CVE-2026-59641

Affected versions of the `org.bouncycastle:bcmail-jdk18on` package are vulnerable to Insufficient Verification of Data Authenticity due to reliance on a signer-controlled attribute for certificate validation. The S/MIME validator uses the `signingTime` attribute, which is asserted by the signer and located within the signed data, as the reference point for certificate path validation. An attacker with an expired or revoked certificate can exploit this by selecting a `signingTime` when their certificate was valid, allowing them to produce a message that appears to validate successfully.