Email Server and SMTP Relay: How it Works and How to Prevent Spam

1. CAP6135: Malware and Software Vulnerability Analysis Spam and PhishingCliff ZouSpring 2015

2. Acknowledgement • This lecture uses some contents from the lecture notes from: • Dr. Dan Boneh (Stanford): CS155:Computer and Network Security • Jim Kurose, Keith Ross. Computer Networking: A Top Down Approach Featuring the Internet, 5th edition.

3. Three major components: user agents mail servers simple mail transfer protocol: SMTP User Agent a.k.a. “mail reader” composing, editing, reading mail messages e.g., Eudora, Outlook, elm, Netscape Messenger outgoing, incoming messages stored on server user agent user agent user agent user agent user agent user agent SMTP SMTP SMTP mail server mail server mail server outgoing message queue user mailbox Electronic Mail

4. How email works: SMTP(RFC 821, 1982) • Some SMTP Commands: MAIL FROM: <reverse-path> RCPT TO: <forward-path> RCPT TO: <forward-path> If unknown recipient: response “550 Failure reply” DATA email headers and contents • Use TCP port 25 for connections Repeated for each recipient .

5. Sample fake email sending S: 220 longwood.cs.ucf.edu C: HELO fake.domain S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <alice@crepes.fr> S: 250 alice@crepes.fr... Sender ok C: RCPT TO: <czou@cs.ucf.edu> S: 250 czou@cs.ucf.edu ... Recipient ok C: DATA S: 354 Enter mail, end with "." on a line by itself C: from: “fake man” <fake@fake.fake.fake> C: to: “dr. who” <who@who> C: subject: who am I? C: Do you like ketchup? C: How about pickles? C: . S: 250 Message accepted for delivery C: QUIT S: 221 longwood.cs.ucf.edu closing connection

6. Try SMTP interaction for yourself: • telnet servername 25 • see 220 reply from server • enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands • “mail from:” the domain may need to be existed • “rcpt to:” the user needs to be existed • A mail server may or may not support “relay” • CS email server supports relay from Eustis machine • “from:”“to:”“subject:” are what shown in normal email display

7. Using Telnet • On department eustis Linux machine: • telnet longwood.cs.ucf.edu 25 • In telnet interaction, “backspace” is not supported. You can type “ctrl+backspace” to erase previous two characters • On Windows 7 machine: • Telnet is not installed by default, check this tutorial for install: • http://technet.microsoft.com/en-us/library/cc771275%28v=ws.10%29.aspx

8. Outside campus network, department email server does not accept: • You need to first setup VPN to campus network, then use telnet • How to set up VPN: • https://publishing.ucf.edu/sites/itr/cst/Pages/NSvpn.aspx • Even inside campus network, directly telnet EECS email server will not work now because of the CS server’s new restriction • You can connect to Eustis machine, then run telnet command inside Eustis machine.

9. Email in the early 1980’s Network 1 Network 2 Mail relay Network 3 Mail relay sender • Mail Relay: forwards mail to next hop. • Sender path includes path through relays. recipient

10. Why Email Server Support Relay? • Wiki tutorial: • http://en.wikipedia.org/wiki/Open_mail_relay • Old days network constraint makes it necessary • Email agent uses SMTP to send email on behalf of a user • The user could choose which email address to use as the sender • Email server supports email group list: • The “sender” shown in email is the group list address, but the real sender is a different person • Closing Relay: • Messages from local IP addresses to local mailboxes • Messages from local IP addresses to non-local mailboxes • Messages from non-local IP addresses to local mailboxes • Messages from clients that are authenticated and authorized

11. Spoofed email • SMTP: designed for a trusting world … • Data in MAIL FROM totally under control of sender • … an old example of improper input validation • Recipient’s mail server: • Only sees IP address of direct peer • Recorded in the first Fromheader

12. From relays The received header • Sending spoofed mail to myself: From someone@somewhere.com (172.24.64.20) ... Received: from cs-smtp-1.stanford.edu Received: from smtp3.stanford.edu Received: from cipher.Stanford.EDU • Received header inserted by relays --- untrustworthy • From header inserted by recipient mail server

13. Spam Blacklists • RBL: Realtime Blackhole Lists • Includes servers or ISPs that generate lots of spam • spamhaus.org , spamcop.net • Effectiveness (stats from spamhaus.org): • RBL can stop about 15-25% of incoming spam at SMTP connection time, • Over 90% of spam with message body URI checks • Spammer goal: • Evade blacklists by hiding its source IP address.

14. Spamming techniques

15. Open relays • SMTP Relay forwards mail to destination • Bulk email tool connects via SMTP (port 25) • Sends list of recipients (via RCPT TO command) • Sends email body --- once for all recipients • Relay delivers message • Honest relay: • Adds Received header revealing source IP • Hacked relay does not

16. Example: bobax worm • Infects machines with high bandwidth • Exploits MS LSASS.exe buffer overflow vulnerability • Slow spreading: • Spreads on manual command from operator • Then randomly scans for vulnerable machines • On infected machine: (spam zombie) • Installs hacked open mail relay. Used for spam. • Once spam zombie added to RBL: • Worm spreads to other machines

17. ClientHello CONNECT xyz.com 443 ClientHello ServerHello ServerHello Open HTTP proxies • Web cache (HTTP/HTTPS proxy) -- e.g. squid • To spam: CONNECT SpamRecipient-IP 25 SMTP Commands Squid becomes a mail relay … xyz.com URL: HTTPS://xyz.com WebServer SquidWebCache

18. Finding proxies • Squid manual: (squid.conf) acl Safe_ports port 80 443 http_access deny !Safe_ports • URLs for other ports will be denied • Similar problem with SOCKS proxies • Some open proxy and open relay listing services: • http://www.multiproxy.org/ http://www.stayinvisible.com/ http://www.blackcode.com/proxy/ http://www.openproxies.com/ (20$/month)

19. Open Relays vs. Open Proxies • HTTP proxy design problem: • Port 25 should have been blocked by default • Otherwise, violates principal of least privilege • Relay vs. proxy: • Relay takes list of address and send msg to all • Proxy: spammer must send msg body to each recipient through proxy.  zombies typically provide hacked mail relays.

20. TCP handshake TCP Seq #s SMTP bulk mail (Source IP = LSZ) Thin pipe / Thick pipe method • Spam source has • High Speed Broadband connection (HSB) • Controls a Low Speed Zombie (LSZ) • Assumes no egress filtering at HSB’s ISP • Hides IP address of HSB. LSZ is blacklisted. LSZ TargetSMTPServer HSB

21. Bulk email tools (spamware) • Automate: • Message personalization • Also test against spam filters (e.g. spamassassin) • Mailing list and proxy list management

22. Send-Safe bulk emailer

23. Anti-spam methods

24. The law: CAN-SPAM act(Jan. 2004) • Bans false or misleading header information • To: and From: headers must be accurate • Prohibits deceptive subject lines • Requires an opt-out method • Requires that email be identified as advertisement • ... and include sender's physical postal address • Also prohibits various forms of email harvesting and the use of proxies

25. Effectiveness of CAN-SPAM • Enforced by the FTC: • FTC spam archive spam@uce.gov • Penalties: 11K per act • Dec ’05 FTC report on effectiveness of CAN-SPAM: • 50 cases in the US pursued by the FTC • No impact on spam originating outside the US • Open relays hosted on bot-nets make it difficult to collect evidence http://www.ftc.gov/spam/

26. MAIL FROM xyz@hotmail.com hotmail.com 64.4.33.7 64.4.33.8 Sender verification I: SPF (sender policy framework) • Goal: prevent spoof email claiming to be from HotMail • Why? Bounce messages flood HotMail system DNS hotmail.com:SPF record: 64.4.33.7 64.4.33.8 Sender Recipient Mail Server (MUA) Is SenderIP in list? More precisely: hotmail.com TXT v=spf1 a:mailers.hotmail.com -all

27. Sender verification II: DKIM • Domain Keys Identified Mail (DKIM) • Same goal as SPF. Harder to spoof. • Basic idea: • Sender’s MTA signs email • Including body and selected header fields • Receiver’s MUA checks signature • Rejects email if invalid • Sender’s public key managed by DNS • Subdomain: _domainkey.hotmail.com

28. Graylists • Recipient’s mail server records triples: • (sender email, recipient email, peer IP) • Mail server maintains DB of triples • First time: triple not in DB: • Mail server sends 421 reply: “I am busy” • Records triple in DB • Second time(after 5 minutes): allow email to pass • Triples kept for 3 days (configurable) • Easy to defeat but currently works well.

29. Puzzles and CAPTCHA • General DDoS defense techniques • Puzzles: slow down spam server • Every email contains solution to puzzle where challenge = (sender, recipient, time) • CAPTCHA: • Completely Automated Public Turing test to tell Computers and Humans Apart • Every email contains a token • Sender obtains tokens from a CAPTCHA server • Say: 100 tokens for solving a CAPTCHA • CAPTCHA server ensures tokens are not reused • Either method is difficult to deploy.

30. SpamAssasin • Wiki tutorial: • http://en.wikipedia.org/wiki/SpamAssassin • Mainly a rule-based spam filter • Many rules to give scores for all fields in an email • Email header, special keywords in email, URLs in email, images in email, ….. • Final decision is the combined score compared with a threshold • Has false positive (treat normal as spam), and false negative (treat spam as normal) • False positive is very damaging! • Nobody wants to lose an important email! • Also contains Bayesian filtering to match a user’s statistical profile • Need known “ham” and “spam” email samples for training

31. Part II:Phishing & Pharming

32. Oct. 2004 to July 2005 APWG

34. Note: no SSL. Typically: short lived sites.

35. Common Phishing Methods • Often phishing sites hosted on bot-net drones. • Move from bot to bot using dynamic DNS. • Use domain names such as: www.ebay.com.badguy.com • Use URLs with multiple redirections: http://www.chase.com/url.php?url=“http://www.phish.com” • Use randomized links: • http://www.some-poor-sap.com/823548jd/

36. Industry Response • Anti-phishing toolbars: Netcraft, EBay, Google, IE7 • IE7 phishing filter: • Whitelisted sites are not checked • Other sites: (stripped) URL sent to MS server • Server responds with “OK” or “phishing”

37. Check Browser for HTTP or HTTPS HTTP HTTPS The server’s digital Certificate has been verified

38. Pharming • Cause DNS to point to phishing site • Examples: • DNS cache poisoning • Write an entry into machine’s /etc/hosts file: “ Phisher-IP Victim-Name ” • URL of phishing site is identical to victim’s URL • … will bypass all URL checks

39. Response: High assurance certs • More careful validation of cert issuance • On browser (IE7) : … but most phishing sites do not use HTTPS

40. Other industry responses: BofA, PassMark ING bank login

41. Industry Response: Bank of Adelaide

42. ING PIN Guard

43. T.G.s: The next phishing wave • Transaction generation malware: • Wait for user to login to banking sites • Issue money transfer requests on behalf of user. • Reported malware in UK targeting all four major banks. • Note: These are social engineering attacks. Not just a windows problem.

44. Some ID Protection Tools • SpoofGuard: (NDSS ’04) • Alerts user when viewing a spoofed web page. • Uses variety of heuristics to identify spoof pages. • Some SpoofGuard heuristics used ineBay toolbarandEarthlink ScamBlocker. • PwdHash: (Usenix Sec ’05) • Browser extension for strengthening pwd web auth. • Being integrated with RSA SecurID.

45. pwdA = pwdB Password Hashing (pwdhash.com) • Generate a unique password per site • HMACfido:123(banka.com)  Q7a+0ekEXb • HMACfido:123(siteb.com)  OzX2+ICiqc • Hashed password is not usable at any other site hash(pwdA, BankA) Bank A hash(pwdB, SiteB) Site B

46. Problems of Password Hashing • Need to install a client program on user’s machine • It means the user cannot use other machines to log in to her accounts • Different websites have different requirements on password format • # of characters • Special characters, capital characters,…. • This means that the pwdHash client program must know the formats of all users’ accounts

47. Take home message • Deployed insecure services (proxies, relays) • Quickly exploited • Cause trouble for everyone • Current web user authentication is vulnerable to spoofing • Users are easily fooled into entering password in an insecure location