New HTTP/2 Vulnerability Exposes Web Servers to DoS Attacks

In the ever-evolving landscape of cybersecurity, researchers continuously uncover vulnerabilities that threaten the stability and security of digital infrastructures. The latest discovery by security expert Bartek Nowotarski sheds light on a critical flaw in the widely used HTTP/2 protocol, presenting a significant risk to web servers worldwide. Dubbed the CONTINUATION Flood, this exploit enables attackers to launch denial-of-service (DoS) attacks, potentially causing server crashes or severe performance degradation.

Understanding the CONTINUATION Flood Vulnerability

At the heart of this vulnerability lies a flaw in how HTTP/2 implementations handle CONTINUATION frames within a single stream. Unlike its predecessor HTTP/1, which transmitted headers in a single block, HTTP/2 divides headers into fragments encapsulated within CONTINUATION frames. While this approach offers performance benefits, it also introduces a vulnerability that can be exploited by malicious actors.

Attackers leverage this vulnerability by inundating a server with a continuous stream of CONTINUATION frames, deliberately omitting the END_HEADERS flag. This malicious action forces the server to parse and store an excessive amount of header data in memory, leading to memory exhaustion or crashes. What makes this threat particularly insidious is its ability to disrupt server availability using a single machine or TCP connection, with the attack often remaining undetected in HTTP access logs.

Affected Implementations and Exploitation Scenarios

Numerous HTTP/2 implementations are susceptible to the CONTINUATION Flood exploit, including widely used projects such as Apache HTTP Server, Node.js, and Golang. Attackers can exploit vulnerabilities in these implementations to orchestrate DoS attacks, exploiting weaknesses in how each system handles CONTINUATION frames.

For instance, in the case of Node.js, attackers can render the HTTP/2 server unavailable by sending a limited number of HTTP/2 frame packets containing a few frames, triggering a race condition that leads to memory leaks. Similarly, Envoy’s oghttp codec fails to reset requests when header map limits are exceeded, enabling attackers to trigger unlimited memory consumption.

Mitigation Strategies and Best Practices

As the threat landscape evolves, it’s imperative for organizations to implement robust mitigation strategies to protect against emerging threats like the CONTINUATION Flood exploit. Here are some recommended steps:

1. Patch and Update: Ensure that affected software, including web servers and HTTP/2 implementations, are promptly updated to the latest versions that address the vulnerability. This is the most effective way to mitigate the risk of exploitation.
2. Temporary Disabling of HTTP/2: If a patch is unavailable, consider temporarily disabling HTTP/2 support on web servers to mitigate the risk of exploitation. While this may impact performance, it provides a necessary layer of protection until a permanent solution is implemented.
3. Network Monitoring and Anomaly Detection: Implement robust network monitoring solutions capable of detecting anomalous patterns indicative of a DoS attack, such as a sudden influx of CONTINUATION frames. Early detection enables swift response and mitigation.
4. Resource Limiting: Configure web servers to enforce resource limits, such as maximum header size or connection thresholds, to mitigate the impact of potential DoS attacks.
5. Regular Security Audits: Conduct regular security audits of web servers and HTTP/2 implementations to identify and address potential vulnerabilities proactively.

Removal Guide

If your server has been compromised by a CONTINUATION Flood attack, follow these steps to remove the threat:

1. Isolate Affected Systems: Immediately disconnect the compromised server from the network to prevent further spread of the attack.
2. Terminate Suspicious Processes: Identify and terminate any suspicious processes associated with the attack using system monitoring tools.
3. Restore from Backup: If feasible, restore the affected server from a clean backup to ensure the removal of any malicious code or artifacts.
4. Implement Security Updates: Once the server is restored, apply any necessary security updates and patches to address the vulnerability exploited by the attack.
5. Monitor for Recurrence: Continuously monitor server logs and network traffic for signs of recurrence or additional malicious activity.

By taking proactive measures to address vulnerabilities and implement robust security practices, organizations can safeguard their web servers against emerging threats like the CONTINUATION Flood exploit. Stay vigilant, stay updated, and prioritize cybersecurity to mitigate the risk of DoS attacks and protect critical digital assets.