CVE-2026-59638

CRITICAL

JSSE hostname verifier CN-fallback enabled by default despite documented opt-in

Title source: CVE List
Search associated code STIX 2.1 vulnerability STIX 2.1 with PoCs

Record summary

CVE-2026-59638 has a selected CVSS score of 9.3 (critical); EIP currently links 1 repository PoC.

Description

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), 2.0.24 (2.0.X series) and 2.1.24 (2.1.X series).

Description source: CVE List

Exploitation context

Available material

Repository PoCs
1

CISA SSVC decision

ExploitationNone
AutomatableYes
Technical impactTotal

CISA Coordinator · SSVC 2.0.3 · Evaluated Aug 3, 2026 · Source: CVE List

Affected products and versions

3
ProductSourceVersion range

BC-FJA

Browse Legion of the Bouncy Castle Inc. / BC-FJAbctls-fips

Default status: unaffected

CVE List1.0.7 to < 1.0.24affected
2.0.0 to < 2.0.24affected
2.1.0 to < 2.1.24affected

BC-JAVA

Browse Legion of the Bouncy Castle Inc. / BC-JAVAbctls

Default status: unaffected

CVE List1.61 to < 1.85affected

BC-LTS-JAVA

Browse Legion of the Bouncy Castle Inc. / BC-LTS-JAVAbctls-lts8on

Default status: unaffected

CVE List2.73.0 to < 2.73.12affected

Proofs of concept

1

Repository PoCs

GitHubxiaoqiMikko/bc-checkRepository PoCby xiaoqiMikko0Scanner19 files

149.7 KiB · linked to 4 vulnerabilities

GitHub
PoC details
Analysisdeepseek-v4-pro:cloud ·

Technical assessment

A Java-based offline scanner that identifies Bouncy Castle artifacts in JAR/WAR files and directories, determines their product line (BC, BC-LTS, FIPS), and checks them against a hardcoded table of 37 CVE rules to report which vulnerabilities apply. It does not exploit any vulnerability.

Backdoor review

No backdoor observed in reviewed code

The repository is a Java command-line tool (bc-check) that scans local JAR/WAR files for Bouncy Castle dependencies and checks them against a hardcoded table of 2026 CVEs. All reviewed source code (README, Java classes, Python generator) performs only local file scanning, version parsing, and CVE matching. No network I/O, command execution, data exfiltration, or concealed payloads were found. The tool's behavior is consistent with its documented purpose.

ClassificationScanner
Model confidence98%
AuthenticationNot required
LanguagesJavaPython
Target softwareBouncy Castle for JavaBouncy Castle for Java LTSBouncy Castle for Java FIPS
Attack typesCWE-295 Improper Certificate ValidationCWE-297 Improper Validation of Certificate with Host MismatchCWE-20 Improper Input Validation
Evidence & reasoningClassification basis · observed behavior · safety review
Technical evidence

Classification basis and observed behavior

Classification basis

The artifact is a vulnerability scanner. It reads local files to detect the presence and version of Bouncy Castle libraries, then compares them against a built-in CVE database to report exposure. It does not contain any code to trigger, exploit, or demonstrate the vulnerabilities; it only checks version ranges and reports matches.

README.md:3-4src/main/java/io/mikko/bccheck/Judge.java:107-150src/main/java/io/mikko/bccheck/Main.java:41-123

Requirements

  • Java runtime (targets Java 8) to execute the JARREADME.md:155-156
  • Access to target JAR/WAR files or directories to scanREADME.md:7-8

Observed behavior

  • Scans JAR/WAR/EAR files and directories recursively to locate Bouncy Castle artifactssrc/main/java/io/mikko/bccheck/JarScanner.java:69-81
  • Identifies artifacts via MANIFEST.MF, Maven pom.properties, or filename patternssrc/main/java/io/mikko/bccheck/JarScanner.java:104-108
  • Classifies each artifact into product line (BC, LTS, FIPS) and modulesrc/main/java/io/mikko/bccheck/Coordinate.java:56-91
  • Compares detected version against a hardcoded table of 37 CVE rules to determine vulnerability statussrc/main/java/io/mikko/bccheck/Judge.java:107-150
  • Outputs a report listing matched CVEs, severity, and recommended fix versionREADME.md:89-97
  • Operates entirely offline with no network access or data exfiltrationREADME.md:3-4
Safety-review evidence

Behaviors behind the backdoor verdict

Observables

Email Address
sikongjuechen@gmail.comContact email in README for consulting services, not embedded in code or used for data exfiltration.README.md:175
Review boundaries

What the analysis did not establish

  • Evidence includes only 8 of 19 repository files; 11 files are omitted (unclassified_bytes: 67151). The missing files may include additional logic, tests, or data files not analyzed.
  • The CVE rules are hardcoded in CveTable.java, generated from tools/data/bc_cves.json, which is not included in the provided text. The accuracy of the scanner depends on the correctness of that external data file.
  • The scanner only covers 2026 CVEs; earlier CVEs are explicitly out of scope per README.md lines 127-134.
  • 11 files in the repository were not provided as text and were not reviewed, including the pre-built JAR (bc-check.jar) and binary data files. The review is based solely on the 8 provided source/documentation files.
  • The tool's behavior when processing maliciously crafted JAR files (e.g., zip bombs) is bounded by documented limits (MAX_NESTED_ENTRY_BYTES, MAX_DEPTH) but was not tested.
Model interpretation

This review is limited to the supplied PoC code and context. It does not assert that the code works or is safe to execute.

References

5