---
id: CVE-2026-16001
title: >-
  Exposure of the message authentication key through the encryption keystream in
  the stream mode of IesEngine (an IesEngine constructed without a block cipher)
  in Legion of the Bouncy Castle Inc
summary: >-
  Exposure of the message authentication key through the encryption keystream in
  the stream mode of IesEngine (an IesEngine constructed without a block cipher)
  in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows a remote
  atta…
severity: high
cvss: 8.2
cvssVector: 'CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N'
cwe:
  - CWE-354
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: '2026-10-02T07:16:36.537'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-16001'
references:
  - url: >-
      https://github.com/bcgit/bc-csharp/commit/f881400b761134160ec2a05fd27c3e569453788d
    label: 91579145-5d7b-4cc5-b925-a0262ff19630
  - url: 'https://github.com/bcgit/bc-csharp/wiki/CVE-2026-16001'
    label: 91579145-5d7b-4cc5-b925-a0262ff19630
tags:
  - nvd
  - cve.org
cvssSource: cna
ingestedAt: '2026-10-02T07:13:06.672Z'
---

## Overview

Exposure of the message authentication key through the encryption keystream in the stream mode of IesEngine (an IesEngine constructed without a block cipher) in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows a remote attacker who has observed one encrypted message with known plaintext to forge shorter messages of their choosing that the recipient accepts as authentic, via a crafted ciphertext and MAC tag, because the MAC key was taken from the key derivation output directly after a keystream as long as the message, while the derivation input depends only on the static key pair and fixed parameters. The keystream revealed by that one message therefore contains the MAC key for every sufficiently shorter message.

## Remediation

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