The vulnerability (CVE-2026-15999) arises from improper validation of the integrity check value in the AES-CCM implementation within Legion of the Bouncy Castle Inc. bc-csharp library versions before 2.7.0. Specifically, the CcmParameters class accepts any authentication tag length, including zero or lengths outside the RFC 5084 specification, and the CcmBlockCipher class only validates tag length during encryption, not decryption. This allows an attacker positioned on the network path to modify CCM-encrypted content undetected by exploiting an AlgorithmIdentifier with a CCMParameters tag length of zero or an invalid length. The flaw impacts the ParameterUtilities.GetCipherParameters method used by CmsEnvelopedData and CmsEnvelopedDataParser for AES-CCM encrypted data, as well as any decryption calls passing unchecked tag lengths to CcmBlockCipher.

An on-path attacker can modify AES-CCM encrypted content without detection due to acceptance of invalid authentication tag lengths during decryption. This undermines the integrity guarantees of AES-CCM encryption in affected versions, potentially allowing undetected tampering with encrypted messages.

A fix is available in Legion of the Bouncy Castle Inc. bc-csharp version 2.7.0 and later. Users should upgrade to version 2.7.0 or newer to ensure proper validation of authentication tag lengths during AES-CCM decryption. Patch status is not explicitly confirmed in the provided data, but the version boundary indicates remediation. Until upgrading, avoid using affected versions for AES-CCM encrypted data decryption or apply additional validation on tag lengths if possible.