# CVE-2026-63567: Bouncy Castle C# IesEngine padding oracle in CBC mode

Source: https://junglewise.ai/threats/cve-2026-63567-observable-discrepancy-in-iesengine-decryptblock-in-legion-of-the  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-02. Updated: 2026-10-05  
License: CC BY 4.0 with a link back

CVE-2026-63567 · Severity: info · Published 2026-10-02

The Bouncy Castle cryptography library for .NET contains a flaw in its IES (Integrated Encryption Scheme) implementation that weakens encryption when used with padded block ciphers like AES in CBC mode. An attacker who can intercept encrypted messages and submit modified versions for decryption can exploit timing and error message differences to gradually recover the original plaintext without knowing the decryption key. Applications using unpadded or stream-mode encryption are unaffected.

Technologies: [Legion of the Bouncy Castle Inc. BouncyCastle.Cryptography](https://junglewise.ai/threats/technologies/legion-of-the-bouncy-castle-inc-bouncycastle-cryptography). Vendors: [Legion of the Bouncy Castle Inc.](https://junglewise.ai/threats/vendors/legion-of-the-bouncy-castle-inc).

## Quick answers

**Is CVE-2026-63567 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-63567 being exploited in the wild.

**How severe is CVE-2026-63567?** Informational severity, published 2 October 2026.

**Which products and versions are affected?** Legion of the Bouncy Castle Inc. BouncyCastle.Cryptography before 2.7.0.

**Is there a fix for CVE-2026-63567?** Yes: fixed in 2.7.0. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-63567 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-63567?** Check whether you run Legion of the Bouncy Castle Inc. BouncyCastle.Cryptography at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in 2.7.0

## Technical details

The vulnerability is a CBC padding oracle in IesEngine.DecryptBlock that occurs because the implementation decrypts the ciphertext, removes PKCS#7 padding, and then verifies the MAC—rather than verifying the MAC first. This causes padding failures to produce a different error message than MAC failures, enabling an attacker with repeated decryption access under the same key to mount a padding oracle attack on AES-CBC and similar padded block ciphers. Stream-mode IES implementations are not vulnerable.

## Affected products

- Legion of the Bouncy Castle Inc. BouncyCastle.Cryptography before 2.7.0

## Timeline

- 2026-10-02: disclosed

## References

- [https://www.nuget.org/packages/BouncyCastle.Cryptography](https://www.nuget.org/packages/BouncyCastle.Cryptography)
- [https://github.com/bcgit/bc-csharp/commit/eaa42e1e8125b3e2e25e365c4194c7cd5932360b](https://github.com/bcgit/bc-csharp/commit/eaa42e1e8125b3e2e25e365c4194c7cd5932360b)

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