PoC Exploit Released for Apache Tomcat DoS Vulnerability

A critical memory leak vulnerability in Apache Tomcat’s HTTP/2 implementation (CVE-2025-31650) has been weaponized, enabling unauthenticated denial-of-service attacks through malformed priority headers.

The flaw affects Tomcat versions 9.0.76–9.0.102, 10.1.10–10.1.39, and 11.0.0-M2–11.0.5, with public exploits already circulating 12.

Vulnerability Mechanics and Attack Vector

According to the report, the vulnerability stems from the improper cleanup of failed HTTP/2 requests containing invalid priority headers.

When Tomcat rejects these malformed requests:

1. Memory allocations for stream objects aren’t fully released
2. Heap space gradually depletes due to the accumulation of OutOfMemoryError.
3. The server becomes unresponsive within minutes under sustained attack

The exploit leverages HTTP/2’s stream prioritization mechanism by sending headers like:

textpriority: u=-1, q=NaN

Attack scripts generate thousands of concurrent requests with 22 distinct invalid priority patterns, overwhelming Tomcat’s connection handlers.

Exploit Code Analysis

The publicly available Python PoC uses asynchronous requests to maximize attack efficiency:

pythonasync def send_invalid_priority_request(self, host, port, num_requests, task_id):
    async with httpx.AsyncClient(http2=True) as client:
        for _ in range(num_requests):
            headers = {
                "priority": random.choice(self.invalid_priorities),
                "user-agent": f"TomcatKiller-{task_id}-{random.randint(1,1e6)}"
            }
            await client.get(f"https://{host}:{port}/", headers=headers)

Key features include:

• 300+ concurrent tasks by default
• 100,000 requests/task throughput
• Randomized User-Agent strings to evade basic detection

Mitigation Strategies and Version Comparison

Recommended actions:

1. Immediate upgrade to Tomcat 9.0.104/10.1.40/11.0.6
2. Implement rate limiting (max 100 req/IP/second)
3. Monitor catalina.out for OutOfMemoryError entries
4. Use WAF rules to block requests containing: text^priority:s*(u=[^d]+|q=[^0-9.]+)

Security teams should validate HTTP/2 support using:

bashcurl -vI --http2 https://target:8443/ -o /dev/null

and monitor for HTTP/2 200 Responses indicating potential exposure.

This vulnerability highlights the risks of protocol implementation flaws in enterprise web infrastructure.

With a CVSSv3.1 score of 7.5 (HIGH) and working exploits in circulation, organizations must prioritize patching or risk extended service outages from trivial-to-execute memory exhaustion attacks.

The incident underscores the importance of rigorous fuzz testing for HTTP/2 implementations in application servers.

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