High 8.2Published Oct 2, 2026

CVE-2026-15999AES-CCM decryption accepts zero or out-of-range tag length, bypassing authentication

Updated under an hour ago (2026-10-19 07:30 UTC)

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It affects Legion of the Bouncy Castle Inc. bc-csharp. Add what you run to find out whether that includes you.

EPSS puts the chance of exploitation in the next 30 days at 0.23%, higher than 12% of scored CVEs.

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Every CVE for Legion of the Bouncy Castle Inc. bc-csharp

Exploited in the last 30 days

Improper validation of integrity check value in the AES-CCM implementation (CcmParameters and CcmBlockCipher) in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows an on-path attacker to modify CCM-encrypted content without detection via an AlgorithmIdentifier whose CCMParameters declare an authentication tag (aes-ICVlen) of zero or another length outside the RFC 5084 set, because CcmParameters accepted any value and CcmBlockCipher validated the tag length only when encrypting, so decryption compared a zero-length or very short tag. Affected paths include ParameterUtilities.GetCipherParameters, used by CmsEnvelopedData and CmsEnvelopedDataParser for EnvelopedData encrypted with AES-CCM, and any caller passing an unchecked tag length to CcmBlockCipher for decryption.

CVSS: 8.2 HIGH (v4.0, CNA)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
Weakness (CWE): CWE-354
Known exploited: Not in CISA's KEV catalog
EPSS (exploitation in the next 30 days): 0.23% (percentile 12)
Assigned by: bcorg
NVD analysis: Undergoing Analysis
Record last updated: Oct 2, 2026
First seen by InfoSec Signals: Oct 9, 2026

Affected products

Affected products

Legion of the Bouncy Castle Inc. bc-csharp legion_of_the_bouncy_castle:bc_csharp (from the name)	before 2.7.0	CNA record

References

https://github.com/bcgit/bc-csharp/wiki/CVE-2026-15999vendor-advisory
https://github.com/bcgit/bc-csharp/commit/2818bdfa55efd58b0d6a1c75f360b8404d879780patch
https://github.com/bcgit/bc-csharp/commit/9d284d8f379614def853b590c62bbb31c48f7af0patch

The CVE record on cve.org NVD's entry

The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.