What’s Vulnerable › Vulnerabilities

CVE-2026-103601: CcmBlockCipher and KCcmBlockCipher leave unverified plaintext in the output buffer after a failed tag check

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-10-02 · updated 2026-10-28
• Weakness: CWE-354

Description

Release of unverified plaintext in the CCM (CcmBlockCipher) and DSTU 7624 CCM (KCcmBlockCipher) AEAD modes in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows a remote attacker to obtain decryptions of ciphertexts of their choosing via forged messages sent to an application that lets the output buffer of a failed decryption be observed, for example through buffer reuse or logging, because decryption wrote the recovered plaintext into the caller-supplied output buffer before checking the authentication tag and left it there when the check failed. Only decryption into a caller-supplied buffer is affected; methods that return a newly allocated array are not.

Affected products

• Legion of the Bouncy Castle Inc. bc-csharp: before 2.7.0 — fixed in 2.7.0
• Legion of the Bouncy Castle Inc. bc-csharp: before 2.7.0 — fixed in 2.7.0

How to fix

Upgrade to a fixed version: Legion of the Bouncy Castle Inc. bc-csharp 2.7.0; Legion of the Bouncy Castle Inc. bc-csharp 2.7.0. Check the vendor advisory for mitigations.

References

• https://github.com/bcgit/bc-csharp/commit/4ddb1d0bfeae3a81b18f4446d8cc8a7871bd2d9f
• https://github.com/bcgit/bc-csharp/commit/e0868323645a063cb2479646e93c1449dc338de5
• https://github.com/bcgit/bc-csharp/wiki/CVE-2026-103601

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