Whitepaper: Command Injection in XML Digital Signatures and Encryption

[brad () isecpartners com]

iSEC Partners and Brad Hill are pleased to announce the availability of a new whitepaper describing design flaws and 
new attacks against the XML Digital Signature and XML Encryption standards.  It accompanies recent advisories and 
provides detailed guidance for auditors and implementers of these products.

Full Paper Available at:
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http://www.isecpartners.com/files/XMLDSIG_Command_Injection.pdf

Abstract: 
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The XML Digital Signature (XMLDSIG) and XML Encryption (XMLENC) standards are complex protocols for securing XML and 
other content. Among its complexities, the XMLDSIG standard specifies various “Transform” algorithms to identify, 
manipulate and canonicalize signed content and key material. Unfortunately, the defined transforms have not been 
rigorously constrained to prevent their use as attack vectors, and denial of service or even arbitrary code execution 
are probable in implementations that have not specifically guarded against such risks. 

Attacks against the processing application can be embedded in the KeyInfo portion of a signature, making them 
inherently unauthenticated, or in the SignedInfo block. Although tampering with the SignedInfo should be detectable, a 
defective implied order of operations in the specification may still allow unauthenticated attacks here. 

The ability to execute arbitrary code and perform file system operations with a malicious, invalid signature has been 
confirmed by the researcher in at least two independent XMLDSIG implementations, and other implementations may be 
similarly vulnerable. This paper describes the vulnerabilities in detail and offers advice for remediation. The most 
damaging attack is also likely to apply in other contexts where XSLT is accepted as input, and should be considered by 
all implementers of complex XML processing systems.

About iSEC Partners:
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iSEC Partners is a full-service security consulting firm that provides
penetration testing, secure systems development, security education
and software design verification.

115 Sansome Street, Suite 1005
San Francisco, CA 94104
Phone: (415) 217-0052