Observable discrepancy in the CMS RSA PKCS#1 v1.5 key-transport unwrap…

Why it is Yellow

• CVSS 8.2 high
• Remotely exploitable without logging in

References

• https://github.com/bcgit/bc-csharp/commit/85679cc7023adf4e2a0650b88a1f4f98679c7463
• https://github.com/bcgit/bc-csharp/commit/c38b4790e2aef68f116a551f73b94ea05cb5590c
• https://github.com/bcgit/bc-csharp/wiki/CVE-2026-63573

Original description

Observable discrepancy in the CMS RSA PKCS#1 v1.5 key-transport unwrap (KeyTransRecipientInformation.UnwrapKey) in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows a remote attacker who holds a captured CMS EnvelopedData message, and who can submit many modified messages to an application that decrypts them with the recipient's RSA private key and reveals how decryption failed, to recover the captured message's content-encryption key and so its content, via a Bleichenbacher-style adaptive chosen-ciphertext attack, because a key-transport ciphertext with invalid PKCS#1 v1.5 padding is rejected during unwrap with a distinct "bad padding in message." CmsException instead of being replaced by a random key, so it can be told apart from a correctly padded ciphertext, which fails only later at content decryption.