CVE-2026-63567 in bc-csharp
Summary
by MITRE • 10/02/2026
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.
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Analysis
by VulDB Data Team • 10/02/2026
The vulnerability identified within the Legion of the Bouncy Castle Inc bc-csharp library prior to version 2.7.0 represents a critical implementation flaw in the Integrated Encryption Scheme engine, specifically affecting its DecryptBlock method when operating with block ciphers such as AES in Cipher Block Chaining mode with PKCS#7 padding. This issue stems from an observable discrepancy in how the cryptographic engine handles decryption errors, creating a side-channel that allows for a CBC padding-oracle attack. The core technical flaw lies in the order of operations during the decryption process; specifically, the engine decrypts the ciphertext and removes its padding before verifying the Message Authentication Code or MAC. This sequence is fundamentally insecure because it exposes the internal state of the padding validation to an attacker who can observe error messages returned by the application.
In a secure cryptographic implementation, any failure in integrity verification should be handled uniformly to prevent information leakage. However, in this vulnerable version, a padding failure results in a distinct error message and occurs without computing the MAC, whereas a valid ciphertext with an incorrect MAC triggers a different error path that includes MAC computation. This difference allows a remote attacker who has captured IES or ECIES ciphertexts to submit modified versions of these ciphertexts for decryption under the same key pair. By analyzing whether the system returns a padding-related error or a general authentication failure, the attacker can iteratively deduce the plaintext bytes one block at a time without possessing the private key. This is a classic example of an oracle attack where the application itself acts as the oracle by leaking information through its response patterns.
The operational impact of this vulnerability is severe for any system relying on IES or ECIES with padded block ciphers like AES-CBC-PKCS7. An attacker can fully recover the plaintext from encrypted communications, leading to a complete compromise of confidentiality and integrity. This undermines the security guarantees provided by the encryption scheme, potentially exposing sensitive data such as session tokens, personal information, or proprietary business logic depending on the application context. It is important to note that this vulnerability only affects applications that construct IesEngine directly with padded block ciphers; stream-mode implementations of IES are not susceptible because they do not rely on padding mechanisms in the same way.
To mitigate this risk, organizations must upgrade their bc-csharp dependencies to version 2.7.0 or later where the implementation has been corrected to ensure constant-time execution and uniform error handling regardless of whether a padding or MAC failure occurs. In addition to upgrading libraries, developers should review any custom cryptographic implementations to ensure that decryption operations do not leak timing information or distinct error codes based on specific validation failures. Adhering to industry standards such as CWE-209 which addresses the generation of an error message containing sensitive information is crucial for preventing such side-channel leaks. Furthermore, this vulnerability aligns with MITRE ATT&CK techniques related to Cryptographic Failures and potentially exploitation via Oracle attacks, emphasizing the need for rigorous code review and adherence to secure coding practices that prioritize constant-time algorithms and uniform error handling in cryptographic modules.