CVE-2026-59638Network attack vector

TLS host impersonation via default CN fallback

Published Aug 3, 2026 · Updated Aug 3, 2026

Certificate hostname validation in affected BC Java, BC-LTS, and BC-FJA TLS releases allows remote attackers to impersonate trusted hosts. HostnameUtil supplies true as the default to Properties.isOverrideSet for JSSE_HOSTNAME_CHECK_CN_FALLBACK, so an unset opt-in property enables Common Name matching when no dNSName SAN exists. A trusted chain with a DNS-constrained intermediate and a leaf lacking a dNSName SAN lets an attacker-chosen Common Name pass verification, exposing client traffic to interception or alteration.

CVSS severity9.3
Critical
EPSS probability0.21%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Certificate hostname validation in affected BC Java, BC-LTS, and BC-FJA TLS releases allows remote attackers to impersonate trusted hosts. HostnameUtil supplies true as the default to Properties.isOverrideSet for JSSE_HOSTNAME_CHECK_CN_FALLBACK, so an unset opt-in property enables Common Name matching when no dNSName SAN exists. A trusted chain with a DNS-constrained intermediate and a leaf lacking a dNSName SAN lets an attacker-chosen Common Name pass verification, exposing client traffic to interception or alteration.

The record

CVE
CVE-2026-59638
Published
Aug 3, 2026
Updated
Aug 3, 2026
Vendor
Legion of the Bouncy Castle Inc.
Product
Bouncy Castle Java
Classifications
CWE-297, T1557
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 3, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=1.61 <1.85

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Legion of the Bouncy Castle Inc. · Bouncy Castle Javaversion=1.61 <1.85
Legion of the Bouncy Castle Inc. · Bouncy Castle FIPS Java APIversion=1.0.7 <1.0.24; version=2.0.0 <2.0.24; version=2.1.0 <2.1.24
Legion of the Bouncy Castle Inc. · Bouncy Castle Java LTSversion=2.73.0 <2.73.12

Published CVSS scores

9.3Legion of the Bouncy Castle Inc.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

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-297Improper Validation of Certificate with Host Mismatch
T1557Adversary-in-the-Middle

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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