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Cassis, March 8 th 2005 Formal Tools for Web Services Security

Cassis, March 8 th 2005 Formal Tools for Web Services Security. 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.

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Cassis, March 8 th 2005 Formal Tools for Web Services Security

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  1. Cassis, March 8th 2005Formal Tools for Web Services Security 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

  2. Our starting point (2003) Two parallel trends over past five years: • 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

  3. 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

  4. 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”

  5. 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…

  6. 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)

  7. 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,…

  8. 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)

  9. 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

  10. 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?)

  11. 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

  12. 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”

  13. 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

  14. Pi Calculus & Cryptography • Milner, Parrow, Walker (1989) • Computation is name-passingbetween parallel processes onnamed channels. Each namehas a mobile scope. • Spi calculus: Pi + cryptographicoperations (Abadi Gordon 1999) • Mobile scopes can representlocal keys and fresh nonces • Processes represent protocol configurations • Contexts represent active attackers • Applied Pi: Pi + equational theory (Abadi Fournet 2001) • There is a generally-useful theory (equivalences, proofs) • Using tools such as ProVerif (Blanchet 2001—), we can mix manual and automated proofs for various security properties

  15. The Samoa Project, Briefly • If misconfigured or mis-implemented, WS-Securityprotocols vulnerable to XML rewriting attacks • TulaFale — shows the absence of such attacksgiven a description of the protocol • First 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 • First source-based formal verification of interoperable implementations of crypto protocols • Policy advisor — runs 35+ queries for securityerrors found in reviews of sample policies

  16. 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

  17. 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

  18. 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

  19. 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:

  20. 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

  21. 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

  22. 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

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

  24. 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

  25. 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

  26. A TulaFale Summer 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

  27. 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

  28. 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

  29. Discussion • A first 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

  30. Policy-Based Security

  31. 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”)</>

  32. 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]

  33. 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…

  34. 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)

  35. 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

  36. Policy Advisor Demo

  37. Summary • Web services security specs encourage extreme flexibility • Message formats, composable protocols, configurations • Specs and implementations are only just emerging • Attacks and proofs are subtle: tool support needed • We bridge the gap between theoretical pi threat modeland XML as used in WS security protocols • Put effort into real samples & implementations, found bugs • Obtained theorems about wire-level protocols • Exploited automation for authentication & secrecy properties • We develop tools for the automated analysis ofsecurity for deployed systems based on crypto protocols • Proving protocols secure in isolation is not enough • Our tools find attacks, verify configs, generate safe configs • Good place to develop formal tools, get positive results • Standard message formats, composition, wide applicability Details, papers, tools, pointers at http://Securing.WS

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