Apache Tomcat Coyote Flaw Allows Attackers to Launch DoS Attacks

The Apache Software Foundation has revealed a vulnerability in the Tomcat Coyote module, specifically within the Maven artifact org.apache.tomcat:tomcat-coyote, that could enable malicious actors to orchestrate denial-of-service (DoS) attacks.

This flaw stems from an uncontrolled resource consumption issue tied to HTTP/2 protocol handling, potentially allowing attackers to overwhelm server resources by manipulating stream concurrency limits.

System administrators and developers relying on Apache Tomcat are urged to assess their deployments immediately, given the widespread use of Tomcat in enterprise Java environments for web application hosting.

Critical HTTP/2 Vulnerability Exposed

The vulnerability arises when an HTTP/2 client fails to acknowledge the initial SETTINGS frame, which is responsible for negotiating parameters like the maximum number of concurrent streams permitted on a connection.

In affected versions, Tomcat’s Coyote HTTP/2 implementation does not properly enforce these limits if the acknowledgment is absent, leading to a scenario where the server might process an excessive number of streams without bounds.

According to the Report, this uncontrolled behavior can exhaust server threads, memory, or CPU resources, effectively rendering the server unresponsive to legitimate requests.

Technically, HTTP/2’s multiplexing feature, designed to improve performance by allowing multiple requests over a single TCP connection, becomes a vector for abuse here.

If exploited, an attacker could initiate a connection, ignore the SETTINGS acknowledgment, and flood the server with stream requests, bypassing the intended concurrency caps defined in the protocol’s specification (RFC 7540).

This not only disrupts service availability but also amplifies the risk in high-traffic environments where Tomcat serves as the embedded servlet container in applications built with frameworks like Spring Boot.

Affected Versions

The issue impacts a broad range of Tomcat releases, specifically versions from 11.0.0-M1 up to but not including 11.0.9, from 10.1.0-M1 up to but not including 10.1.43, and from 9.0.0.M1 up to but not including 9.0.107.

These milestones and stable releases, spanning several years of development, highlight the persistence of this flaw across Tomcat’s evolution.

For instance, the 9.x branch, which is still in use for legacy systems supporting Java 8, is particularly vulnerable, as are the more recent 10.x and 11.x lines that incorporate modern features like enhanced HTTP/2 support and improved security hardening.

The Apache team has confirmed that no workarounds exist short of upgrading, as the root cause lies deep within the Coyote connector’s stream management logic.

To mitigate this risk, users are strongly advised to upgrade to the patched versions: 11.0.9 for the latest stable branch, 10.1.43 for those on the 10.x series, or 9.0.107 for environments constrained to older releases.

These updates incorporate fixes that enforce strict acknowledgment checks and robustly limit concurrent streams, preventing resource exhaustion even in the face of non-compliant clients.

Developers should verify their Maven dependencies, as tomcat-coyote is often pulled in transitively via tomcat-embed-core or full Tomcat distributions.

In production settings, implementing additional safeguards such as rate limiting at the network layer or using reverse proxies like Apache HTTP Server or NGINX can provide interim protection, though they do not address the core vulnerability.

This disclosure underscores ongoing challenges in HTTP/2 implementations, where protocol complexities can lead to subtle denial-of-service vectors.

Historically, Tomcat has faced similar issues, such as past CVEs related to header parsing and connection handling, emphasizing the need for rigorous testing in protocol stacks.

Organizations should conduct vulnerability scans using tools like OWASP Dependency-Check or integrate automated updates into their CI/CD pipelines to stay ahead of such threats.

With Tomcat powering countless web applications globally, prompt action is essential to avoid potential outages, especially in sectors like finance and e-commerce where downtime translates to significant losses.

The Apache Foundation continues to monitor for related exploits, and no active in-the-wild attacks have been reported as of this date, but vigilance remains key in an era of increasingly sophisticated cyber threats.

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