Vulnerability · Published Aug 3, 2026 · Assigned by bcorg
CVE-2026-59638JSSE hostname verifier CN-fallback enabled by default despite documented opt-in
Critical 9.3
In plain English
A critical-severity security flaw in BC-FJA from Legion of the Bouncy Castle Inc.. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 28% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
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).
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | bouncycastle | no fix listed | DEBIAN-CVE-2026-59638 |
| Debian:13 | bouncycastle | no fix listed | DEBIAN-CVE-2026-59638 |
| Debian:14 | bouncycastle | 1.86-1 | DEBIAN-CVE-2026-59638 |
| Ubuntu:26.04:LTS | bouncycastle | no fix listed | UBUNTU-CVE-2026-59638 |
| Ubuntu:26.10 | bouncycastle | no fix listed | UBUNTU-CVE-2026-59638 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Legion of the Bouncy Castle Inc. | BC-JAVA | 1.61 – before 1.85 |
| Legion of the Bouncy Castle Inc. | BC-LTS-JAVA | 2.73.0 – before 2.73.12 |
| Legion of the Bouncy Castle Inc. | BC-FJA | 1.0.7 – before 1.0.242.0.0 – before 2.0.242.1.0 – before 2.1.24 |
Weakness: CWE-297 · 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/U:Amber