Internet DRAFT - draft-wrapping-attack-mitigation
draft-wrapping-attack-mitigation
Network Working Group Jitendra Kumar
Internet-Draft Balaji Rajendran
Intended status: Standards Track Bindhumadhava BS
Expires: September 23, 2018 C-DAC Bangalore
March 22, 2018
Enhanced XML Digital Signature Algorithm to Mitigate Wrapping Attacks
draft-wrapping-attack-mitigation-03
Abstract
XML signature standard as described in [RFC3275] and defined by IETF/
W3C references or identifies signed elements by their unique
identities in the given XML document. Hence, signed XML elements can
be shifted from one location to another location in a XML
document,and still, it does not have any effect on its ability to
verify its signature. This flexibility paves the way for an attacker
to tweak original XML message without getting noticed by the
receiver. This document proposes to use absolute XPath as an
"Positional Token" and modifies existing XML Digital Signature
algorithm to overcome the XML Signature wrapping/rewriting attacks on
XML ignatures.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on September 23, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. XML Digital Signature structure . . . . . . . . . . . . . . . 3
3. Suggested Modified Algorithm . . . . . . . . . . . . . . . . 3
3.1. Algorithm for signing SOAP Request . . . . . . . . . . . 4
3.2. Algorithm for verification of Signature . . . . . . . . . 4
3.2.1. Verifying SignedInfo Element Digest with Decrypted
Digest from SignatureValue element . . . . . . . . . 5
4. Simple Example . . . . . . . . . . . . . . . . . . . . . . . 5
5. Algorithm Validation . . . . . . . . . . . . . . . . . . . . 9
5.1. Mitigation of XML Signature wrapping attacks . . . . . . 9
5.2. Mitigation of XML elements jumbling type of wrapping
attacks . . . . . . . . . . . . . . . . . . . . . . . . . 9
6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 9
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
9.1. Normative References . . . . . . . . . . . . . . . . . . 10
9.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
Many researchers have shown that even a signed SOAP messages are
vulnerable to interception and further manipulation of its content.
McIntosh and Austel (described in wrapping_attack [wrapping_attack])
have illustrated that a SOAP message content, protected by an XML
Digital Signature, as specified in WS-Security(refer, WS-Security
[WS-Security]) can be forged without invalidating the signature.
These attacks are termed as XML Signature wrapping attacks or XML
rewriting attacks.These types of attacks are possible because the XML
Digital Signature refers to a signed element in XML document in a way
that does not take care of its location inside the XML document into
consideration.Attackers inject additional nodes replacing signed
nodes while still preserving the signed nodes inside the document but
at different level in the hierarchy of the XML tree such that it
results in successful signature verification thereby resulting in XML
Re-Writing/Wrapping attack.
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1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. XML Digital Signature structure
XML Signatures (described in RFC3275 [RFC3275]) are applied to
arbitrary digital content (data objects).Data objects are digested,
the resulting value is placed in an element (with other information)
and that element is then digested and cryptographically signed.XML
digital signatures are represented by the Signature element which has
the following structure (where "?" denotes zero or one occurrence;
"+" denotes one or more occurrences; and "*" denotes zero or more
occurrences):
<Signature ID?>
<SignedInfo>
<CanonicalizationMethod/>
<SignatureMethod/>
(<Reference URI? >
(<Transforms>)?
<DigestMethod>
<DigestValue>
</Reference>)+
</SignedInfo>
<SignatureValue>
(<KeyInfo>)?
(<Object ID?>)*
</Signature>
Signatures are related to data objects via URIs [URI].Within an XML
document, signatures are related to local data objects via fragment
identifiers.
3. Suggested Modified Algorithm
As, SOAP requests are prone to XML wrapping attacks and this
vulnerabilities stems mostly because of usage of ID (Identity) to
identify the signed XML sub tree. There are many solutions proposed
to mitigate such attacks but still such attacks can't be fully
eliminated because of inherent limitation present in XML Digital
Signature standard.In this document, we have proposed an addition of
"Positional Token" as a doping to the XML element getting signed to
mitigate XML Signature wrapping attacks. We are also proposing a
little modification of existing XML Signature standard as to use of
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"Absolute XPath" instead of ID in <Reference> node's "URI" attribute
to refer the signed element. Use this absolute XPath as a
"Positional Token", as this token exactly points to the position of
element getting signed. Also, during signing process, add this
"Positional Token" as an attribute (e.g. PosToken= "Absolute XPath")
to the element subjected to be signed. This absolute XPath as a
"Positional Token" would identify the signed element in XML Signature
and addition of this "Positional Token" as an attribute to the
element getting signed eliminate the chances of XML Wrapping attacks
as in the case of forged SOAP requests, calculated digest of signed
element will not match with the respective digest value in
<DigestValue> node during signature validation process.We propose a
modified XML signature algorithm which suggests usage of absolute
XPath as a "Positional Token" and it will be used during signing as
well as during signature validation process. The algorithms are as
follows.:
3.1. Algorithm for signing SOAP Request
1. KS=Load(Keystore.JKS) //Load certificates and keys
2. For each element subjected to be signed(represented
by its "id" attribute value) {
3. ABSXpath= "Absolute XPath" of element to be signed
as identified with its "Id" attribute value
4. ProtectTree=Node as identified by ABSXpath
5. MixedElement=AppendSyntacticToken(ProtectTree, ABSXpath)
/*Append a Positional Token as an attribute,
"PosToken= ABSXpath" to the ProtectTree */
6. H=Hash(MixedElement)
7. Add ABSXpath to <Reference> node as "URI" attribute value
8. Enclose H to <DigestValue> node inside the <Reference> node,
as defined in XML Signature standard.
9. }
10. SignedInfoHash=calculate hash of <SignedInfo> element
/* Calculate the digest of the <SignedInfo> element */
11. SignedSOAP=Encrypt(SignedInfoHash , KS.PrivateKey)
/*Signing that digest and enclosing the signature value
in a <SignatureValue> element */
3.2. Algorithm for verification of Signature
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1. SignInfoDigest=Calculate digest of the <SignedInfo> element
2. SignatureValueContent= content inside <SignatureValue> node
3. Flag=VerifySignature(Public Key, SignatureValueContent, SignInInfoDigest)
4. If(Flag){
5. Ids=All URI's in <Reference> nodes inside the <SignedInfo> node
6. For each Id from Ids){
7. ABSXpath=Get the content of Id
8. Subtree=Get the sub tree identified by ABSXpath
9. MixedElement =AppendSyntacticTokenSubTree(Subtree, ABSXpath)
/* Append a Positional Token as an attribute,
"PosToken= ABSXpath" to the Subtree */
10. H=Hash (MixedElement)/* generate hash value of signed elements. */
11. Digest=Get digest value under the <Reference>
node and inside <DigestValue> node, whose "URI" is equal to Id
12. If(H!=Digest){
13. return "Signature Validation Failed"
14. }else{
15. return "Signature Validation Successful"
16. }
17. } //For loop
18. else
19. return "Signature Validation Failed"
20. }
3.2.1. Verifying SignedInfo Element Digest with Decrypted Digest from
SignatureValue element
1. VerifySignature(PublicKey, SignatureValueContent, SignInInfoDigest){
2. DecryptedDigest=Decrypt SignatureValueContent with PublicKey
3. If(DecryptedDigest!=SignInInfoDigest){
4. return False
5. }
6. else{
7. return True
8. }
9. }
4. Simple Example
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The <Signature> Lets consider an XML document for the example
purpose:
<?xml version="1.0"?>
<PatientRecord>
<Visit date="10pm March 2018">
<Account id="id1">1234</Account>
<Name>ABC</Name>
<Diagnosis>Kidney Test</Diagnosis>
</Visit>
<Visit date="12pm May 2018">
<Account id="id2">1235</Account>
<Name>DEF</Name>
<Diagnosis>Liver Test</Diagnosis>
</Visit>
</PatientRecord>
Figure 1
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Existing XML Signature algorithm would produce a <Signature> element
for the XML document mentioned in Figure 1, as follows:
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod
Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#WithComments" />
<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" />
<Reference URI="#id1">
<Transforms>
<Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
<Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
</Transforms>
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
<DigestValue>.................</DigestValue>
</Reference>
<Reference URI="#id2">
<Transforms>
<Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
<Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
</Transforms>
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
<DigestValue>................</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue>
..........
</SignatureValue>
<KeyInfo>
<X509Data>
<X509Certificate>
.............................
</X509Certificate>
</X509Data>
</KeyInfo>
</Signature>
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The proposed XML Signature algorithm would produce a <Signature>
element for the XML document mentioned in Figure 1, which is
described in Figure 2. Also, during signing process, "Positional
Token" as an attribute e.g.(PosToken= "Absolute XPath") has been used
in it, as per proposed algorithm in Section 3.1. Now, <DigestValue>
elements inside <Signature> element will also contain the trace of
"Positional Token", hence the relative position of signed elements in
the given XML document:
<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
<SignedInfo>
<CanonicalizationMethod
Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#WithComments" />
<SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" />
<Reference URI="/PatientRecord/Visit[1]/Account[@id='id1']">
<Transforms>
<Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
<Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
</Transforms>
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
<DigestValue>.................</DigestValue>
</Reference>
<Reference URI="/PatientRecord/Visit[2]/Account[@id='id2']">
<Transforms>
<Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
<Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
</Transforms>
<DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
<DigestValue>................</DigestValue>
</Reference>
</SignedInfo>
<SignatureValue>
............
</SignatureValue>
<KeyInfo>
<X509Data>
<X509Certificate>
.............................
</X509Certificate>
</X509Data>
</KeyInfo>
</Signature>
Figure 2
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5. Algorithm Validation
In this section we will discuss as how the suggested algorithm can
mitigate the various scenarios of XML wrapping attacks.
5.1. Mitigation of XML Signature wrapping attacks
This kind of attacks are possible because signature verification
algorithm identifies signed element using identity i.e. ID and
identifying position of signed element using ID has inherent flaw as
the signed element can easily be moved within the document and still
the document retains the ability to verify its signature. So, in our
algorithm, we have suggested the usage of absolute XPath in place of
ID for identifying the position of signed elements. Absolute XPath
has two fold advantages as it can easily identify the position of
signed element within the XML document and it fixes both the vertical
and horizontal axes of the signed element exactly. The absolute
XPath expression to identify signed element will not be same as
absolute XPath expression for signed element in forged document.The
signature validation will fail at step-8, of algorithm in
Section 3.2, as there is no such node, Further, if the attacker
modifies the URI attribute and tries to perform XML wrapping attack,
the digest of <SignedInfo> will not match and signature validation
will fail at step-4 of algorithm in Section 3.2.
5.2. Mitigation of XML elements jumbling type of wrapping attacks
This kind of wrapping attacks are possible as the attacker jumbles
the position of signed elements within the document as XML signature
process defined by specification takes only ID into consideration for
referencing the signed elements. The proposed algorithm suggests
using "Absolute XPath" for referencing the signed elements as well as
doping the elements subjected to be signed with it. Hence, the
digest of the signed element inside <DigestValue> node has a trail of
the position of element; refer step-6 of algorithm in Section 3.1.
Hence, any changes in the position of signed elements by the
attackers will invalidate the signature validation; refer step-12 of
algorithm in Section 3.2, because calculated digest during signature
validation will not match with the digest contained in <DigestValue>
of the forged SOAP request.
6. Conclusion
XML Signature wrapping attacks try to inject forged elements into the
XML document structure in such a way that the valid signature covers
the unmodified elements, while forged elements are processed by the
application logic. This results in a scenario, where an attacker can
perform arbitrary web service requests, while authenticating as a
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legitimate user. The proposed algorithm takes help of the absolute
XPath as a "Positional Token" for identifying the signed elements and
adding this to the elements to be signed as an attribute before the
canonicalization process has a trace of both content of signed
element and its position in the XML document as well. Hence, the
proposed algorithm can solve the issue of XML wrapping attacks
elegantly without much change in the current standard.
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
This draft proposes a modification to the existing algorithm of XML
signature to counter XML Signature wrapping attacks. However other
forms of attack may be possible that could not be mitigated.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2807] Reagle, J., "XML Signature Requirements", RFC 2807,
DOI 10.17487/RFC2807, July 2000,
<https://www.rfc-editor.org/info/rfc2807>.
[RFC3275] Eastlake 3rd, D., Reagle, J., and D. Solo, "(Extensible
Markup Language) XML-Signature Syntax and Processing",
RFC 3275, DOI 10.17487/RFC3275, March 2002,
<https://www.rfc-editor.org/info/rfc3275>.
9.2. Informative References
[I-D.narten-iana-considerations-rfc2434bis]
Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", draft-narten-iana-
considerations-rfc2434bis-09 (work in progress), March
2008.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
DOI 10.17487/RFC2629, June 1999,
<https://www.rfc-editor.org/info/rfc2629>.
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[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
[wrapping_attack]
McIntosh, Michael. and Paula. Austel, "XML signature
element wrapping attacks and countermeasures", 2005,
<https://dl.acm.org/citation.cfm?id=1103026>.
[WS-Security]
OASIS., "OASIS Web Services Security (WSS) TC", 2006,
<https://www.oasis-open.org/committees/
tc_home.php?wg_abbrev=wss>.
Authors' Addresses
Jitendra Kumar
C-DAC Bangalore
#68, Electronics City Hosur Road
Bangalore 560100
India
Email: jitendra@cdac.in
Balaji Rajendran
C-DAC Bangalore
#68, Electronics City Hosur Road
Bangalore 560100
India
Email: balaji@cdac.in
Bindhumadhava BS
C-DAC Bangalore
Old Madras Road, Opposite Hal Aero Engine Division
Bangalore 560038
India
Email: bindhu@cdac.in
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