Combatting Joe Jobs and Spam Backscatter Attacks

1. NZNOG �07�Dealing with Joe Jobs�or how to cope with spam backscatter attacks

2. Presentation Overview Definition Amplifier Target Bounce Verification Technologies Issues to Consider Questions

3. Joe Doll Attack on Joe Doll, webmaster of Joe's Cyberpost. User had their account removed for advertising spam In retaliation, forged an email from Joe Doll Caused joes.com to be DoS�ed [Jan 1997] Also defined in Wayne's World as a sub-standard job

4. Definition Your email address/domain used as the envelope sender in a spam run. Your mail systems end up receiving bounce messages vacation/out-of-office notices challenge-responses etc. Resulting in huge mail gateway, server and administrative overhead

5. Envelope Headers $ telnet 192.168.1.1 25 Connected to 192.168.1.1 Escape character is '^]'. helo dm220-mail70.yourcompany.com ESMTP 220 Welcome to yourcompany.com�s email system helo domain.com 250 mail70.yourcompany.com mail from: jane@domain.com 250 sender <jane@domain.com> ok rcpt to: tracey@yourcompany.com 250 recipient <tracey@yourcompany.com> ok data 354 go ahead

6. Email Structure

7. Why am I being Joe Job�ed The spammer is using your credentials to legitimise their marketing campaign Spam Phishing Discredit your company (Competitive sabotage) Random � In order to bypass reverse DNS lookup controls. Side-effect of a mass-mailing virus Blatant Denial of Service

8. Amplifier

9. How Amplification Works

10. Amplifier Implications Addition to a DNSBL (MAPS/Spamhaus/Spamcop) Denial of Service Leaking of sensitive information

11. Amplification of NDR�s Gateway accepts all email and relies on downstream servers to generate the NDR (non-delivery report) RFC-821 requires an NDR for unsuccessful deliveries to a final destination Notification to failed recipient + reason for the failure Above + original email (or part of it) Above + original email + all attachments

12. Further Amplification of NDR�s For NDR's of messages sent to multiple recipients, RFC-821 provides two options A single notification which lists all failed recipients of that failed message Separate notification for each failed recipient

13. NDR Payload Payload Viruses Spam Large files Zip-bombs "With 105 outbound emails (containing 1000 invalid recipients) totalling 3.60MB of traffic we caused the mail servers under study to generate more than 80,000 emails, totalling 1.15GB of traffic" http://www.techzoom.net/paper-mailbomb.asp?id=mailbomb

14. Lessening The Noise Hard bounce mail for invalid recipients at the gateway Limit the maximum number of recipients per message Generate minimalistic NDR�s Generate one NDR for all failed recipients Send bounces from a server you can afford to have blacklisted Disable NDR�s altogether� (maybe not)

15. Target

16. Target of a Phishing Attack

17. Example Phishing Message mail-from: admin@bank.com rcpt-to: <millions of users@domains> subject: Your Account Details

18. Implications for the Target Denial of Service User inbox restrictions Mail queues exploding / mail delays Unhappy users

19. Invalid Users Accept mail for valid users only local_recipient_maps (postfix) Recipient Access Table (Ironport RAT) LDAP integration Sometimes we don�t know who our valid users are or it can be too expensive to maintain. Backend user directory incompatible with front-end MTA This will stop backscatter for invalid addresses� What about valid ones?

20. Valid Users Message Headers Received-from Message-ID Looking for signs that it didn�t actually leave your network in the first place Resource Intensive

21. Bounce Verification Technologies We can�t rely on session verification techniques e.g. SPF Bounce Verification BATV (Bounce Address Tag Validation) Authbounce ABBS (Anti-Bogus-Bounce-Scheme) SES (Signed Envelope Sender)

22. BATV (Bounce Address Tag Validation)

23. BATV Specification The envelope sender address is signed. mail-from: mailbox@domain.com becomes mail-from: prvs=mailbox/tag-val@domain.com tag-val = K DDD SSSSSS K = key number DDD = low 3 digits of the number of days since 1970 when the address will expire SSSSSS = Hex of the first three bytes of the SHA-1 HMAC of <hash-source> and a key hash-source = K DDD <orig-mailfrom> orig-mailfrom = {original RFC2821.MailFrom address}

24. BATV cont� Supported on the following MTAs netqmail Ironport AsyncOS Exim Documentation available for other MTAs Pursuing IETF standardisation

25. How Authbounce works

26. Authbounce for Exim Addition of a signed X-Header: X-bounce-key:example.net-1;you@example.com;1077198109;fb7e6ffa; (1) (2) (3) (4) A key identifier, typically the ISP's domain plus a number. The E-mail address to which a bounce may be sent. The time when the message was sent out. Bounces older than a certain age are ignored. 32-bit cryptographic checksum, calculated as a hash over (1), (2), (3) and a secret value

27. ABBS (Anti-Bogus-Bounce-Scheme) Similar to BATV Signed envelope sender mail-from: user-b@b.example.com becomes mail-from: user-b=timestamp-hmac@b.example.com timestamp is time() hmac is HMAC-SHA1-nn Timeout defaults to 1296000 seconds (15 days) Supported in qmail safari

28. SES (Signed Envelope Sender) Challenge response system for SMTP UDP call back service Send hash value to the server that claims to have originated the message If the query is positive, mail is accepted as valid, if not, its rejected Website is dead, probably a good thing

29. Advantages / Disadvantages BATV ? Modifies the envelope sender address ? Stops invalid bounces after the rcpt-to header is received Authbounce ? Doesn�t modify the envelope sender address ? More overhead as X-headers need to be processed ABBS ? BATV for qmail with a few differences SES ? project is dead?

30. Security Considerations Cryptographic weaknesses Replay attacks

31. Issues to Consider Too many different standards, none settled on CPU overhead for large mail volumes All outbound messages tagged, all inbound checked Greylisting (451) Mail-listings validate on envelope sender Challenge-Response systems

32. More Issues� Legitimate DSNs are rejected unless the original mail has been sent via your server Roaming users�

33. Knowing Your Environment Which servers send out email?

34. Conclusion Know and control your environment Determine if you are an amplifier Decide which technology fits best Implement technology before you are targeted

35. Conclusion You don�t want to be hacking up custom Sendmail rules at 3:00am Kphishsrc regex -a@MATCH ^(customercare|customerssupport|customersupport|custservice|custsupport|infonum|online_support|onlinesupport|operate|operator|reference|support|supprefnum)(\-ref|\_ref|\-reference|\_reference|\-id|\_id)?(\-|\_)?[0-9]+$ SLocal_check_rcpt R$* $: $>Parse0 $>3 $1 R$+ < @ domain.co.nz. > $* $: $(phishsrc $1 $)

36. Questions?

37. References ABBS - http://msgs.securepoint.com/cgi-bin/get/qmail0403/161.html Anti-Phishing Workging Group: http://www.antiphishing.org/phishing_archive.html Anti-spam Email Research http://spamlinks.net/prevent-research.htm Authbounce - http://psg.com/%7Ebrian/software/authbounce/configure-authbounce.txt BATV - http://mipassoc.org/batv/ Mail Non-Delivery Notice Attacks: http://www.techzoom.net/paper-mailbomb.asp?id=mailbomb Postfix Backscatter Howto: http://www.postfix.org/BACKSCATTER_README.html Signed Envelope Sender: http://www.advogato.org/proj/Signed%20Envelope%20Sender/ Sender Policy Framework http://www.openspf.org/ Signed Return Address: http://www.tuffmail.com/backscatter.php Why are auto responders bad? : http://www.spamcop.net/fom-serve/cache/329.html