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Vulnerability record · CVE-2026-63566 · published 2 October 2026

CVE-2026-63566: Bouncycastle bc-csharp vulnerability

Bouncycastle · Bc Csharp

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.

8.7 CVSS 4.0 High EPSS 0.43% · top 62.0% CWE-789 · CWE-789
8.7CVSS 4.0 base score
0.43%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
3References
27 Oct 2026Last modified by NVD

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.

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://github.com/bcgit/bc-csharp/commit/212d06474166d5e85be72c2dd4898313aa5dd5f3 Patch
https://github.com/bcgit/bc-csharp/commit/99afef612458f70f28e7824a09719bfc3a169bda Patch
https://github.com/bcgit/bc-csharp/wiki/CVE-2026-63566 PatchVendor Advisory

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

9.1CVE-2026-63569Bouncycastle bc-csharp improper input validation vulnerabilityImproper input validation in DHAgreement.CalculateAgreement (MTI/A0 two-pass Diffie-Hellman) in Legion of the Bouncy Castle Inc. bc-csharp before 2.7…EPSS 0.33%8.7CVE-2026-63574Bouncycastle bc-csharp vulnerabilityMemory allocation with excessive size value in the OpenPGP signature and user attribute subpacket parsers (SignatureSubpacketsParser.ReadPacket, User…EPSS 0.43%8.7CVE-2026-63571Bouncycastle bc-csharp improper verification of cryptographic signature vulnerabilityImproper verification of cryptographic signature in the attribute certificate path validator (PkixAttrCertPathValidator, also used by PkixAttrCertPat…EPSS 0.14%8.7CVE-2026-103600Bouncycastle bc-csharp vulnerabilityUncontrolled recursion in the ASN.1 parser (Asn1InputStream, Asn1StreamParser) in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allows a re…EPSS 0.36%8.7CVE-2026-103603Bouncycastle bc-csharp vulnerabilityMemory allocation with excessive size value in the HSS/LMS signature code (HssPublicKeyParameters, HssSignature) in Legion of the Bouncy Castle Inc. …EPSS 0.52%8.7CVE-2026-103604Bouncycastle bc-csharp vulnerabilityInefficient algorithmic complexity in X.509 distinguished name string conversion (X509Name.ToString and IetfUtilities.ValueToString) in Legion of the…EPSS 0.32%8.7CVE-2026-63568Bouncycastle bc-csharp allocation without limits vulnerabilityAllocation of resources without limits or throttling in the CMP/CRMF password-based MAC verifier (PKMacBuilder) in Legion of the Bouncy Castle Inc. b…EPSS 0.36%8.7CVE-2026-16000Bouncycastle bc-csharp vulnerabilityMissing cryptographic step in the DSTU 7624 CCM mode implementation (KCcmBlockCipher) in Legion of the Bouncy Castle Inc. bc-csharp before 2.7.0 allo…EPSS 0.22%

Source: NIST National Vulnerability Database (record CVE-2026-63566), CISA KEV, FIRST EPSS (scores of 2026-11-14). This page is refreshed as NVD updates the record.