# CVE-2026-59638: JSSE hostname verifier CN-fallback enabled by default despite documented opt-in

> Bouncy Castle for Java enables hostname verifier CN-fallback by default, bypassing intended security controls and allowing potential man-in-the-middle attacks.

- **Severity:** Critical 9.3 (`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/U:Amber`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 3 Aug 2026
- **Updated:** 3 Aug 2026
- **Also known as:** CLEANSTART-2026-OW17581, DEBIAN-CVE-2026-59638, GHSA-8rxj-3p7p-rq39, UBUNTU-CVE-2026-59638

## What is CVE-2026-59638?

CVE-2026-59638 is a critical vulnerability in Bouncycastle Bc-Java, published on 3 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official 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).

Weakness: CWE-297 (Improper Validation of Certificate with Host Mismatch)

## Which versions are affected by CVE-2026-59638?

It affects 3 products.


Products: Bouncycastle Bc-Java, Bouncycastle Bctls-Fips, Bouncycastle Bouncy Castle for Java Lts.

## How do I fix CVE-2026-59638?

Upgrade to a fixed version.

## Is CVE-2026-59638 being exploited?

There is no known exploitation of CVE-2026-59638 in the wild and no public exploit code.

## Other vulnerabilities in Bouncycastle Bc-Java

- [CVE-2007-6721](https://insights.sbomify.com/CVE-2007-6721/): Legion of the Bouncy Castle Java Cryptography API Bleichenbacher Oracle Vulnerability (critical 10.0)
- [CVE-2018-1000613](https://insights.sbomify.com/CVE-2018-1000613/): Deserialization of Untrusted Data in Bouncy castle (critical 9.8)
- [CVE-2026-58062](https://insights.sbomify.com/CVE-2026-58062/): Stapled OCSP response accepted without binding to the checked certificate (critical 9.3)
- [CVE-2026-59650](https://insights.sbomify.com/CVE-2026-59650/): MTI/A0 DH agreement exponentiates unvalidated peer value (critical 9.3)
- [CVE-2026-8763](https://insights.sbomify.com/CVE-2026-8763/): Name Constraints bypass via trailing dot in rfc822Name and URI (critical 9.3)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-59638 and fix it if it is affected.

What it is: CVE-2026-59638 is a critical vulnerability in Bouncycastle Bc-Java, published on 3 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-59638 in the wild and no public exploit code.

Affected product: Bouncycastle Bc-Java. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-59638/ (Markdown: https://insights.sbomify.com/CVE-2026-59638/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-59638
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-59638
- OSV: https://osv.dev/vulnerability/CVE-2026-59638

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-59638/
Summaries are generated and may contain mistakes; the official description above is the record.