EUVD-2026-80331
GHSA-4pv3-cr63-m4ph
Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain
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AccessObtain CA-trusted cert with mismatched SAN
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DeliveryConnect to FIPS-mode quorum TLS port
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ExploitSSLSocket accepts peer without hostname check
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ExecutionJoin leader election and replication
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ImpactInject/corrupt replicated coordination state
Vulnerability Assessment
| Weakness |
CWE
OWASP Top 10:2025
A07:2025 - Authentication Failures
ASVS 5.0
V12 - Secure Communication Certificate host validation
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| Exploitation Conditions | Exploitation requires a specific, non-default quorum-TLS configuration in which ALL of these are enabled simultaneously: sslQuorum=true, zookeeper.fips-mode=true, ssl.quorum.hostnameVerification=true, and ssl.quorum.clientHostnameVerification=true - i.e., FIPS-mode quorum TLS. The attacker must also possess a certificate that chains to a CA already trusted by the quorum truststore (a misissued or wrong-host CA-trusted cert); a random self-signed cert will not work. Limiting factors: the FIPS-mode configuration is a minority deployment, the CA-trusted certificate requirement is a substantial barrier, and impact is confined to integrity of the quorum (leader election/replication) with no confidentiality or availability scored. Plain (non-FIPS) TLS deployments and ensembles without quorum TLS are unaffected. |
| Impact | Signals converge on this being a genuine but narrowly-scoped integrity issue rather than a broad emergency. The vendor CVSS 3.1 vector (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N = 7.5) rates it network-reachable and unauthenticated, but the description contradicts AC:L: exploitation is gated behind FOUR non-default settings all being enabled together (sslQuorum=true, zookeeper.fips-mode=true, ssl.quorum.hostnameVerification=true, ssl.quorum.clientHostnameVerification=true) AND the attacker must present a certificate signed by a CA already in the quorum truststore (misissued or otherwise CA-trusted) - that is a real attack requirement, so AC:H is more honest (my assessed ~5.9). EPSS is very low at 0.12% (2nd percentile), there is no CISA KEV listing and no public exploit identified, consistent with a config-dependent, hard-to-reach flaw. Impact is integrity-only: a rogue peer joining leader election and replication can corrupt/inject coordination state - vendor set C:N/A:N, though a hostile quorum member could plausibly also affect availability of the ensemble (not scored). Net: real for FIPS-mode ZooKeeper operators to patch, but not a widespread automated-exploitation priority. Note the 'Information Disclosure' tag conflicts with the C:N/I:H vector - this is an integrity flaw, not confidentiality. |
| Exploit ScenarioDerived | An operator runs a ZooKeeper 3.8.x/3.9.x ensemble with quorum TLS in FIPS mode and hostname verification nominally enabled. An attacker who obtains a certificate trusted by the quorum CA (e.g., a misissued cert or one issued for a different host) connects to the quorum port; because the FIPS SSLSocket path skips SAN/host matching, the peer is accepted, joins leader election, and injects or manipulates replicated data. No public exploit code is identified at time of analysis. |
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Alert meAnalysisAIv2 – Sep 17, 2026
Improper certificate hostname validation in Apache ZooKeeper's quorum TLS path allows a CA-trusted peer certificate whose SAN does not match the connected host to be accepted when FIPS mode and quorum hostname verification are enabled, letting the holder of such a certificate join quorum traffic, participate in leader election, and enter replication flows. Only deployments that simultaneously set sslQuorum=true, zookeeper.fips-mode=true, ssl.quorum.hostnameVerification=true and ssl.quorum.clientHostnameVerification=true on affected releases (3.8.0 through 3.8.6 and 3.9.0 through 3.9.5) are exposed; plain TLS and non-quorum-TLS ensembles are unaffected. Exploitation additionally requires a certificate chaining to a CA already trusted by the quorum truststore, so the practical barrier is high; no public exploit code has been identified at time of analysis, and EPSS is 0.12% (2nd percentile). Unauthenticated remote attackers are in scope per the CVSS vector (PR:N), though the certificate precondition makes real-world abuse a targeted rather than opportunistic scenario.
Earlier versions of this analysis (1)
Apache ZooKeeper quorum TLS peers running in FIPS mode can skip hostname verification, so a certificate whose SAN does not match the connected host is still accepted, letting a rogue peer join quorum traffic, participate in leader election, and enter replication flows. The flaw affects org.apache.zookeeper:zookeeper 3.8.0 through 3.8.6 and 3.9.0 through 3.9.5, but only in deployments where sslQuorum=true, zookeeper.fips-mode=true, and both ssl.quorum.hostnameVerification and ssl.quorum.clientHostnameVerification are enabled. Exploitation additionally requires the attacker to hold a certificate issued by a CA the cluster already trusts, making this a misissuance or CA-compromise scenario rather than a generic remote attack; no public exploit code or CISA KEV entry was identified at time of analysis, and fixes are released in 3.8.7 and 3.9.6.
RemediationAI
Vendor-released patch: Apache ZooKeeper 3.8.7 and 3.9.6 both fix CVE-2026-59969, so upgrade to the branch-appropriate minimum (3.8.7 for the 3.8 line, 3.9.6 for the 3.9 line) and see the Apache advisory at https://zookeeper.apache.org/ and the release thread at https://lists.apache.org/thread/qf9kwsgtgbgtvwls6f41281846xg0rss. If immediate upgrade is not possible, the most direct mitigation is to turn off quorum TLS hostname-verification dependence by disabling FIPS mode (zookeeper.fips-mode=false) - but this side-steps the FIPS compliance posture many regulated environments require, so it should be treated as a temporary measure and not applied where FIPS validation is mandatory; disabling sslQuorum entirely is worse, as it removes transport encryption and peer authentication altogether. Compensating controls with meaningful effect: harden the quorum truststore so it contains only CAs that issue strictly for the ZooKeeper quorum hosts, shrink or split the issuing CA to eliminate wildcard or multi-purpose certificates (a wrong-host cert is the core prerequisite for exploitation), pin and inventory every certificate in the truststore, and rotate anything that was issued with broad SAN coverage. Network-side, restrict quorum ports (2888/3888 by default) to the known ensemble peers via host firewalls or network policy, and monitor quorum connection logs and peer certificate subjects for unexpected members joining leader election or replication. Note that none of these controls fully restores hostname binding the way the patched code does; they reduce the likelihood that a usable CA-trusted certificate lands in an attacker's hands.
Affected VersionsENISA
Affected Packages
Source: OSV (Open Source Vulnerabilities)
Affects your stack
Affects 1 package in euvd
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Technical ContextAI · show details
The flaw is rooted in CWE-297 (Improper Validation of Certificate with Host Mismatch), implemented in the Java SSLSocket-based quorum communication layer. ZooKeeper's quorum transport, when sslQuorum is enabled, uses SSLSocket for server-to-server (and quorum client) connections; hostname verification is intended to be enforced via ssl.quorum.hostnameVerification and ssl.quorum.clientHostnameVerification. In FIPS-mode deployments (zookeeper.fips-mode=true), the verification path fails to compare the peer certificate's Subject Alternative Name against the host actually connected, so the identity binding that hostname verification is meant to provide is effectively skipped. The result is that authentication degenerates into pure trust-chain validation: any certificate signed by a CA present in the quorum truststore is accepted regardless of which host it was issued for. This matters because ZooKeeper quorum membership drives leader election and the replication pipeline, so an accepted rogue peer can influence the metadata/state consistency of the ensemble - an integrity issue (C:N/I:H/A:N) with no scored confidentiality or availability impact. The assessed vector (AV:N/AC:H) reflects that the attack is network-reachable but requires the specific FIPS quorum-TLS configuration plus a misissued or wrong-host CA-trusted certificate; the vendor's published CVSS of 7.5 with AC:L is more permissive than our assessment.
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Severity by source
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
Network-reachable and unauthenticated to ZooKeeper (PR:N), but AC:H because exploitation demands both a rare FIPS-mode config and a CA-trusted mismatched cert; impact is integrity-only (I:H, C:N/A:N).
Primary rating from Vendor (CNA).
CVSS VectorVendor
Lifecycle Timeline
3Description PRE-NVD
Executive ViewAI
Apache ZooKeeper - the coordination service that Kafka, Hadoop, and many internal distributed platforms rely on for leader election and configuration - contains a hostname-validation flaw in its quorum TLS path that, under a narrow set of FIPS-mode configurations, can allow a certificate holder to impersonate a legitimate quorum peer. The exposure is limited to organizations running ZooKeeper 3.8.0-3.8.6 or 3.9.0-3.9.5 with quorum TLS, FIPS mode, and quorum hostname verification all enabled simultaneously, which typically means teams operating under FIPS compliance mandates. If abused, unauthorized participation in leader election and replication could compromise the integrity of the clusters that depend on ZooKeeper, so leadership should treat this as a cluster-integrity risk rather than a routine library update.
No vendor-released patch identified at time of analysis; interim mitigations are to restrict quorum port 2888/3888 access to known peer addresses, narrow the quorum truststore to explicit per-peer certificates or a tightly constrained intermediate CA rather than a broad root, monitor quorum membership and leader election events for anomalies, and if policy permits, disable FIPS mode for the quorum TLS listener since the flaw only manifests with FIPS mode enabled.
Threat Intelligence Overview
NVD
CVE.org
EPSS
VulDB
How this decision was reached
The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.
What every action level means
SSVC AssessmentENISA
Priority Score
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Apache ZooKeeper 3.8.5 and 3.9.4 improperly log sensitive client configuration data at INFO level, allowing unauthentica
No authentication/authorization is enforced when a server attempts to join a quorum in Apache ZooKeeper before 3.4.10, a
An issue is present in Apache ZooKeeper 1.0.0 to 3.4.13 and 3.5.0-alpha to 3.5.4-beta. Rated medium severity (CVSS 5.9),
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Exploit-DB