CVE-2026-59638
Bouncycastle Bctls-Fips 1.0.0 – 1.0.24
Raw vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/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:X/V:X/RE:X/U:AmberCVSS and EPSS are reproduced from their sources (NVD, FIRST EPSS). Risk Priority is our own derived reading, not an NVD score.
Summary
CVE-2026-59638 is a critical-severity Improper Validation of Certificate with Host Mismatch (CWE-297) vulnerability in Bouncycastle Bctls-Fips. Its CVSS base score is 9.3 (Critical).
Operationally, exploitation aligns with the MITRE ATT&CK technique Adversary-in-the-Middle (T1557); ranked at the 12th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified map to SC-23 (Session Authenticity) and SC-8 (Transmission Confidentiality and Integrity) — see the control section below for these in your framework.
OWASP Top 10 for Web (2025)
EU & UK References
- 🇪🇺 ENISA EUVD: EUVD-2026-52016
Vulnerability Data
In Bouncy Castle for Java before 1.85, JSSE hostname verifier CN-fallback enabled by default despite documented opt-in. This issue also affects Bouncy Castle for Java LTS before 2.73.12, and Bouncy Castle for Java FIPS (BC-FJA) before bctls-fips 1.0.24 (1.0.X series),…
more
2.0.24 (2.0.X series) and 2.1.24 (2.1.X series).
- CWE(s)
Related Threats
MITRE ATT&CK Enterprise Techniques
CVEs Like This One
Affected Assets
Mitigating Controls
Control response
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V17.2.8
Mitigating Controls (NIST 800-53 r5) AI
Protecting session authenticity structurally requires proper certificate validation to bind the presented certificate to the intended host.
Requiring confidentiality and integrity of transmitted data necessitates correct TLS certificate validation with host-name checking.
Security engineering principles require correct implementation of certificate validation (including hostname matching) when cryptography is used.
Mitigating Controls (NIST CSF 2.0) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.
Server/service authentication via certificates directly requires correct host validation.
Protecting data-in-transit integrity depends on proper certificate validation to prevent MITM.
Secure development practices include coding correct certificate validation logic.
Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI
Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.
Network security controls include certificate validation and host-name verification to prevent man-in-the-middle attacks.
Security of network services requires proper TLS configuration and certificate validation for service endpoints.
Use of cryptography mandates correct implementation of certificate validation and hostname verification.
Application security requirements include secure communication and certificate validation for external services.
Secure system architecture principles require proper certificate validation to protect communications.
Secure coding practices include implementing proper certificate and hostname validation in code.