CVE-2026-63567 IesEngine block-cipher mode checks padding before MAC (CBC padding oracle)
Remote attackers can recover plaintext from IES/ECIES ciphertexts via a padding oracle attack in Bouncy Castle C# when using padded block ciphers.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-63567?
CVE-2026-63567 is a high-severity vulnerability in Legion of the Bouncy Castle Inc. bc-csharp, published on 2 Oct 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-203
Which versions are affected by CVE-2026-63567?
It affects 1 product.
Products include Bouncycastle Bc-Csharp.
How do I fix CVE-2026-63567?
No fix has been published for CVE-2026-63567 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-63567 being exploited?
There is no known exploitation of CVE-2026-63567 in the wild and no public exploit code.
Other vulnerabilities in Legion of the Bouncy Castle Inc. bc-csharp
- CVE-2026-63569 MTI/A0 DHAgreement does not validate the peer's ephemeral value
- CVE-2026-103600 Unbounded ASN.1 nesting depth causes process-terminating stack overflow
- CVE-2026-103603 Unbounded HSS public key level count allows huge array allocation during signature verification
- CVE-2026-103604 Quadratic-time escaping when converting X.509 distinguished names to strings
- CVE-2026-16000 KCcmBlockCipher (DSTU 7624 CCM) tag not bound to nonce when no associated data is used
Sources
Also known as GHSA-6w4x-4wpp-m6r4.