ZeptoClaw: Email Sender Spoofing to bypass Header-Only From Allowlist Validation

Summary

The email channel authorizes senders based on the parsed From header identity only. If upstream email authentication/enforcement is weak (for example, relaxed SPF/DKIM/DMARC handling), an attacker can spoof an allowlisted sender address and have the message treated as trusted input.

Details

Relevant code paths:

• src/channels/email_channel.rs:311 extracts sender identity from parsed message headers:
  • let from = parsed.from() ... a.address() ...
• src/channels/email_channel.rs:328 authorizes using that from value:
  • if !self.is_sender_allowed(&from) { ... }
• src/channels/email_channel.rs:87 onward (is_sender_allowed) performs allowlist/domain matching against the same header-derived value.
• There is no in-channel validation of sender authenticity indicators such as SPF/DKIM/DMARC results before allowlist trust decisions.

Result:

• Trust decision is based on a potentially spoofable header field unless mailbox/provider-side anti-spoofing controls are strong and enforced.

PoC

1. Configure email channel with strict sender allowlist:
   • channels.email.enabled = true
   • channels.email.allowed_senders = ["[email protected]"]
   • channels.email.deny_by_default = true
2. Ensure the monitored mailbox accepts or forwards a spoofed message (for testing, use a local SMTP path that does not enforce sender authentication strongly).
3. Send an email to the monitored inbox with forged header identity:

python - <<'PY'
import smtplib
from email.message import EmailMessage

msg = EmailMessage()
msg["From"] = "[email protected]"   # forged trusted sender
msg["To"] = "[email protected]"
msg["Subject"] = "forged control message"
msg.set_content("FORGED EMAIL CONTENT")

# Example test SMTP endpoint
with smtplib.SMTP("127.0.0.1", 25) as s:
    s.send_message(msg)
PY

4. Wait for IMAP fetch/IDLE processing.
5. Observe the message is accepted as allowlisted sender [email protected] and published as inbound channel input.

Impact

• Vulnerability type: sender identity spoofing risk due to header-based authorization.
• Affected deployments: those using email channel allowlists where upstream anti-spoof controls are weak, misconfigured, or bypassed.
• Security effect:
  • Spoofed From headers may bypass logical sender allowlist.
  • Malicious content can enter trusted automation/agent flows as if sent by authorized identities.
• Risk is reduced in environments with strict SPF/DKIM/DMARC enforcement and strong inbound mail hygiene, but not eliminated at application layer.

Patch Recommendation

Add a sender-authentication gate in src/channels/email_channel.rs immediately after parsing from (src/channels/email_channel.rs:311) and before allowlist enforcement (src/channels/email_channel.rs:328). The gate should require trusted SPF/DKIM/DMARC evidence with domain alignment (for example, DMARC=pass, or aligned SPF/DKIM pass) before is_sender_allowed is evaluated. For backward compatibility, add a configurable mode in EmailConfig (for example, sender_verification_mode), but recommend hardened settings in production: dmarc_aligned, exact-address allowlists, and deny_by_default=true.