CVE-2026-63573 Legion of the Bouncy Castle Inc. CVE debrief

Who should care

Defenders handling encrypted messages with bc-csharp, especially in systems where decryption failure can reveal information about the encrypted content, should assess exposure and prioritize patching or mitigation.

Why it matters

CVE-2026-63573 in bc-csharp before 2.7.0 allows a Bleichenbacher-style attack, enabling recovery of encrypted content. Defenders handling encrypted messages should assess exposure, prioritize patching, and implement additional monitoring to prevent potential decryption and content exposure.

• Potential decryption and exposure of sensitive message content
• Information disclosure through decryption failure analysis
• Increased risk of targeted attacks using Bleichenbacher-style attacks
• Verification of patch deployment and configuration to prevent exploitation

Technical summary

The CMS RSA PKCS#1 v1.5 key-transport unwrap in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 is vulnerable to a Bleichenbacher-style adaptive chosen-ciphertext attack. This allows a remote attacker who holds a captured CMS EnvelopedData message and can submit many modified messages to an application that decrypts them with the recipient's RSA private key to recover the captured message's content-encryption key and its content.

Defensive priority

Defenders should prioritize patching or mitigating this vulnerability, especially in systems handling encrypted messages, to prevent potential decryption and content exposure.

Recommended defensive actions

• Patch bc-csharp to version 2.7.0 or later
• Implement additional monitoring for encrypted message handling
• Review and update incident response plans for potential decryption and content exposure
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The CVE description and source references indicate that bc-csharp versions before 2.7.0 are vulnerable to a Bleichenbacher-style attack, allowing recovery of the content-encryption key and message content. Official patches are available in versions 2.7.0 and later.

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.