Forge: Certificate signature verified with the issuer's RSA signature scheme

Summary

When verifying a certificate, node-forge chose the RSA padding scheme (PKCS#1 v1.5 or RSASSA-PSS) and PSS parameters from the issuer certificate's own signature algorithm instead of the certificate being verified. A certificate labeled as signed with RSASSA-PSS could pass verifyCertificateChain() with a PKCS#1 v1.5 signature. Fixed in node-forge 1.4.1.

Details

certificate.verify(child) in lib/x509.js hashes the child's TBSCertificate with the digest of the child's signature algorithm, then calls _verifySignature() with the issuer certificate. _verifySignature() selected the padding scheme from that certificate's signatureOid and signatureParameters, so the child was verified with the issuer's own scheme rather than the one the child declares.

As a result, a child certificate whose signature algorithm says RSASSA-PSS was accepted with a PKCS#1 v1.5 signature when the issuer itself was signed with PKCS#1 v1.5. The outer signatureAlgorithm field of a certificate is not covered by the signature, so an existing PKCS#1 v1.5 (SHA-256) certificate can be relabeled as RSASSA-PSS and still pass verification. The signature itself must still be made with the issuer's private key over the certificate's contents; no certificate can be forged.

The same cause made correctly signed chains fail: a PKCS#1 v1.5 certificate issued by an RSASSA-PSS-signed certificate, or the reverse, was rejected.

The fix verifies with the child certificate's signature scheme and parameters and the issuer's public key.

Patches

Fixed in node-forge 1.4.1. Certificate signatures are verified with the scheme of the certificate's own signature algorithm.

Workarounds

Applications that make decisions based on a certificate's signature algorithm can verify the signature themselves with that algorithm's scheme and the issuer's public key.

References

• https://www.rfc-editor.org/rfc/rfc4055#section-3 (RSASSA-PSS signature algorithm in certificates)
• https://www.rfc-editor.org/rfc/rfc5280#section-4.1.1.2 (Certificate signatureAlgorithm)
• https://cwe.mitre.org/data/definitions/347.html (CWE-347: Improper Verification of Cryptographic Signature)

Severity

CVSS 3.1: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N (3.7, Low) CVSS 4.0 (informational): CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N (6.3) CVSS 4.0 rates this Moderate because its integrity weighting is higher; the severity rating follows CVSS 3.1.

Credits

Reported by Brian Lee (@hibrian827, Georgia Tech SSLab, Team Atlanta), who also suggested the fix.

Proof of concept

Create a CA certificate signed with sha256WithRSAEncryption, and a leaf certificate whose signature algorithm fields (outer and in the TBSCertificate) are RSASSA-PSS with SHA-256/MGF1-SHA-256/salt 32, but sign the leaf's TBSCertificate with the CA key using PKCS#1 v1.5 (caKey.sign(sha256)).

On 1.4.0, forge.pki.verifyCertificateChain(forge.pki.createCaStore([ca]), [leaf]) returns true, although verifying the leaf's signature with RSASSA-PSS fails. On 1.4.1 it throws "Certificate signature is invalid.".

No private key is needed for the same result: take any certificate signed by the CA with sha256WithRSAEncryption and replace only its outer signatureAlgorithm (outside the signed TBSCertificate) with the RSASSA-PSS identifier above. On 1.4.0 it still verifies and reports signatureOid as RSASSA-PSS; on 1.4.1 it is rejected.

Regression tests are in tests/unit/x509.js ("signature scheme of the child certificate").

Impact

Applications that rely on a certificate's declared signature algorithm, for example a policy that only accepts RSASSA-PSS-signed certificates, can be given a certificate labeled RSASSA-PSS whose signature was verified as PKCS#1 v1.5. The certificate must still have been signed by the trusted issuer's key, so the identity it certifies cannot be forged. There is no confidentiality or availability impact.