The vulnerability CVE-2026-63567 affects the IesEngine.DecryptBlock implementation in bc-csharp versions before 2.7.0. When IesEngine is constructed with a padded block cipher (e.g., AES-CBC with PKCS#7 padding), the engine decrypts ciphertext and removes padding prior to MAC verification. This leads to different error messages for padding errors compared to MAC errors, enabling a padding-oracle attack. An attacker who has captured an IES or ECIES ciphertext and can submit modified ciphertexts for decryption under the same key pair can exploit this to recover plaintext. Applications using stream-mode IES are not vulnerable.

An attacker with the ability to submit modified ciphertexts for decryption under the same key pair can exploit this vulnerability to recover plaintext from encrypted messages via a CBC padding-oracle attack. This compromises the confidentiality of encrypted data processed by vulnerable versions of bc-csharp when using padded block ciphers in IesEngine. There is no indication of privilege escalation or denial of service impacts.

A fixed version 2.7.0 or later of bc-csharp addresses this vulnerability. Users should upgrade to version 2.7.0 or newer to mitigate this issue. If upgrading is not immediately possible, avoid constructing IesEngine directly with padded block ciphers such as AES in CBC mode with PKCS#7 padding. Use stream-mode IES instead, which is not affected by this vulnerability. Patch status is confirmed by the affectedVersions field indicating versions before 2.7.0 are vulnerable.