CVE-2025-52434: Apache Tomcat Race Condition Vulnerability

CVE-2025-52434 Overview

CVE-2025-52434 is a race condition vulnerability in Apache Tomcat affecting the APR/Native connector. The flaw stems from concurrent execution using a shared resource with improper synchronization [CWE-362]. The issue is most observable when clients initiate closes of HTTP/2 connections, causing denial of service conditions on the server.

The vulnerability affects Apache Tomcat versions 9.0.0.M1 through 9.0.106, as well as end-of-life versions 8.5.0 through 8.5.100. Older EOL releases may also be affected. Apache has released version 9.0.107 to address the issue.

Critical Impact
Remote attackers can trigger the race condition over the network without authentication, resulting in availability impact on Tomcat instances using the APR/Native connector.

Affected Products

Apache Tomcat 9.0.0.M1 through 9.0.106
Apache Tomcat 8.5.0 through 8.5.100 (EOL)
Older EOL Apache Tomcat versions using the APR/Native connector

Discovery Timeline

2025-07-10 - CVE-2025-52434 published to the National Vulnerability Database
2025-07-10 - Vendor advisory published on the Apache mailing list
2025-11-04 - Last updated in NVD database

Technical Details for CVE-2025-52434

Vulnerability Analysis

The vulnerability resides in Apache Tomcat's APR/Native connector, which provides native I/O and SSL support through the Apache Portable Runtime (APR) library. Tomcat handles HTTP/2 connections using multiplexed streams over a single TCP connection. When multiple threads operate on shared connection state without proper synchronization, the race condition manifests during connection teardown.

The issue surfaces most reliably when a client initiates closure of an HTTP/2 connection. Concurrent threads access shared connection resources during the close sequence, producing inconsistent state. This inconsistent state can lead to resource exhaustion or service disruption on the Tomcat server.

Root Cause

The root cause is improper synchronization between threads operating on shared connection objects in the APR/Native connector's HTTP/2 handling path. The connector did not adequately serialize access to connection state during client-initiated close events. The defect is classified under CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization).

Attack Vector

The vulnerability is exploitable over the network without authentication or user interaction. An attacker establishes HTTP/2 connections to a vulnerable Tomcat server configured with the APR/Native connector and then closes those connections in a manner designed to trigger the race window. Repeated exploitation degrades availability of the affected service. No proof-of-concept exploit is publicly available, and the issue is not listed in the CISA Known Exploited Vulnerabilities catalog.

Detection Methods for CVE-2025-52434

Indicators of Compromise

Unexpected Tomcat connector thread errors or stack traces referencing HTTP/2 stream handling and APR/Native components in catalina.out
Abnormal volumes of short-lived HTTP/2 connections from a single source followed by client-initiated GOAWAY or RST_STREAM frames
Service availability degradation, hung worker threads, or connector restarts coinciding with HTTP/2 traffic spikes

Detection Strategies

Inventory all Apache Tomcat instances and identify those using the APR/Native connector with HTTP/2 enabled
Compare deployed Tomcat versions against the affected ranges (9.0.0.M1 through 9.0.106 and 8.5.x)
Review web server logs for anomalous HTTP/2 connection churn patterns indicative of probing for race conditions

Monitoring Recommendations

Monitor connector thread pool utilization, request queue depth, and JVM thread counts for sudden anomalies
Alert on repeated connector exceptions, native library crashes, or unscheduled Tomcat process restarts
Forward Tomcat access and error logs to a centralized logging or SIEM platform for correlation with network telemetry

How to Mitigate CVE-2025-52434

Immediate Actions Required

Upgrade Apache Tomcat to version 9.0.107 or later, which contains the synchronization fix
Retire end-of-life 8.5.x deployments and migrate workloads to a supported, patched release branch
Audit reverse proxy and load balancer configurations to confirm HTTP/2 traffic terminates on patched Tomcat instances

Patch Information

Apache Tomcat 9.0.107 resolves the race condition in the APR/Native connector. Release details are documented in the Apache Tomcat Mailing List Advisory. Linux distribution updates are available through the Debian LTS Announcement and the Openwall OSS-Security Update.

Workarounds

Switch the connector from APR/Native to the NIO or NIO2 Java-based connectors as an interim measure if patching is delayed
Disable HTTP/2 on the affected connector by removing the UpgradeProtocol configuration in server.xml until the upgrade is applied
Place rate limiting in front of Tomcat to reduce the volume of rapid HTTP/2 connection open/close sequences from individual clients

bash

# server.xml - remove or comment the HTTP/2 UpgradeProtocol on the APR connector
# <Connector port="8443" protocol="org.apache.coyote.http11.Http11AprProtocol"
#            SSLEnabled="true" maxThreads="200">
#     <UpgradeProtocol className="org.apache.coyote.http2.Http2Protocol" />
# </Connector>

# Verify installed Tomcat version
$CATALINA_HOME/bin/version.sh