CVE-2026-15999 Legion of the Bouncy Castle Inc. CVE debrief

Who should care

Defenders and developers using Bouncycastle's bc-csharp library for encryption should assess exposure and prioritize remediation. This includes reviewing and updating bc-csharp to version 2.7.0 or later, verifying the integrity of encrypted content, and monitoring for potential tampering with encrypted data. Security teams and operators using affected versions should verify encrypted data integrity and plan for remediation.

Why it matters

CVE-2026-15999 allows an on-path attacker to modify CCM-encrypted content without detection in Bouncycastle's bc-csharp before version 2.7.0. Defenders and developers using this library should assess exposure, verify encrypted data integrity, and prioritize remediation.

• Potential tampering with encrypted content without detection
• Verification of encrypted data integrity is necessary
• Remediation requires updating bc-csharp to version 2.7.0 or later

Technical summary

The AES-CCM implementation in Bouncycastle's bc-csharp before version 2.7.0 is vulnerable to tampering with encrypted content due to improper validation of the integrity check value. This vulnerability affects CcmParameters and CcmBlockCipher, used by CmsEnvelopedData and CmsEnvelopedDataParser for EnvelopedData encrypted with AES-CCM. The vulnerability 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.

Defensive priority

High

Recommended defensive actions

• Review and update bc-csharp to version 2.7.0 or later
• Verify the integrity of encrypted content using authenticated encryption modes
• Monitor for and respond to potential tampering with encrypted data
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up
• Review the supplied official advisory or CVE record to validate affected scope, severity, and vendor guidance
• Plan vendor-supported updates or mitigations through normal change control where exposure is confirmed
• Check relevant monitoring, detection, and logs for exposed assets that need extra review

Evidence notes

The CVE record and NVD entry provide details on the vulnerability in Bouncycastle's bc-csharp before version 2.7.0. The vulnerability allows an on-path attacker to modify CCM-encrypted content without detection.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.