CVE-2026-59969: Apache ZooKeeper: Improper validation of certificate with host mismatch in FIPS mode

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.3% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-16 · updated 2026-09-18
Weakness: CWE-297

Description

Apache ZooKeeper quorum TLS fails to enforce peer hostname verification in FIPS-mode deployments. When sslQuorum=true, zookeeper.fips-mode=true, ssl.quorum.hostnameVerification=true, and ssl.quorum.clientHostnameVerification=true are enabled, the Java SSLSocket quorum path accepts a CA-trusted peer certificate whose SAN does not match the connected host. A malicious or misissued peer certificate can therefore join quorum traffic, participate in leader election, and enter replication flows. Users are recommended to upgrade to version 3.8.7 or 3.9.6, which fixes the issue.

Affected products

Apache Software Foundation Apache ZooKeeper: from 3.8.0 before 3.8.7; from 3.9.0 before 3.9.6 — fixed in 3.8.7, 3.9.6

How to fix

Upgrade to a fixed version: Apache Software Foundation Apache ZooKeeper 3.8.7, 3.9.6. Check the vendor advisory for mitigations.

References

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