CVE-2026-63566 Legion of the Bouncy Castle Inc. CVE debrief

Who should care

Defenders responsible for DTLS servers and clients using the Bouncycastle bc-csharp library should assess exposure and prioritize patching or mitigation to prevent potential memory exhaustion attacks.

Why it matters

The vulnerability in the Bouncycastle bc-csharp library can allow a remote unauthenticated DTLS peer to cause a denial of service through memory exhaustion. Defenders responsible for DTLS servers and clients should assess exposure and prioritize patching or mitigation to prevent potential memory exhaustion attacks.

• Denial of service through memory exhaustion
• Potential for significant memory allocation and exhaustion
• Need for patching or mitigation to prevent attacks
• Verification of DTLS server and client configurations required

Technical summary

The Bouncycastle bc-csharp library before version 2.7.0 has a vulnerability in the DTLS handshake reassembly process. The reassembly buffer for each incoming handshake message was allocated at the 24-bit length declared in the fragment header, without the check against the peer's maximum handshake message size that TLS already applied. This allows a remote unauthenticated DTLS peer to cause a denial of service through memory exhaustion via crafted handshake message fragments.

Defensive priority

Defenders should prioritize patching or mitigating this vulnerability, especially for DTLS servers and clients, to prevent potential memory exhaustion attacks.

Recommended defensive actions

• Patch or upgrade bc-csharp to version 2.7.0 or later
• Implement rate limiting or monitoring for DTLS handshake messages
• Consider using compensating controls such as IP blocking or traffic filtering
• Review and update DTLS server and client configurations to ensure secure settings
• Verify and enforce secure DTLS handshake message handling
• Conduct regular security audits to identify potential vulnerabilities
• Monitor system resources for signs of memory exhaustion attacks

Evidence notes

The vulnerability is caused by the excessive size of memory allocation in the DTLS handshake reassembly process. A crafted handshake message fragment can force an allocation of almost 16 MB for each of up to 16 pending messages per handshake, before the handshake is authenticated.

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.