xml-crypto vulnerable to XML signature verification bypass due improper verification of signature/signature spoofing

Summary

Default configuration does not check authorization of the signer, it only checks the validity of the signature per section 3.2.2 of https://www.w3.org/TR/2008/REC-xmldsig-core-20080610/#sec-CoreValidation. As such, without additional validation steps, the default configuration allows a malicious actor to re-sign an XML document, place the certificate in a <KeyInfo /> element, and pass xml-crypto default validation checks.

Details

Affected xml-crypto versions between versions >= 4.0.0 and < 6.0.0.

xml-crypto trusts by default any certificate provided via digitally signed XML document's <KeyInfo />.

xml-crypto prefers to use any certificate provided via digitally signed XML document's <KeyInfo /> even if library was configured to use specific certificate (publicCert) for signature verification purposes.

Attacker can spoof signature verification by modifying XML document and replacing existing signature with signature generated with malicious private key (created by attacker) and by attaching that private key's certificate to <KeyInfo /> element.

Vulnerability is combination of changes introduced to 4.0.0 at

• https://github.com/node-saml/xml-crypto/pull/301
• https://github.com/node-saml/xml-crypto/commit/c2b83f984049edb68ad1d7c6ad0739ec92af11ca

Changes at PR provided default method to extract certificate from signed XML document.

• https://github.com/node-saml/xml-crypto/blob/c2b83f984049edb68ad1d7c6ad0739ec92af11ca/lib/signed-xml.js#L405-L414
• https://github.com/node-saml/xml-crypto/blob/c2b83f984049edb68ad1d7c6ad0739ec92af11ca/lib/signed-xml.js#L334

and changes at PR prefer output of that method to be used as certificate for signature verification even in the case when library is configured to use specific/pre-configured signingCert

• https://github.com/node-saml/xml-crypto/blob/c2b83f984049edb68ad1d7c6ad0739ec92af11ca/lib/signed-xml.js#L507

Name of the signingCert was changed later (but prior to 4.0.0 release) to publicCert:

• https://github.com/node-saml/xml-crypto/commit/78329fbae34c9b25ba25882604e960f506d7c0e7
• https://github.com/node-saml/xml-crypto/blob/78329fbae34c9b25ba25882604e960f506d7c0e7/lib/signed-xml.js#L507

Issue was fixed to 6.0.0 by disabling implicit usage of default getCertFromKeyInfo implementation:

• https://github.com/node-saml/xml-crypto/pull/445
• https://github.com/node-saml/xml-crypto/commit/21201723d2ca9bc11288f62cf72552b7d659b000

Possible workarounds for versions 4.x and 5.x:

• Check the certificate extracted via getCertFromKeyInfo against trusted certificates before accepting the results of the validation.
• Set xml-crypto's getCertFromKeyInfo to () => undefined forcing xml-crypto to use an explicitly configured publicCert or privateKey for signature verification.

PoC

https://github.com/node-saml/xml-crypto/discussions/399

Impact

An untrusted certificate can be used to pass a malicious XML payload through an improperly configured installation of xml-crypto.