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

• #301
• c2b83f9

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

• xml-crypto/lib/signed-xml.js

  Lines 405 to 414 in c2b83f9

  	SignedXml.getCertFromKeyInfo = function (keyInfo) {
  	if (keyInfo != null && keyInfo.length > 0) {
  	const certs = xpath.select(".//*[local-name(.)='X509Certificate']", keyInfo[0]);
  	if (certs.length > 0) {
  	return utils.derToPem(certs[0].textContent.trim(), "CERTIFICATE");
  	}
  	}
  	return null;
  	};

• xml-crypto/lib/signed-xml.js

  Line 334 in c2b83f9

  this.getCertFromKeyInfo = SignedXml.getCertFromKeyInfo;

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

• xml-crypto/lib/signed-xml.js

  Line 507 in c2b83f9

  this.getCertFromKeyInfo(this.keyInfo) || this.signingCert || this.signingKey,

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

• 78329fb

• xml-crypto/lib/signed-xml.js

  Line 507 in 78329fb

  this.getCertFromKeyInfo(this.keyInfo) || this.publicCert || this.privateKey,

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

• #445
• 2120172

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

#399

Impact

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