation handler in internal/integrations/ingestors/sns/sns.go makes an unvalidated
http.Get() call to payload.UnsubscribeURL without any URL restriction. Because UnsubscribeURL is intentionally
excluded from the BuildSignature() signed field list, an attacker can replace this field in a legitimately
AWS-signed message with an arbitrary internal URL, bypassing the VerifyPayload() signature check entirely and
triggering a Server-Side Request Forgery (SSRF) against internal infrastructure — including the EC2 Instance Metadata
Service (IMDS) at 169.254.169.254.

Impact

This is a Server-Side Request Forgery (SSRF) vulnerability. An attacker with a Hatchet account and an AWS Free Tier
account can:

• Reach the EC2 IMDS endpoint and retrieve IAM temporary credentials if Hatchet is deployed on AWS
• Port-scan and probe internal services not exposed to the internet
• Access internal HTTP APIs (e.g., Kubernetes API server, internal dashboards) from the Hatchet server's network
  context