CVE-2026-63567

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.

• CWE-203

• https://github.com/bcgit/bc-csharp/commit/eaa42e1e8125b3e2e25e365c4194c7cd5932360bPatch
• https://github.com/bcgit/bc-csharp/commit/f881400b761134160ec2a05fd27c3e569453788dPatch
• https://github.com/bcgit/bc-csharp/wiki/CVE-2026-63567Patch, Vendor Advisory

Published

Oct 2, 2026

Last Modified

Oct 27, 2026

Status

Analyzed

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