Observable discrepancy in IesEngine.DecryptBlock in Legion of the Bouncy Castle Inc

Why it is Amber

• CVSS 8.2 high
• Remotely exploitable without logging in

References

• https://github.com/bcgit/bc-csharp/commit/eaa42e1e8125b3e2e25e365c4194c7cd5932360b
• https://github.com/bcgit/bc-csharp/commit/f881400b761134160ec2a05fd27c3e569453788d
• https://github.com/bcgit/bc-csharp/wiki/CVE-2026-63567

Original description

Observable discrepancy in IesEngine.DecryptBlock in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows a remote attacker who has captured an IES or ECIES ciphertext, and who can submit modified ciphertexts for decryption under the same key pair, to recover its plaintext via a CBC padding-oracle attack, because in block-cipher mode the engine decrypts the ciphertext and removes its padding before verifying the MAC. A padding failure is therefore reported with a different error message, and without the MAC computation, compared with a MAC failure. Only applications that construct IesEngine directly with a padded block cipher, such as AES in CBC mode with PKCS#7 padding, are affected; stream-mode IES is not.