Apache HTTP Server is one of the most widely used web servers on the Internet and forms the foundation of a significant portion of web services, from simple websites to complex application backends. Its performance and stability are critical to operational continuity, making its security a top priority for system administrators and security teams worldwide.

This vulnerability poses a high risk because it allows an unauthenticated remote user to cause a complete Denial of Service (DoS) with minimal effort. The attack requires no special privileges or user interaction. Public exploit code is available, and the attack is simple to execute, significantly increasing the likelihood of exploitation.

Any Internet-exposed Apache HTTP Server instance running a vulnerable version with the HTTP/2 protocol enabled is at risk. A successful attack will render all hosted web services unavailable, resulting in operational disruption, potential financial losses, and reputational damage. Given the server’s widespread use, the potential impact is considerable.

Technical Summary

The technical cause of the vulnerability is improper handling of the HTTP/2 flow control mechanism, classified as CWE-400: Uncontrolled Resource Consumption. It is a variant of the “slow loris” attack, specifically targeting the server’s HTTP/2 protocol implementation.

The attack unfolds as follows:

1. The attacker initiates an HTTP/2 connection to a vulnerable Apache HTTP Server instance.
2. The attacker then sends a SETTINGS frame with the SETTINGS_INITIAL_WINDOW_SIZE parameter set to 0.
3. This malicious setting causes the server’s connection handling logic to hang indefinitely while processing subsequent data from the client, waiting for a window update that will never arrive.
4. Each of these hung connections occupies a worker thread. By initiating multiple such connections, an attacker can quickly exhaust all available worker threads in the server.
5. Once the worker thread pool is exhausted, the server is no longer able to accept new connections, effectively denying service to legitimate users.

Conceptually, the malicious client configuration would look like this:

# Conceptual representation of a malicious HTTP/2 SETTINGS frame
# Do NOT use as an attack string.
SETTINGS {
    SETTINGS_INITIAL_WINDOW_SIZE = 0
}

• Impacted versions: Apache HTTP Server 2.4.55 through 2.4.57
• Fixed version: Apache HTTP Server 2.4.58

Recommendations

• Apply the patch immediately: Update Apache HTTP Server instances to the fixed version 2.4.58 or later. This is the most effective way to remediate the vulnerability.

• Mitigations:

• As a temporary measure until the patch is applied, consider disabling the HTTP/2 protocol if it is not essential for the environment. This can be done by modifying the Protocols directive in the Apache configuration (e.g., Protocols http/1.1).

• Use a Web Application Firewall (WAF) or perimeter device capable of inspecting and normalizing HTTP/2 traffic to block requests with an initial window size of zero.

• Hunt & Monitor:

• Monitor the Apache server-status page to identify an abnormal number of connections stuck in a persistent “Read” or “Write” state without data transfers.

• Analyze network traffic for a sudden spike in HTTP/2 connections, particularly those that hang after the initial SETTINGS frame exchange.

• Configure alerts for consistently high worker thread usage, which can be an early indicator of the attack.

• Incident Response:

• If a compromise is suspected, gracefully restart the Apache service to terminate all existing connections and free up the worker thread pool.

• Temporarily block the source IP addresses responsible for the attack at the firewall level. Note that attackers can easily change IP addresses, making this measure only temporary.

• Prioritize the immediate deployment of the patch (version 2.4.58) to prevent recurrence.

• Defense in Depth:

• Implement connection and traffic limiting policies on perimeter devices to slow down resource exhaustion attacks.

• Ensure comprehensive server monitoring and logging systems are active to provide visibility into connection states and resource usage.

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