CVE-2026-24281 in ZooKeeper
Summary
by MITRE • 03/07/2026
Hostname verification in Apache ZooKeeper ZKTrustManager falls back to reverse DNS (PTR) when IP SAN validation fails, allowing attackers who control or spoof PTR records to impersonate ZooKeeper servers or clients with a valid certificate for the PTR name. It's important to note that attacker must present a certificate which is trusted by ZKTrustManager which makes the attack vector harder to exploit. Users are recommended to upgrade to version 3.8.6 or 3.9.5, which fixes this issue by introducing a new configuration option to disable reverse DNS lookup in client and quorum protocols.
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Analysis
by VulDB Data Team • 04/09/2026
The vulnerability described in CVE-2026-24281 represents a critical flaw in Apache ZooKeeper's certificate validation mechanism that undermines the security of TLS communications within distributed systems. This issue resides in the ZKTrustManager component responsible for validating hostnames during TLS handshakes, creating a dangerous fallback mechanism that exposes systems to sophisticated impersonation attacks. The vulnerability specifically affects the hostname verification process where the system initially attempts to validate IP addresses against Subject Alternative Names (SAN) but fails to properly enforce certificate validation when this check fails.
The technical flaw manifests through a dangerous fallback behavior that automatically attempts reverse DNS (PTR) lookups when IP SAN validation does not succeed. This design decision creates a fundamental security weakness because PTR records can be controlled or spoofed by malicious actors, allowing them to present valid certificates for arbitrary hostnames that match the PTR record rather than the actual service endpoint. The vulnerability operates under the principle that attackers must already possess a certificate trusted by ZKTrustManager, which adds a layer of complexity to exploitation but does not eliminate the risk entirely. This requirement means that attackers cannot simply generate arbitrary certificates but must obtain legitimate certificates that have been pre-trusted within the ZooKeeper trust store, making the attack more sophisticated but still feasible.
From an operational impact perspective, this vulnerability enables man-in-the-middle attacks where adversaries can impersonate legitimate ZooKeeper servers or clients by leveraging DNS spoofing techniques combined with certificate manipulation. The attack vector becomes particularly dangerous in environments where DNS security is not properly enforced, as attackers can manipulate DNS records to match their malicious certificates. This weakness affects both client-to-server and quorum communication protocols, potentially compromising the entire distributed system's integrity and confidentiality. The vulnerability aligns with CWE-295 which addresses improper certificate validation and falls under ATT&CK technique T1552.001 for credentials from password storage, as compromised trust relationships can lead to unauthorized access to sensitive distributed system communications.
The recommended mitigation strategy involves upgrading to Apache ZooKeeper versions 3.8.6 or 3.9.5 which introduce a new configuration option specifically designed to disable reverse DNS lookups in both client and quorum protocols. This configuration change effectively eliminates the dangerous fallback behavior by preventing the system from automatically attempting PTR record validation when IP SAN validation fails. Organizations should also implement additional security measures such as enforcing strict certificate management policies, monitoring for unusual DNS changes, and ensuring that only necessary certificates are trusted within the ZooKeeper trust stores. The fix addresses the root cause by removing the insecure fallback mechanism and requiring explicit configuration for any reverse DNS validation, thereby aligning with security best practices for TLS certificate validation and reducing the attack surface for sophisticated adversaries targeting distributed systems.