{"id":"CVE-2026-15999","state":"PUBLISHED","title":"AES-CCM decryption accepts zero or out-of-range tag length, bypassing authentication","description":"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.","publishedAt":"2026-10-02T06:52:16.792Z","updatedAt":"2026-10-02T18:02:14.715Z","severity":{"score":8.2,"label":"High","version":"4.0","vector":"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","source":"bcorg"},"assigner":"bcorg","products":["Legion of the Bouncy Castle Inc. bc-csharp"],"affected":[{"vendor":"Legion of the Bouncy Castle Inc.","product":"bc-csharp","packageName":"BouncyCastle.Cryptography","defaultStatus":"unaffected","versions":[{"version":"0","status":"affected","lessThan":"2.7.0","versionType":"semver"}],"collectionURL":"https://www.nuget.org/packages/BouncyCastle.Cryptography","repo":"https://github.com/bcgit/bc-csharp","programFiles":["crypto/src/asn1/cms/CcmParameters.cs","crypto/src/crypto/modes/CcmBlockCipher.cs"]}],"references":["https://github.com/bcgit/bc-csharp/wiki/CVE-2026-15999","https://github.com/bcgit/bc-csharp/commit/2818bdfa55efd58b0d6a1c75f360b8404d879780","https://github.com/bcgit/bc-csharp/commit/9d284d8f379614def853b590c62bbb31c48f7af0"],"weaknesses":["CWE-354"],"ingestedAt":"2026-10-29T20:11:00.674Z","source":{"repository":"CVEProject/cvelistV5","path":"cves/2026/15xxx/CVE-2026-15999.json","commit":"556c9d3a8cd17e218bb8e452ba7cd619a384ff47"},"raw":{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-15999","state":"PUBLISHED","datePublished":"2026-10-02T06:52:16.792Z","dateUpdated":"2026-10-02T18:02:14.715Z","assignerShortName":"bcorg","assignerOrgId":"91579145-5d7b-4cc5-b925-a0262ff19630","dateReserved":"2026-07-16T23:53:17.500Z"},"containers":{"cna":{"title":"AES-CCM decryption accepts zero or out-of-range tag length, bypassing authentication","descriptions":[{"lang":"en","value":"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."}],"affected":[{"vendor":"Legion of the Bouncy Castle Inc.","product":"bc-csharp","packageName":"BouncyCastle.Cryptography","defaultStatus":"unaffected","versions":[{"version":"0","status":"affected","lessThan":"2.7.0","versionType":"semver"}],"collectionURL":"https://www.nuget.org/packages/BouncyCastle.Cryptography","repo":"https://github.com/bcgit/bc-csharp","programFiles":["crypto/src/asn1/cms/CcmParameters.cs","crypto/src/crypto/modes/CcmBlockCipher.cs"]}],"references":[{"url":"https://github.com/bcgit/bc-csharp/wiki/CVE-2026-15999","tags":["vendor-advisory"]},{"url":"https://github.com/bcgit/bc-csharp/commit/2818bdfa55efd58b0d6a1c75f360b8404d879780","tags":["patch"]},{"url":"https://github.com/bcgit/bc-csharp/commit/9d284d8f379614def853b590c62bbb31c48f7af0","tags":["patch"]}],"metrics":[{"format":"CVSS","scenarios":[{"lang":"en","value":"GENERAL"}],"cvssV4_0":{"attackVector":"NETWORK","attackComplexity":"LOW","attackRequirements":"PRESENT","privilegesRequired":"NONE","userInteraction":"NONE","vulnConfidentialityImpact":"NONE","subConfidentialityImpact":"NONE","vulnIntegrityImpact":"HIGH","subIntegrityImpact":"NONE","vulnAvailabilityImpact":"NONE","subAvailabilityImpact":"NONE","exploitMaturity":"NOT_DEFINED","Safety":"NOT_DEFINED","Automatable":"NOT_DEFINED","Recovery":"NOT_DEFINED","valueDensity":"NOT_DEFINED","vulnerabilityResponseEffort":"NOT_DEFINED","providerUrgency":"NOT_DEFINED","version":"4.0","baseSeverity":"HIGH","baseScore":8.2,"vectorString":"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"}}],"problemTypes":[{"descriptions":[{"cweId":"CWE-354","lang":"en","description":"CWE-354 Improper Validation of Integrity Check Value","type":"CWE"}]}],"providerMetadata":{"shortName":"bcorg","orgId":"91579145-5d7b-4cc5-b925-a0262ff19630","dateUpdated":"2026-10-02T06:52:16.792Z"},"impacts":[{"descriptions":[{"lang":"en","value":"Integrity (CCM authentication bypass: modified ciphertext accepted as authentic on decryption)"}]}],"credits":[{"lang":"en","value":"Discovered by Claude, Anthropic's AI assistant, and triaged by the Anthropic security team in collaboration with Anthropic Research.","type":"finder"}],"source":{"discovery":"EXTERNAL"},"x_generator":{"engine":"Claude Opus 5.5"},"x_antReportIds":["ANT-2026-X1VYWFD5"]},"adp":[{"title":"CISA ADP Vulnrichment","metrics":[{"other":{"type":"ssvc","content":{"timestamp":"2026-10-02T18:01:58.140745Z","id":"CVE-2026-15999","options":[{"Exploitation":"none"},{"Automatable":"no"},{"Technical Impact":"partial"}],"role":"CISA Coordinator","version":"2.0.3"}}}],"providerMetadata":{"shortName":"CISA-ADP","orgId":"134c704f-9b21-4f2e-91b3-4a467353bcc0","dateUpdated":"2026-10-02T18:02:14.715Z"}}]}}}