CVE-2026-40970: Spring Boot SSL Hostname Vulnerability

CVE-2026-40970 Overview

CVE-2026-40970 is a certificate validation bypass vulnerability in Spring Boot's Elasticsearch auto-configuration. When configured to use an SSL bundle, the application fails to perform hostname verification when connecting to the Elasticsearch server. This improper certificate validation (CWE-295) could allow an attacker on an adjacent network to intercept or manipulate traffic between the Spring Boot application and the Elasticsearch server through man-in-the-middle attacks.

Critical Impact

Attackers on adjacent networks can potentially intercept sensitive data exchanged between Spring Boot applications and Elasticsearch servers due to missing hostname verification in SSL connections.

Affected Products

• Spring Boot 4.0.0 through 4.0.5

Discovery Timeline

• April 27, 2026 - CVE-2026-40970 published to NVD
• April 27, 2026 - Last updated in NVD database

Technical Details for CVE-2026-40970

Vulnerability Analysis

This vulnerability stems from improper certificate validation in Spring Boot's Elasticsearch auto-configuration component. When applications configure SSL bundles for secure communication with Elasticsearch servers, the implementation fails to verify that the hostname in the server's certificate matches the expected hostname. This creates an opportunity for attackers positioned on the same network segment to perform man-in-the-middle attacks.

The vulnerability requires adjacent network access, meaning an attacker must be on the same local network or network segment as either the Spring Boot application or the Elasticsearch server. While the attack complexity is high due to the positioning requirements and the need to intercept SSL traffic, successful exploitation could compromise the confidentiality, integrity, and availability of data transmitted between the application and Elasticsearch.

Root Cause

The root cause is the omission of hostname verification logic in the SSL bundle configuration path for Elasticsearch connections. When SSL/TLS connections are established, certificate validation typically involves two steps: verifying the certificate chain is trusted and verifying the hostname matches. Spring Boot's Elasticsearch auto-configuration correctly validates the certificate chain but skips the hostname verification step, allowing certificates issued for different hostnames to be accepted.

Attack Vector

The attack vector requires adjacent network positioning (AV:A). An attacker must be able to intercept network traffic between the Spring Boot application and the Elasticsearch server. From this position, the attacker can present their own valid certificate (even one issued for a different hostname) during the SSL handshake. Since hostname verification is not performed, the connection proceeds, allowing the attacker to decrypt, view, and modify traffic before forwarding it to the legitimate server.

The attack scenario involves:

1. An attacker positions themselves on the network path between the Spring Boot application and Elasticsearch server
2. The attacker intercepts the SSL connection attempt
3. The attacker presents a valid certificate (potentially obtained for any domain they control)
4. Due to missing hostname verification, the Spring Boot application accepts the certificate
5. The attacker can now intercept and potentially modify all Elasticsearch queries and responses

Detection Methods for CVE-2026-40970

Indicators of Compromise

• Unexpected certificate warnings or changes in SSL certificate fingerprints for Elasticsearch connections
• Network traffic anomalies showing Elasticsearch communications being routed through unexpected intermediary hosts
• Audit logs showing Elasticsearch queries or responses with unexpected modifications or latency patterns

Detection Strategies

• Monitor network traffic for SSL/TLS connections to Elasticsearch that present certificates with hostnames not matching the configured Elasticsearch server
• Implement certificate pinning validation at the network layer to detect unauthorized certificates
• Review application logs for connection establishment patterns that indicate potential MITM scenarios

Monitoring Recommendations

• Enable detailed SSL/TLS logging in Spring Boot applications to capture certificate information during connections
• Deploy network intrusion detection systems (IDS) to monitor for ARP spoofing or other network-level attacks that could facilitate MITM positioning
• Implement regular certificate auditing to compare expected versus presented certificates for Elasticsearch connections

How to Mitigate CVE-2026-40970

Immediate Actions Required

• Upgrade Spring Boot to version 4.0.6 or later, which includes the security fix for hostname verification
• Review and audit all Spring Boot applications using SSL bundles with Elasticsearch connections
• Consider implementing additional network segmentation between application servers and Elasticsearch clusters to reduce adjacent network attack surface

Patch Information

VMware (Spring) has released Spring Boot version 4.0.6 that addresses this vulnerability. Organizations should upgrade to this version or later to ensure hostname verification is properly performed when using SSL bundles with Elasticsearch. For detailed patch information, refer to the Spring Security Advisory for CVE-2026-40970.

Workarounds

• Implement network-level controls such as firewalls and network segmentation to restrict which hosts can communicate with Elasticsearch servers
• Use mutual TLS (mTLS) authentication to provide an additional layer of verification beyond standard server certificate validation
• Deploy network monitoring to detect potential MITM attacks while awaiting the patch deployment

If immediate patching is not possible, consider implementing a custom HostnameVerifier in your Elasticsearch client configuration:

// Custom hostname verifier configuration
// Consult Spring documentation for proper implementation
// https://spring.io/security/cve-2026-40970