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Cédric Fournet Microsoft Research, Cambridge

Ecole JEUNES CHERCHEURS en PROGRAMMATION 6 au 17 juin 2005 Modèles formels de sécurité logicielle 2eme partie. Protocoles cryptographiques & pi calcul 14h Modélisation de protocoles en pi calcul 16h Application: Services web et sécurité.

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Cédric Fournet Microsoft Research, Cambridge

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  1. Ecole JEUNES CHERCHEURS en PROGRAMMATION 6 au 17 juin 2005 Modèles formels de sécurité logicielle2eme partie Protocoles cryptographiques & pi calcul 14h Modélisation de protocoles en pi calcul 16h Application: Services web et sécurité Cédric FournetMicrosoft Research, Cambridge

  2. Un Stage Chez Microsoft Research? “Summer Internships” a Cambridgeen interlude pendant la thèse. • Objectif: boucler un projet de rechercheen 12 semaines avec un chercheur du labo • Candidatures avant février pour l’été 2006 http://research.microsoft.com/aboutmsr/jobs/internships/cambridge.aspx

  3. Web Services Security& Protocol Verification Cédric Fournet Microsoft Research, Cambridge joint work with Karthik Bhargavan, Andy Gordon, Greg O’Shea, Riccardo Pucella, Ricardo Corin MSRC Samoa: Details, papers, tools, pointers at http://Securing.WS

  4. Web Services and Formal Security Two parallel trends: • Rapid invention and deployment of XML-basedcrypto protocols for securing web services • Flexible message formats for interop • Enables home-grown protocols • New crypto protocols are often wrong, XML or not • Sustained and successful effort to develop formalisms and tools to verify crypto protocols • (Dolev&Yao, BAN,) FDR, Athena, Isabelle, ProVerif, … • At MSRC: spi, sjoin, Cryptyc, applied pi calculus, … Timely opportunity to develop tools for validating standards-based XML crypto protocols

  5. XML Request XML Response What’s a Web Service? • “A web service is a web site intended for use by computer programs instead of human beings.” (Barclay et al) • So XML not HTML Client • Service messages in SOAP format: • Envelope/Header – addressing, security, and transactional headers • Envelope/Body – actual payload • Service metadata in WSDL format: • For each SOAP endpoint, list of operations • For each operation, request and response types Server

  6. (1) On the Public Internet • Aim: outsource user-interface to specialists • Specialist storefronts • Ex: Products sold through Amazon, but associate site gets commission • From Jul’02; 25k developers one year later • Specialist visualizations • Ex: TouchGraph re Google

  7. (2) Within a Private Intranet • Aim: XML as common language to integrate systems obtained from different vendors • Ex: single service to calculate quotes uniformly no matter how customer contacts an insurance company • call centre • sales force • website • Today, biggest growth area for web services, in part because there are few new security concerns

  8. (3) Between Private Intranets • Aim: support inter-institution workflows • E-business transactions • E-science grid • This is the hardest of the three cases to secure • Needs being addressed by crypto-based security specs • Single sign-on: scientist may access 100 sites • Federation: eg can company A permit just certain company B employees to access shared service? • Delegation: eg can I permit online travel agent to consult my calendar service, make bookings?

  9. Standards & Specifications • “XML Web Services” refers to SOAP stack of specifications: • SOAP – message format • Syntax of request, response, fault messages • WSDL – service description • Interface: function name, parameter and return types • UDDI – service discovery • Search for service by attributes (like Yellow Pages) • Not yet widely used in practice • BPEL4WS – service composition • Programming language for automating business processes, such as B2B order processing • Some sort of merger of IBM WSFL, Microsoft XLANG, and Sun WSCI … so quite complex • …

  10. A Sample SOAP Request <soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <soap:Body> <GetOrder xmlns="http://microsoft.com/petshop"> <OrderId>20</orderId> </GetOrder> </soap:Body> </soap:Envelope> • Says: “get me status of order 20”

  11. A Sample SOAP Request <Envelope> <Body> <GetOrder> <OrderId>20</> • Says: “get me status of order 20” • XML not meant to be read by humans, so we’ll omit namespace info, and trailing brackets…

  12. Another SOAP Request <Envelope> <Header> <Action>http://bookshop/ws/OrderStatus</> <To>http://www.bookshop.com/ws/orderstatus.asmx</> <MessageId>uuid:5ba86b04-3c0f-428b-8dd6-17804286fe40</> <Body> <GetOrder> <OrderId>20</> • Optional headers tell SOAP processorswhat to do with the envelope (automatically)

  13. Web Services: What’s New? • Though their core is roughly XML-encoded RPC – rather old! – what’s new about SOAP web services is the combination of: • Standards, Internet-scale, high-level tools • Signs of fervour, • Wide support, easy to deploy • OASIS and W3C • yet reasons for caution, • Cost of SOAP encoding? • Lack of SOAP security?

  14. Web Services Security

  15. The 2002 Security Story • The 2002 best practice was to build secure web services using an SSL transport • SSL gives transport- not application-level security • Messages secured point-to-point not end-to-end • Messages cannot securely be filtered or routed • Messages not encrypted in files or databases • Party line (Web Services Security Roadmap) security within SOAP envelopes is better

  16. Web Services Security • SOAP level security aims to provide end-to-end, compositional application-level security, independently of transport protocol • Fresh standards: • Security Roadmap • WS-Security, May 2004 (Draft: Apr 2002) • WS-Trust, WS-SecureConversation, WS-SecurityPolicy,… • A grammar for SOAP-based security protocols • Automated processing of security headers • Informal semantics except for XML syntax • Security tokens = wire format for claims and evidence • Keys, certificates, x509 signatures, Kerberos tickets,…

  17. Securing SOAP Messages UsernameToken assumes both parties know Alice’s secret password p <Envelope> <Header> <Security> <UsernameToken Id=1> <Username>“Alice" <Nonce>"mTbzQM84RkFqza+lIes/xw==" <Created>"2004-09-01T13:31:50Z" <Signature> <SignedInfo> <SignatureMethod Algorithm=hmac-sha1> <Reference URI=#2> <DigestValue>"U9sBHidIkVvKA4vZo0gGKxMhA1g=“ <SignatureValue>"8/ohMBZ5JwzYyu+POU/v879R01s=" <KeyInfo> <SecurityTokenReference> <Reference URI=#1 ValueType=UsernameToken> <Body Id=2> <StockQuoteRequest> <symbols> <Symbol>"FABRIKAM" <Symbol>"CONTOSO" <Security> header defined by OASIS WS-Security 2004 includes identity tokens, signatures, encrypted message parts Each DigestValue is a cryptographic hash of the URI target Dozens of implementations, including Microsoft Web Services Enhancements (WSE) hmacsha1(key, SignedInfo) where keypsha1(p+nonce+created)

  18. Attacks on SOAP security • Web services vulnerable to same sorts of attacks as conventional websites • Buffer overruns, denial of service, SQL injection, etc • New concerns: flexible, XML-based protocols • Web services developers can design and deploytheir own application-specific security protocols • XML message format open to rewriting attacks • Much like classic active attackers (Needham-Schroeder ’78) • Opponent can redirect, replay, modify, impersonate • New: message processing is driven by a flexible,semi-structured message format • Flexibility is usually bad for security • We have found a range of problems in sample code,thus motivating our research on theory and tools

  19. From: AliceTo: BookshopAction: “Buy Charlie’s book” (signed by Alice) Sent: Monday From: AliceTo: BankAction: “Pay Charlie $20” (signed by Alice) Sent: Tuesday From: AliceTo: BankAction: “Buy Charlie’s book” (signed by Alice) Sent: Wednesday From: AliceTo: BookshopAction: “Buy Charlie’s book” (signed by Alice) An XML Rewriting Attack Alter and replay envelopesto confuse participants Alice’s laptop Alice’s bookshop(Web Service) Someoneon the net(Charlie?)

  20. From: AliceTo: Bookshop“Publish this paper” (encrypted for bookshop) (signed by Alice) From: CharlieTo: Bookshop“Publish this paper” (encrypted for bookshop) (signed by Charlie) Another XML Rewriting Attack Take credit for someone else’s data Alice’s bookshop(Web Service) Alice’s laptop Someoneon the net(Charlie?)

  21. A Signed SOAP Message Before... Message to bank’s web service says: “Transfer $1000 to Bob, signed Alice” <Envelope> <Header> <Security> <UsernameToken Id=2> <Username>Alice</> <Nonce>cGxr8w2AnBUzuhLzDYDoVw==</> <Created>2003-02-04T16:49:45Z</> <Signature> <SignedInfo> <Reference URI= #1><DigestValue>Ego0...</> <SignatureValue>vSB9JU/Wr8ykpAlaxCx2KdvjZcc=</> <KeyInfo> <SecurityTokenReference><Reference URI=#2/> <Body Id=1> <TransferFunds> <beneficiary>Bob</> <amount>1000</> Bank can verify the signature has been computed using key derived from Alice’s secret password

  22. and After an XML Rewriting Attack Charlie has intercepted and rewritten this message <Envelope> <Header> <Security> <UsernameToken Id=2> <Username>Alice</> <Nonce>cGxr8w2AnBUzuhLzDYDoVw==</> <Created>2003-02-04T16:49:45Z</> <Signature> <SignedInfo> <Reference URI= #1><DigestValue>Ego0...</> <SignatureValue>vSB9JU/Wr8ykpAlaxCx2KdvjZcc=</> <KeyInfo> <SecurityTokenReference><Reference URI=#2/> <BogusHeader> <Body Id=1> <TransferFunds> <beneficiary>Bob</> <amount>1000</> <Body> <TransferFunds> <beneficiary>Charlie</> <amount>5000</> The indirect signature of the body, now hidden in BogusHeader, may still appear valid Although Alice’s password has not been broken, the message now reads “Transfer $5000 to Charlie, signed Alice”

  23. A Long History of Attacks We assume that an intruder can interpose a computer on all communication paths, and thus can alter or copy parts of messages, replay messages, or emit false material.While this may seem an extreme view, it is the only safe one when designing authentication protocols. Needham and Schroeder CACM (1978) A B C 1978: N&S propose authentication protocols for “large networks of computers” 1981: Denning and Sacco find attack found on N&S symmetric key protocol 1983: Dolev and Yao first formalize secrecy properties wrt N&S threat model, using formal algebra 1987: Burrows, Abadi, Needham invent authentication logic; neither sound nor complete, but useful 1994: Hickman (Netscape) invents SSL; holes in v2, but v3 fixes these, very widely deployed 1994: Ylonen invents SSH; holes in first versions, now very widely deployed 1995: Abadi, Anderson, Needham, et al propose various informal “robustness principles” 1995: Lowe finds insider attack on N&S asymmetric protocol; rejuvenates interest in FMs circa 1999: Several FMs for “D&Y problem”: tradeoff between accuracy and approximation circa 2004: Many FMs now developed; several deliver both accuracy and automation

  24. The Samoa Project: Tools • If misconfigured or mis-implemented, WS-Securityprotocols vulnerable to XML rewriting attacks • TulaFale — shows the absence of such attacksgiven a description of the protocol • An analysis tool for XML-based crypto protocols • Automatic analysis of hand-written models viaapplied pi calculus and Bruno Blanchet’s ProVerif tool • Policy generator/analyzer — produces TulaFalefrom declarative XML policy files that drive WSE 2.0 • Hence, can directly analyze WSE 2.0 configurations • Source-based formal verification of deployedimplementations of crypto protocols • Policy advisor — runs 35+ queries for securityerrors found in reviews of sample policies

  25. TulaFale

  26. TulaFale: a language for WS-Sec TulaFale = pi + XML + predicates + assertions We designed TulaFale, a programming language to model WSE protocols and hand-wrote models for a series of WSE protocols(POPL’04, FMCO’03) What TulaFale does TulaFale script predicatelibrary WSE 1.0out of the box TulaFale C# code intermediate pi-calculus WSE 1.0 CLR (IL) ProVerif Analyzer[B. Blanchet] OK, orNo because… SOAP processing

  27. Example: A Secure RPC • A typical system model: • A single certification authority (CA) issuing X.509 public-key certificates for services, signed with the CA's private key. • Two servers, each equipped with a public key certified by the CA and exporting an arbitrary number of web services • Multiple clients, acting on behalf of human users • Threat model: an active attacker, in control of network, but knowing none of: • The private key of the CA • The private key of any public key certified by the CA • The password of any user in the database • Security goals: authentication of each message;and correlation of request and response; but not confidentiality

  28. An intended run of the protocol Server(sx,cert,S) Client(kr,U) begin C1 (U,S,id1,t1,b1) isMsg1(-,U,S,id1,t1,b1) end C1 (U,S,id1,t1,b1) begin C2 (U,S,id1,t1,b1,id2,t2,b2) isMsg2(-,S,id1,id2,t2,b2) end C2 (U,S,id1,t1,b1,id2,t2,b2) Msg 1 includes signature of S,id1,t1,b1 under key derived from username token for U Msg 2 includes signature of id1,id2,t2,b2 under public key of S

  29. pi+XML+predicates+assertions TulaFale predicates defined by Horn clauses with message patterns For example, this predicate is usedin two ways, to construct and parse Message 1 TulaFale messages are terms in a many-sorted algebra with sorts:

  30. pi+XML+predicates+assertions TulaFale library includes predefined predicates for XML signatures and encryption For example, this predicate uses these predicates to check structure of Message 1

  31. pi+XML+predicates+assertions By sending a message on init, the attacker can pick any payload and destination Each begin-event marksthe intent to send a message Messages are exchanged on a public SOAP channel Each end-event marksthe intent to accept a message as valid

  32. pi+XML+predicates+assertions The implicit attacker, running in parallel, can: • Send and receive on the soap channel • Generate arbitrarily many users and services • Initiate arbitrarily many sessions

  33. Some Tulafale queries We also run basic reachability queries (sanity checks) We verify two correspondence properties from end-events to begin-event with matching contents (including both messages for C2)

  34. TulaFale Demo Automatic verification of following reachability and safety properties via TulaFale/ProVerif

  35. Suppose a client does not sign the message identifier id1... Opponent Server(sx,cert,S) Client(kr,U) begin C1 (U,S,id1,t1,b1) isMsg1(-,U,S, id1,t1,b1) Copy end C1 (U,S,id1,t1,b1) id1:=id2, Replay isMsg1(-,U,S, id2,t1,b1) end C1 (U,S,id2,t1,b1) Pair (id1,t1) uniquely identifies the message only if id1 and t1 are signed We found and fixed faults like this in preliminary WSE samples

  36. What else might go wrong? Opponent Server(sx,cert,S) Client(kr,U) isMsg1(-,U,S, id1,t1,b1) begin C2 (U,S,id1,t1,b1,id2,t2,b2) Call 1 isMsg2(-,S,id1, id2,t2,b2) SOAP Fault isMsg1(-,U,S, id1,t1’,b1’) Call 2, re-using id1 isMsg2(-,S,id1, id2,t2,b2) end C2 (U,S,id1,t1’,b1’,id2,t2,b2) If the client doesn’t generate fresh id1’s, then message correlation (C2) fails; the tool easily finds this bug

  37. What about insider attacks? Opponent aka O Server(sx,cert,S) Client(kr,u) isMsg1(-,U,S, id1,t1,b1) isMsg1(-,O,S, id1,t1,b1’) begin C2 (O,S,id1,t1,b1’,id2,t2,b2) isMsg2(-,S,id1, id2,t2,b2) isMsg2(-,S,id1, id2,t2,b2) end C2 (U,S,id1,t1,b1,id2,t2,b2) If one or more passwords are compromised, there is an insider attack on message correlation; more extensive changes to the script are needed to model this

  38. Secure Conversations

  39. A TulaFale case study • WS-Security provides basic mechanisms to secure SOAP traffic, one message at a time • Signing and encryption keys derived from long-lived secrets like passwords or private keys • If a SOAP interaction consists of multiple, related messages, WS-Security alone may be inefficient, and does not secure session integrity • Standard idea: establish short-lived session key • Recent specs describe this idea at the SOAP-level • WS-SecureConversation defines security contexts, used to secure sessions between two parties • WS-Trust defines how security contexts are issued and obtained

  40. A Typical System STS 1. RST Trust 2. RSTR Client SCs SCT SC Secure Conv 3. “Session Exchanges” Service … STS = Security Token Server RST = Request Security Token RSTR = RST Response SC = Security Context SCT = SC Token

  41. We prove authentication forwhole sessions We rely on some combination of manual and automated proofs Open-Ended Conversations Client Service get SC get SC begin Cn end Cn begin C’n end C’n for n¸ 0

  42. Discussion • A formal analysis of WS-Trust andWS-SecureConversation • XML syntax and automation very effective,against a demanding, realistic attacker model • Approx 1000 lines of script – too large for manual proofs • As is common, these specs: • focus on message formats for interoperability • are non-committal regarding security,for example, no clear spec of contents of SCs • By making modes, data, and goals explicit, we found design and implementation bugs

  43. Policy-Based Security

  44. Security Policies • Clients, services use XML files to pick security mechanisms • Located in same IIS virtual directory • Describe protocols to use for different services • Simple declarative description of deployed protocols • No need to look at messy C# code • We analyze policy files collected from client and servers • Easy to get them wrong • Many policies are insecure • Combination of policies may have unexpected effects <Policy Id=“Msg1"> <All> <Confidentiality> <TokenInfo> <SecurityToken> <TokenType>X509v3</> <Claims><SubjectName>S</></> <MessageParts>Body()</> <Integrity> <TokenInfo> <SecurityToken> <TokenType>UsernameToken</> <Claims><SubjectName>U</></> <MessageParts>Body() Header("To") Header("MessageId”)</>

  45. Analyzing Policy Configurations Web Server User U Premium Web Service S Client App U’s pwd V’s pwd’ cert key U’s pwd cert Security Layer Security Layer Policy2.xml Policy1.xml User V Regular Web Service T Client App V’s pwd cert Security Layer Security Layer Policy3.xml Policy4.xml Automated tools for collecting, parsing policies from IIS Servers, Clients Config = [Policy1, Policy2, Policy3, Policy4]

  46. Modelling Security Policies What our tools do spec L of a secure link In WSE 2.0, WS-SecurityPolicy files drive security; hence, we can generate TulaFale directly from implementation files(CCS’04) Generator C(-) Analyzer S(-,-) WSE 2.0out of the box Static warnings C# code policy config C(L) predicatelibrary TulaFale script S(C(L),L) WSE 2.0 TulaFale CLR (IL) ProVerif (pi calculus) SOAP processing OK, orNo because…

  47. Translating Policies to Predicates <Policy Id="Msg1"> <All> <Confidentiality> <TokenInfo> <SecurityToken> <TokenType>X509v3</> <Claims><SubjectName>S</></> <MessageParts>Body()</> <Integrity> <TokenInfo> <SecurityToken> <TokenType>UsernameToken</> <Claims><SubjectName>U</></> <MessageParts>Body() Header("To") Header("MessageId")</> Conjunction Encryption Requirement Signature Requirement predicate hasMsg1Policy(msg1:item,U:item,pwd:string, S:item,skS:bytes,id1:string,req:item) :- msg1 = <Envelope> <Header> <To>S</> <MessageId>id1</> <Security> utok sig1</></> <Body>b1</></>, isEncryptedData(b1,req,skS), isUserTokenKey(utok,U,pwd,skU), isSignature(sig1,"hmacsha1",skU, [<Body>b1</> <To>S</> MessageId>id1</>]).

  48. Security for All Generated Configs? • Theorem: All policy configurations generated from link specs enforce their security goals • For all link specifications L, S(C(L),L) provides request & response authentication, correlation, and secrecy. • Hence, at least generated configs can be safely deployed • Proof: • Non-obvious: there are infinitely-many link specs • Use combination of automated proofs and manual reasoning • Hint 1: Reduce to 4 base kinds of link-generated policies • Hint 2: Prove that configs with all links enabled is secure(programmed unfoldings in the pi calculus)

  49. Security for Any Client Policy? • Theorem: If a service uses a link-generated policy, then irrespective of the client policies, the resulting configuration preserves request authentication and response secrecy • Hence, naïve clients cannot break service authentication • Proof: • Combination of automated proofs and manual reasoning • Hint: Even the weakest send policy preservessecrecy of passwords and signing keys

  50. Security in the field:a Policy Advisor for WSE • Advisor guesses intended goals and runs queries that check for: • likely errors in configuration file settings • conformance to conservative policy schema • likely errors in (request,response,fault) mappings • likely errors in particular policies WSE 2.0SP2 out of the box Our plugin WSE Policy Advisor codeC#/VB policy config static queries WSE 2.0 security report CLR (IL) SOAP processing

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