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Securing 7 Layers of Insecurity

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Securing 7 Layers of Insecurity Chapter 62 Simple Mail Transfer Protocol You've got Mail! D 7 eep L “President Bush said for ay security reasons, he's sworn off Se er all e-mail communication. He will s c not be using email at the White o V f I House at all. Is that a good idea? i e n I mean, it's not like that speaking nn thing was working out so good.” sec a 2007 urity -- Jay Leno Copyright Information Some rights reserved / Einige Rechte vorbehalten Michael Kafka, René Pfeiffer, Sebastian Mayer C.a.T. Consulting and Trainings, Vienna, Austria You may freely use, distribute and modify this work under following D agreement: 7 eep Diese Arbeit darf frei genutzt, verbreitet und bearbeitet werden unter L folgenden Bedingungen: ay Se Authors must be referenced (also for modification) er s Autoren müssen genannt werden (auch bei Bearbeitung) c o V Only for non commercial use f I i Nur für nichtkommerzielle Nutzung e n nn Derivative work under same licence sec Derivative Arbeit unter selber Lizenz a 2007 urity http://www.creativecommons.com © November 2007 62 - SMTP Chapter 62 Simple Mail Transfer Protocol Agenda SMTP Basics SMTP Security Risks D 7 eep Anti-Spam Frameworks L ay Se E-Mail Content er s c o V f I i e n nn sec a 2007 urity © November 2007 62 - SMTP SMTP Basics The Foundation of Mail Delivery. D 7 eep L ay Se er s c o V f I i e n nn sec a 2007 urity © November 2007 62 - SMTP SMTP Properties Text based protocol Commands use 7-bit ASCII Data may use 8-bit encodings D 7 eep Message transmission as header+body L ay Se Uses different TCP ports er s c 25/TCP for server-server communication o V f I i 465/TCP for SMTP+SSL e n nn 587/TCP for message submission sec a 2007 urity © November 2007 62 - SMTP Sample SMTP Session 220 agamemnon.example.net ESMTP Postfix (Debian/GNU) EHLO agamemnon.example.net 250-agamemnon.example.net 250-PIPELINING 250-SIZE 52428800 250-VRFY D 250-ETRN 7 250-STARTTLS eep 250-ENHANCEDSTATUSCODES L 250-8BITMIME ay 250 DSN Se MAIL From: <[email protected]> er 250 2.1.0 Ok s c RCPT To: <[email protected]> o 250 2.1.5 Ok V f I DATA i e 354 End data with <CR><LF>.<CR><LF> n nn Hi! sec Bye, a 2007 Lynx. urity . 250 2.0.0 Ok: queued as 2CB7C24008A9 QUIT 221 2.0.0 Bye © November 2007 62 - SMTP Extended SMTP (ESMTP) Adds commands and functionalities Command pipelining SSL/TLS support D 7 eep 8-bit support for data L ay Se On-demand relay for mobile users er s c Many MTAs support ESMTP o V f I i e n nn sec a 2007 urity © November 2007 62 - SMTP SMTP Security Risks Let's Shoot the Messenger. D 7 eep L ay Se er s c o V f I i e n nn sec a 2007 urity © November 2007 62 - SMTP Open / Unauthorised Relays UBE/UCE still a big problem Malware packed into email Spam D 7 eep MTA needs to verify source of email L ay Se Any open relay will be misused er s c Disrupt email delivery o V f I i Put IP range(s) on RBLs e n nn Risk: medium sec a 2007 Impact: high urity © November 2007 62 - SMTP SMTP EHLO/HELO Pipe Bombs Imagine you inspect HELO strings Your logs look like this someday 2007-…rblsmtp: 43 EHLO “|http://a.b.cc/cgi/put.cgi” D 7 eep 2007-…rblsmtp: 43 HELO “|http://a.b.cc/cgi/put.cgi” L ay Scripting security also applies to MTAs! Se er s Risk: medium c o V f I Impact: high i e n nn sec a 2007 urity © November 2007 62 - SMTP SMTP VRFY SMTP VRFY tests email addresses Existing accounts will be confirmed (Ab)use for reconnaissance D 7 eep Some clients require VRFY L ay Se MTAs may allow VRFY without announcement er s c Risk: medium o V f I i Impact: medium e n nn sec a 2007 urity © November 2007 62 - SMTP SMTP Authentication Protocol offers no authentication Sender/recipient is arbitrary Mail relaying must be tightly controlled D 7 eep MITM attacks can be easily done L ay Se Spoofing of messages very easy er s c o V f I i e n nn sec a 2007 urity © November 2007 62 - SMTP SMTP AUTH ESMTP offers authentication Used for MUA→MTA / MTA→MTA Email addresses stay unauthenticated D 7 eep Reduces open relay problem L ay Se Lost/brute-forced password critical er s c Useful for mobile users o V f I i Most MTA support SASL mechanisms e n nn sec a 2007 urity © November 2007 62 - SMTP SMTP + SSL/TLS Works with keys & certificates Provides everything that SSL/TLS promises MTAs usually have no trust relationship D 7 eep Self-signed certificates L ay Se Certificates are next to never verified er s c MITM still possible o V f I i Risk: medium e n nn Impact: medium sec a 2007 urity © November 2007 62 - SMTP Anti-Spam Frameworks Enhanced Complexity as Security Measure. D 7 eep L ay Se er s c o V f I i e n nn sec a 2007 urity © November 2007 62 - SMTP Domain Keys Adds header with signature Signed content of message Signature linked to domain name D 7 eep Cryptographic checksums per message L ay Se In-transit modification breaks checks er s c Mailing list managers o V f I i Content filter e n nn Envelope not part of signature sec a 2007 Replay injection possible urity © November 2007 62 - SMTP Domain Keys & DNS DomainKey-Signature header has fields: …d=example.net; s=dkim.key;… PK lookup: dkim.key._domainkey.example.net D 7 eep DNS security determines DKIM security L ay DNS attacks apply as well Se er s Attacker may try to offer own key c o V f I i e n nn sec a 2007 urity © November 2007 62 - SMTP Sender Policy Framework (SPF) Based on published policies in DNS Domains name valid email origins Domains name default trust level D 7 eep SPF works in SMTP dialogue L ay Se Early blocking possible er s c SPF completely breaks email forwarding o V f I i Message must be rewritten e n nn Sender Rewriting Scheme (SRS) sec a 2007 urity © November 2007 62 - SMTP Real Time Blacklisting (RBL) RBL are simply based on lists Distribution by DNS is common Listing may have arbitrary parameters D 7 eep Most RBLs are based on reputation L ay Se Metrics help measurement er s c Scores can change dynamically o V f I i Using any RBL outsources access lists e n nn sec a 2007 urity © November 2007 62 - SMTP Chapter 62 Simple Mail Transfer Protocol Summary . SMTP needs protection. SMTP relies on DNS infrastructure. D 7 eep . SMTP + SSL/TLS isn't necessarily safe. L ay . Se SMTP servers on RBL are useless. er s . c Pick RBLs carefully. o V f I i e n nn sec a 2007 urity © November 2007 62 - SMTP Thank You Questions? D 7 eep L ay Se er s c o V f I i e n nn sec a 2007 urity © November 2007 62 - SMTP DeepSec Conference © November 2007 Chapter 62 Simple Mail Transfer Protocol You've got Mail! D 7 eep L “President Bush said for ayers o security reasons, he's sworn off Sec all e-mail communication. He will not be using email at the White Vien House at all. Is that a good idea? f I I mean, it's not like that speaking n secu thing was working out so good.” n a 2007 rity -- Jay Leno © November 2007 62 - SMTP 1 99 - Things in Life 1 DeepSec Conference © November 2007 Copyright Information Some rights reserved / Einige Rechte vorbehalten Michael Kafka, René Pfeiffer, Sebastian Mayer C.a.T. Consulting and Trainings, Vienna, Austria You may freely use, distribute and modify this work under following D agreement: 7 eep Diese Arbeit darf frei genutzt, verbreitet und bearbeitet werden unter L folgenden Bedingungen: ayers o Sec Authors must be referenced (also for modification) Autoren müssen genannt werden (auch bei Bearbeitung) Vien Only for non commercial use f I Nur für nichtkommerzielle Nutzung n secu Derivative work under same licence n Derivative Arbeit unter selber Lizenz a 2007 http://www.creativecommons.com rity © November 2007 62 - SMTP 2 This presentation is published under the CreativeCommons License which can be viewed in detail on their hompage: http://creativecommons.org/licenses/by-nc-sa/2.0/at/ Read more on http://www.creativecommons.com You are free: to Share — to copy, distribute and transmit the work to Remix — to adapt the work Under the following conditions: Attribution. 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