Headline
CVE-2023-49087: Validation of SignedInfo
xml-security is a library that implements XML signatures and encryption. Validation of an XML signature requires verification that the hash value of the related XML-document matches a specific DigestValue-value, but also that the cryptographic signature on the SignedInfo-tree (the one that contains the DigestValue) verifies and matches a trusted public key. If an attacker somehow (i.e. by exploiting a bug in PHP’s canonicalization function) manages to manipulate the canonicalized version’s DigestValue, it would be possible to forge the signature. This issue has been patched in version 1.6.12 and 5.0.0-alpha.13.
Validation of an XML Signature requires verification that the hash value of the related XML-document (after any
optional transformations and/or normalizations) matches a specific DigestValue-value, but also that the cryptografic
signature on the SignedInfo-tree (the one that contains the DigestValue) verifies and matches a trusted public key.
Within the simpleSAMLphp/xml-security library (https://github.com/simplesamlphp/xml-security), the hash is being
validated using SignedElementTrait::validateReference, and the signature is being verified in
SignedElementTrait::verifyInternal
https://github.com/simplesamlphp/xml-security/blob/master/src/XML/SignedElementTrait.php:
What stands out is that the signature is being calculated over the canonical version of the SignedInfo-tree. The validateReference
method, however, uses the original non-canonicalized version of SignedInfo.
Impact
If an attacker somehow (i.e. by exploiting a bug in PHP’s canonicalization function) manages to manipulate the canonicalized version’s DigestValue, it would be potentially be possible to forge the signature. No possibilities to exploit this were found during the investigation.
Patches
N/A
Workarounds
N/A
References
This potential vulnerability was found during a code review by Secura.com
Related news
Validation of an XML Signature requires verification that the hash value of the related XML-document (after any optional transformations and/or normalizations) matches a specific DigestValue-value, but also that the cryptografic signature on the SignedInfo-tree (the one that contains the DigestValue) verifies and matches a trusted public key. Within the simpleSAMLphp/xml-security library (https://github.com/simplesamlphp/xml-security), the hash is being validated using SignedElementTrait::validateReference, and the signature is being verified in SignedElementTrait::verifyInternal https://github.com/simplesamlphp/xml-security/blob/master/src/XML/SignedElementTrait.php:  What stands out is that the signature is being calculated over the canonical version of the SignedInfo-tree. The validateReference method, however, uses the original non-canonicalized version of SignedInf...