---
id: CVE-2026-63567
title: >-
  Observable discrepancy in IesEngine.DecryptBlock in Legion of the Bouncy
  Castle Inc
summary: >-
  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 unde…
severity: high
cvss: 8.2
cvssVector: 'CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N'
cwe:
  - CWE-203
vendor: Legion of the Bouncy Castle Inc.
product: BouncyCastle.Cryptography
affected:
  - BouncyCastle.Cryptography < 2.7.0
published: '2026-10-02'
updated: '2026-10-02'
sourceUpdated: '2026-10-02T07:16:37.127'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-63567'
references:
  - url: >-
      https://github.com/bcgit/bc-csharp/commit/eaa42e1e8125b3e2e25e365c4194c7cd5932360b
    label: 91579145-5d7b-4cc5-b925-a0262ff19630
  - url: >-
      https://github.com/bcgit/bc-csharp/commit/f881400b761134160ec2a05fd27c3e569453788d
    label: 91579145-5d7b-4cc5-b925-a0262ff19630
  - url: 'https://github.com/bcgit/bc-csharp/wiki/CVE-2026-63567'
    label: 91579145-5d7b-4cc5-b925-a0262ff19630
tags:
  - nvd
  - cve.org
cvssSource: cna
ingestedAt: '2026-10-02T07:13:06.668Z'
---

## Overview

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.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.