What’s Vulnerable › Vulnerabilities

CVE-2026-63566: DTLS handshake reassembler allocates buffer from unchecked 24-bit length

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-10-02 · updated 2026-10-27
• Weakness: CWE-789

Description

Memory allocation with excessive size value in the DTLS handshake reassembly (DtlsReliableHandshake, DtlsReassembler) in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows a remote unauthenticated DTLS peer to cause a denial of service through memory exhaustion via crafted handshake message fragments, because 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. A fragment carrying no payload can force an allocation of almost 16 MB, for each of up to 16 pending messages per handshake, before the handshake is authenticated. DTLS servers and DTLS clients are both affected; TLS is not.

Affected products

• Legion of the Bouncy Castle Inc. bc-csharp: before 2.7.0 — fixed in 2.7.0
• Legion of the Bouncy Castle Inc. bc-csharp: before 2.7.0 — fixed in 2.7.0

How to fix

Upgrade to a fixed version: Legion of the Bouncy Castle Inc. bc-csharp 2.7.0; Legion of the Bouncy Castle Inc. bc-csharp 2.7.0. Check the vendor advisory for mitigations.

References

• https://github.com/bcgit/bc-csharp/commit/212d06474166d5e85be72c2dd4898313aa5dd5f3
• https://github.com/bcgit/bc-csharp/commit/99afef612458f70f28e7824a09719bfc3a169bda
• https://github.com/bcgit/bc-csharp/wiki/CVE-2026-63566

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