A Deep Dive into the Tomcat HTTP/2 Vulnerability (CVE-2025-31650)

In the world of web infrastructure, Apache Tomcat is a titan, powering countless Java-based web applications across the globe. But even titans can have an Achilles' heel. Recently, a critical Denial of Service (DoS) vulnerability, identified as CVE-2025-31650, emerged in Tomcat version 10.1.39. This flaw is particularly dangerous because it allows a remote, unauthenticated attacker to reliably and swiftly crash a server with a flood of specially crafted requests, causing complete service unavailability.

This is not a theoretical vulnerability. To understand its real-world impact, I set up a vulnerable Tomcat instance in a Proxmox container and, by leveraging a public proof-of-concept, successfully triggered the crash.

In this deep dive, we will deconstruct the vulnerability from the ground up. We will explore the technical nuances of the HTTP/2 protocol that the exploit abuses, walk through a step-by-step replication of the attack, and provide a robust playbook for detection and mitigation using modern SIEM, EDR, and network security tools.

The Technical Heart of the Matter: Abusing HTTP/2 Stream Prioritization

The root cause of CVE-2025-31650 lies not in Tomcat's application logic, but in its implementation of a core feature of the HTTP/2 protocol: stream prioritization.

HTTP/2 was designed for efficiency, allowing multiple requests (streams) to be sent over a single TCP connection. To manage this, the protocol includes a PRIORITY frame. This frame is a message from the client that tells the server how to allocate resources, suggesting which streams are more important than others (e.g., "load the CSS before the images").

How the Exploit Works

The public proof-of-concept exploit, originally authored by xaitax and available on Exploit-DB as EBD-ID 52318, turns this feature into a weapon. The attack is a textbook example of flawed input handling and resource exhaustion:

1. Establish Connection: The attacker first establishes a standard HTTP/2 connection with the vulnerable Tomcat server.
2. The Malformed Flood: Instead of sending valid priority information, the exploit script unleashes a high-volume flood of PRIORITY frames. These frames are intentionally malformed with nonsensical and invalid data, such as negative numbers, floating-point values where integers are expected, or out-of-bounds numbers.
3. Flawed Processing Logic: A secure server should immediately identify these malformed frames, reject them, and potentially drop the connection. However, the vulnerable Tomcat 10.1.39 attempts to process this invalid data.
4. Resource Exhaustion: This flawed processing is catastrophic. Each attempt to interpret the garbage data triggers an error-handling routine that consumes significant CPU and memory resources. When the attacker sends thousands of these requests per second, the server becomes locked in a death spiral of spawning threads that it cannot manage.
5. The Crash: The rapid and uncontrolled resource consumption quickly exhausts the Java Virtual Machine's (JVM) allocated memory heap. This culminates in a fatal java.lang.OutOfMemoryError, which crashes the worker threads and brings the entire Tomcat service to a grinding halt, resulting in a complete Denial of Service.

Seeing is Believing: A Practical Lab Replication

Warning and Ethical Disclaimer: The following steps are for educational purposes only and should NEVER be performed on any system you do not own or have explicit, written permission to test. All actions should be confined to a secure, isolated lab environment.

To witness the exploit firsthand, I configured a victim server in a Proxmox container. The process highlighted a key requirement for the attack to succeed: the vulnerable HTTP/2 protocol handler must be explicitly enabled, as it is often disabled by default.

1. Victim Server Setup (Ubuntu 22.04)

1. Install Dependencies: First, install Java, which is required to run Tomcat.

sudo apt update && sudo apt install openjdk-17-jdk -y

2. Download and Extract Vulnerable Tomcat:

wget https://archive.apache.org/dist/tomcat/tomcat-10/v10.1.39/bin/apache-tomcat-10.1.39.tar.gz
tar -xvzf apache-tomcat-10.1.39.tar.gz

3. Enable the Vulnerable Service (Crucial Step): To make the server vulnerable, you must edit the server.xml configuration file to enable the HTTP/2 protocol. The exploit will not work without this change. By default, this protocol upgrade line is commented out.

nano apache-tomcat-10.1.39/conf/server.xml

You must find the TLS-enabled Connector for port 8443 and uncomment the <UpgradeProtocol> line. This action exposes the vulnerable HTTP/2 processing logic that the exploit targets.

<UpgradeProtocol className="org.apache.coyote.http2.Http2Protocol" />

4. Start the Server:

cd apache-tomcat-10.1.39/bin/
chmod +x *.sh
./startup.sh

At this point, the Tomcat service was running and accessible at https://<VM_IP_ADDRESS>:8443.

2. Attacker Machine Setup

1. Install Python Libraries: On a separate attacker machine, install the libraries required by the PoC script.

pip install httpx colorama

2. Download the PoC Script: Save the raw Python code from Exploit-DB ID: 52318 as 52318.py. Note: If you can't make the script work, you shouldn't be running this!

3. Execution and Confirmation

1. Run the Exploit:

python 52318.py

2. Enter Target URL: When prompted, I provided the HTTPS endpoint: https://<VM_IP_ADDRESS>:8443, 'enter', 'enter'.

Within seconds, the terminal running the exploit script showed a flurry of requests. The server immediately became unresponsive. Attempts to load the Tomcat page resulted in a connection timeout.

The definitive proof was in the server logs. Tailing the catalina.out log file on the victim VM revealed the "smoking gun": a java.lang.OutOfMemoryError, confirming the resource exhaustion crash.

The Blue Team Response: Detection and Defense

Understanding the attack is only half the battle. For defenders, the focus is on detection and mitigation. The exploit's noisy nature provides several opportunities for defenders to identify and block it.

Detection & Hunting with SIEM/EDR

The attack generates two primary indicators: an anomalous volume of traffic from a single source IP and, in the case of the public PoC, a unique User-Agent string (TomcatKiller).

Here are practical hunting queries for common security platforms:

• Splunk
• Hunt for the specific User-Agent:

index=web sourcetype=tomcat:access "TomcatKiller"
| stats count by src_ip, user_agent

• Look for a high volume of requests from a single source, which would catch attackers who change the User-Agent:

index=web sourcetype=tomcat:access
| stats count by src_ip
| where count > 1000

• Kibana (Elasticsearch Query Syntax)
• Find the malicious User-Agent:

user_agent.original : "TomcatKiller"

• Aggregate by client IP to find potential flood attacks (e.g., more than 500 requests in the last 15 minutes):

{
"query": { "range": { "@timestamp": { "gte": "now-15m/m" }}},
"aggs": {
"ip_flood": {
"terms": { "field": "client.ip", "size": 10, "min_doc_count": 500 }
}
}
}

• Microsoft Sentinel (Kusto Query Language - KQL)
• Find requests with the PoC's User-Agent:

CommonSecurityLog
| where DeviceVendor == "Apache" and DeviceProduct == "Tomcat"
| where HttpUserAgent == "TomcatKiller"
| summarize RequestCount = count() by SourceIP, HttpUserAgent

• Identify anomalous traffic spikes from a single IP:

CommonSecurityLog
| where DeviceVendor == "Apache" and DeviceProduct == "Tomcat"
| summarize RequestCount = count() by SourceIP
| where RequestCount > 1000

Building Resilient Defenses: Mitigation Strategies

Defending against CVE-2025-31650 requires a multi-layered approach, starting with the definitive fix and supplemented by network-level protections.

1. Upgrade (Primary Mitigation): The most effective and permanent solution is to upgrade Apache Tomcat to any version higher than 10.1.39. Patching the vulnerability at the source removes the attack vector entirely.

2. Web Application Firewall (WAF) / Load Balancer Rules:

• Aggressive Rate-Limiting: Implement a rate-limiting rule on your WAF, load balancer, or reverse proxy. A rule that blocks a source IP after it makes an excessive number of requests (e.g., 200 requests in one minute) can stop the attack before it exhausts the server's resources.
• Block Malicious User-Agent: A simple but effective rule is to block any incoming request where the User-Agent header is TomcatKiller. While trivial for a determined attacker to bypass, this will block the public PoC and other low-effort attempts.

3. Intrusion Prevention System (IPS) Signature: For a more robust and sophisticated defense, an IPS signature can detect the attack's underlying mechanism. The signature should focus on identifying a high frequency of malformed HTTP/2 PRIORITY frames from a single source.

Here is a conceptual IPS rule in Snort 3 syntax that targets an invalid weight value in the PRIORITY frame:

alert http2 any any -> $HTTP_SERVERS any (
msg:"ET EXPLOIT Apache Tomcat 10.1.39 HTTP/2 PRIORITY Frame DoS (CVE-2025-31650)";
flow:to_server,established;
http2.type:2; /* Match on HTTP/2 PRIORITY frame */
content:"|00|", depth:1, offset:5; /* Match on an invalid weight of 0 at byte offset 5 */
detection_filter:track by_src, count 50, seconds 10; /* Thresholding for flood detection */
reference:cve,2025-31650;
classtype:denial-of-service;
sid:202531650; rev:1;
)

Conclusion

CVE-2025-31650 is a powerful reminder that critical vulnerabilities often hide in the complex implementations of established protocols. The ease with which an unauthenticated attacker can trigger a complete Denial of Service makes this a clear and present danger to organizations running the affected Tomcat version.

While the exploit is potent, it is also detectable and preventable. We urge all administrators to prioritize upgrading their Apache Tomcat instances immediately. By complementing this patch with layered defenses—such as intelligent rate-limiting, robust monitoring, and specific IPS signatures—organizations can build a more resilient infrastructure capable of withstanding this and future attacks.

What are your thoughts on protocol-level vulnerabilities like this? Share your insights or defensive strategies in the comments below.