OpenSSL Improper Certificate Validation Vulnerability - CVE-2026-42769

Issue Summary: An error in the callback used to verify the certificate provided in a Root CA key update Certificate Management Protocol (CMP) message response rendered the certificate validation ineffectual which could lead to escalation of credentials from the Registration Authority (RA) level to the root Certification Authority (root CA) level. Impact Summary: The Registration Autority could replace the root CA certificate for the CMP clients with an arbitrary root CA certificate. One of the parts of the Certificate Management Protocol (CMP) specified in RFC 9810 is Root Certification Authority (root CA) key Rollover which is sent by the server in a message with type 39id-it-rootCaKeyUpdate39. As part of these messages 39newWithOld39 certificate the new root CA certificate signed with the old root CA key is provided and verifying its signature is crucial for transferring the trust from the old CA key to the new one. The 39id-it-rootCaKeyUpdate39 messages are expected to be processed with OSSL_CMP_get1_rootCaKeyUpdate() that is expected to verify the 39newWithOld39 certificate. A typo in the certificate chain building code led to adding an incorrect certificate (39newWithOld39 instead of 39oldRoot39) to the certificate chain rendering the certificate verification process ineffectual (only the issuer name and the algorithm OIDs were verified by other parts of the verification code). An attacker who already has credentials that satisfy the CMP message protection checks can generate a new key pair and use a crafted self-signed certificate in its 39id-it-rootCaKeyUpdate39 CMP messages which affected CMP clients would accept as a new trust anchor. Significant preconditions for the attack (having valid RA-level credentials) are the reason the issue was assigned Low severity. The FIPS modules are not affected by this issue as the affected code is outside the OpenSSL FIPS module boundary.