Improper Validation of Certificate with Host Mismatch Affecting org.bouncycastle:bctls-fips package, versions [,1.0.24)[2.0.19, 2.0.24)[2.1.20, 2.1.24)
Threat Intelligence
The probability is the direct output of the EPSS model, and conveys an overall sense of the threat of exploitation in the wild. The percentile measures the EPSS probability relative to all known EPSS scores. Note: This data is updated daily, relying on the latest available EPSS model version. Check out the EPSS documentation for more details.
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Test your applications- Snyk IDSNYK-JAVA-ORGBOUNCYCASTLE-18518000
- published4 Aug 2026
- disclosed3 Aug 2026
- creditAlex Gaynor
Introduced: 3 Aug 2026
CVE-2026-59638 (opens in a new tab)How to fix?
Upgrade org.bouncycastle:bctls-fips to version 1.0.24, 2.0.24, 2.1.24 or higher.
Overview
Affected versions of this package are vulnerable to Improper Validation of Certificate with Host Mismatch in the HostnameUtil class, whose "match CN when no dNSName SAN exists" fallback is gated on Properties.isOverrideSet(JSSE_HOSTNAME_CHECK_CN_FALLBACK, true), which returns true when the property is unset and so ships enabled by default, contrary to the documented opt-in behavior. An attacker can impersonate a target host and pass verification during a TLS handshake by presenting a certificate that carries no dNSName SAN and places the target hostname in its Common Name field. This applies only to certificates that lack a dNSName SAN and still chain to a trusted anchor, in deployments that have not explicitly set the fallback property to false.
Workaround
This vulnerability can be avoided by explicitly setting the JSSE_HOSTNAME_CHECK_CN_FALLBACK override property to false, which disables the Common Name fallback that is otherwise active by default.