Proof-of-Concept Released for Apache Tomcat CVE-2025-31650 Denial-of-Service Vulnerability

A new proof-of-concept exploit targeting CVE-2025-31650 in Apache Tomcat versions 10.1.10-10.1.39 has been publicly released, demonstrating a critical HTTP/2 priority header vulnerability that enables memory exhaustion attacks.

The exploit leverages malformed header injection to trigger catastrophic memory leaks in Tomcat’s HTTP/2 implementation.

Exploit Mechanism and Technical Analysis

The Python-based tool (TomcatKiller.py) uses asynchronous requests with intentionally corrupted HTTP/2 priority parameters:

pythoninvalid_priorities = [
    "u=-1, q=2",
    "u=4294967295, q=-1",
    "u=-2147483648, q=1.5",
    "u=0, q=invalid",
    # 17 additional malformed variants
]

Key attack components:

1. Concurrency Engine: Uses httpx.AsyncClient with 300 parallel tasks (configurable) sending 100,000 requests each
2. Header Bombardment: Randomly selects invalid priority combinations to bypass parsing safeguards
3. Memory Stress Test: Forces JVM heap allocation through recursive priority queue buildup

The attack manifests as OutOfMemoryError in Tomcat logs:

textSEVERE [http-nio-8443-exec-7] org.apache.coyote.http2.Http2Parser.upgrade Http2Parser
java.lang.OutOfMemoryError: Java heap space

Impact and Mitigation Strategies

Affected Systems:

• Tomcat 10.1.x deployments with HTTP/2 enabled
• Reverse proxies forwarding HTTP/2 traffic to Tomcat backends

Immediate Mitigations:

1. Apply Tomcat’s official patch (10.1.40+) when available
2. HTTP/2 Disable Workaround (server.xml):

xml<Connector port="8443" protocol="org.apache.coyote.http11.Http11NioProtocol"
    maxThreads="150" SSLEnabled="true">
    <UpgradeProtocol className="org.apache.coyote.http2.Http2Protocol"
        enabled="false" />
</Connector>

3. Implement WAF rules to block malformed priority headers:

bashSecRule REQUEST_HEADERS:priority "@rx (?:u\s*=\s*-|q\s*=\s*[^0-1\.])" \
    "id:1005,log,deny,msg:'Invalid HTTP/2 Priority Header'"

Detection Methods:

• Monitor JVM memory usage: jstat -gcutil <tomcat_pid> 5s
• Network traffic analysis using Wireshark filter:
  http2.headers.priority.urgency == 4294967295 || http2.headers.priority.weight < 0

This vulnerability highlights critical risks in HTTP/2 implementation edge cases. Enterprises should conduct immediate memory stress tests on Tomcat instances and consider temporary HTTP/1.1 rollbacks for high-value targets until patched versions become available.

The PoC’s release underscores the urgency for proactive header validation and protocol implementation hardening in web servers.