CVE-2026-40972: VMware Spring Boot RCE Vulnerability

CVE-2026-40972 Overview

CVE-2026-40972 is a timing attack vulnerability in VMware Spring Boot's DevTools remote secret comparison mechanism. An attacker positioned on the same network as the target application can exploit timing differences in secret validation to gradually discover the remote secret. In extreme circumstances, this could allow an attacker to determine the complete secret value and upload modified classes, ultimately achieving remote code execution in the vulnerable application.

Critical Impact

Successful exploitation could allow adjacent network attackers to discover the DevTools remote secret and achieve remote code execution by uploading malicious classes to the target Spring Boot application.

Affected Products

• VMware Spring Boot 4.0.0 through 4.0.5 (fixed in 4.0.6)
• VMware Spring Boot 3.5.0 through 3.5.13 (fixed in 3.5.14)
• VMware Spring Boot 3.4.0 through 3.4.15 (fixed in 3.4.16)
• VMware Spring Boot 3.3.0 through 3.3.18 (fixed in 3.3.19)
• VMware Spring Boot 2.7.0 through 2.7.32 (fixed in 2.7.33)
• Unsupported versions are also affected per vendor advisory

Discovery Timeline

• 2026-04-28 - CVE-2026-40972 published to NVD
• 2026-04-30 - Last updated in NVD database

Technical Details for CVE-2026-40972

Vulnerability Analysis

This vulnerability falls under CWE-208 (Observable Timing Discrepancy), a category of side-channel attacks where timing differences in cryptographic operations can leak sensitive information. The Spring Boot DevTools remote functionality provides a mechanism for developers to remotely restart applications during development. This feature requires a shared secret for authentication, but the secret comparison implementation was susceptible to timing analysis.

When an attacker can measure the time taken to compare a submitted secret against the stored secret, subtle differences in response times can reveal how many characters of the submitted value matched the actual secret. By systematically probing with different values and measuring response latencies, an attacker can incrementally reconstruct the secret character by character.

Root Cause

The vulnerability stems from a non-constant-time string comparison used when validating the DevTools remote secret. Standard string comparison operations typically terminate early when a mismatch is found, creating measurable timing variations based on how many characters match before failure. A constant-time comparison algorithm processes all characters regardless of match status, eliminating this timing side-channel.

Attack Vector

The attack requires the adversary to be positioned on the same network as the target application (adjacent network access). The attacker must be able to send requests to the DevTools remote endpoint and precisely measure response times. The attack complexity is high due to the need for accurate timing measurements and the statistical analysis required to distinguish signal from network noise.

The attacker would repeatedly submit authentication attempts with controlled secret values while measuring response latencies. Over many iterations, statistical patterns emerge that reveal the correct secret value. Once the secret is discovered, the attacker can authenticate to the DevTools remote endpoint and upload modified class files, leading to remote code execution.

For detailed technical information on the timing attack methodology, refer to the Spring Security Advisory CVE-2026-40972.

Detection Methods for CVE-2026-40972

Indicators of Compromise

• Unusual volume of failed authentication attempts to DevTools remote endpoints
• Repeated requests from the same source with slight variations in timing patterns
• Network traffic analysis showing systematic probing behavior against Spring Boot applications
• Unexpected class uploads or application restarts via DevTools remote

Detection Strategies

• Monitor and alert on repeated authentication failures to DevTools endpoints with timing-based patterns
• Implement rate limiting on DevTools remote authentication endpoints
• Analyze network traffic for sequential probing patterns characteristic of timing attacks
• Review application logs for anomalous DevTools remote activity

Monitoring Recommendations

• Enable detailed logging for Spring Boot DevTools remote authentication events
• Configure network intrusion detection systems to identify timing attack patterns
• Set up alerts for multiple failed DevTools authentication attempts from the same source
• Monitor for unexpected class modifications or uploads through DevTools

How to Mitigate CVE-2026-40972

Immediate Actions Required

• Upgrade Spring Boot to a patched version immediately: 4.0.6, 3.5.14, 3.4.16, 3.3.19, or 2.7.33
• Disable DevTools remote functionality in production environments if not strictly required
• Restrict network access to DevTools remote endpoints using firewall rules
• Implement strong, randomly-generated secrets for DevTools remote authentication

Patch Information

VMware has released patched versions of Spring Boot that implement constant-time secret comparison to eliminate the timing side-channel. The fixed versions are:

• Spring Boot 4.0.6 for the 4.0.x branch
• Spring Boot 3.5.14 for the 3.5.x branch
• Spring Boot 3.4.16 for the 3.4.x branch
• Spring Boot 3.3.19 for the 3.3.x branch
• Spring Boot 2.7.33 for the 2.7.x branch

For complete details, see the Spring Security Advisory CVE-2026-40972.

Workarounds

• Disable DevTools remote entirely by removing spring-boot-devtools from production dependencies
• Restrict network access to development machines only through firewall configuration
• Use network segmentation to isolate development environments from untrusted network segments
• Consider VPN or other secure tunneling for any necessary remote DevTools access

# Configuration example - Disable DevTools remote in application.properties
spring.devtools.remote.secret=
spring.devtools.remote.restart.enabled=false

# Alternative: Remove devtools dependency in pom.xml for production builds
# Set scope to runtime and optional in Maven:
# <optional>true</optional>
# <scope>runtime</scope>