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.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
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
Which versions are affected by CVE-2026-59638?
It affects 3 products.
Products include 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 Legion of the Bouncy Castle Java Cryptography API Bleichenbacher Oracle Vulnerability
- CVE-2018-1000613 Deserialization of Untrusted Data in Bouncy castle
- CVE-2026-58062 Stapled OCSP response accepted without binding to the checked certificate
- CVE-2026-59650 MTI/A0 DH agreement exponentiates unvalidated peer value
- CVE-2026-8763 Name Constraints bypass via trailing dot in rfc822Name and URI
Sources
Also known as CLEANSTART-2026-OW17581, DEBIAN-CVE-2026-59638, GHSA-8rxj-3p7p-rq39, UBUNTU-CVE-2026-59638.