CVE-2026-78383 in Tomcat
Summary
by MITRE • 09/23/2026
Allocation of resources without limits or throttling vulnerability in Apache Tomcat allows an unauthenticated AJP request to pin an AJP processing thread leading to denial of service.
This issue affects Apache Tomcat: from 11.0.0-M1 through 11.0.25, from 10.1.0-M1 through 10.1.59, from 9.0.0.M1 through 9.0.121.
The following versions were EOL at the time the CVE was created but are known to be affected: from 8.5.0 through 8.5.100, from 7.0.0 through 7.0.109. Other unsupported versions may also be affected.
Users are recommended to upgrade to version 11.0.26, 10.1.60 or 9.0.122, which fix the issue.
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Analysis
by VulDB Data Team • 09/23/2026
The vulnerability identified in Apache Tomcat represents a critical resource exhaustion flaw within the AJP (Apache JServ Protocol) connector implementation. This protocol is primarily designed for communication between web servers like Apache HTTP Server and application servers running Java-based applications such as Tomcat. The core technical issue stems from an allocation of resources without limits or throttling mechanisms, specifically affecting how processing threads are managed during AJP request handling. When a client sends an AJP request to the server, the system allocates a dedicated thread to process that specific connection and its associated data stream. In vulnerable versions ranging from 11.0.0-M1 through 11.0.25, 10.1.0-M1 through 10.1.59, and 9.0.0.M1 through 9.0.121, as well as older end-of-life versions like 8.5.x and 7.0.x series, there is no upper bound on the number of concurrent AJP threads that can be spawned or retained for a single connection or session. This lack of throttling allows an attacker to exploit the mechanism by initiating multiple connections or maintaining long-lived connections with slow data transmission rates, effectively pinning processing threads indefinitely.
From a technical perspective, this flaw aligns closely with CWE-770: Allocation of Resources Without Limits or Throttling and CWE-400: Uncontrolled Resource Consumption. The absence of proper concurrency controls means that the server does not enforce limits on how many threads can be held by AJP connections simultaneously. An unauthenticated attacker, who only needs network access to port 8009 where the AJP connector typically listens, can send a request and then keep the connection open while sending data at an extremely slow rate or simply holding the thread without completing the transaction. Because each active AJP connection consumes a thread from Tomcat's internal thread pool, this behavior leads directly to resource exhaustion. As more threads are pinned by malicious actors, legitimate requests cannot be processed because no available threads remain in the pool to handle them. This results in a complete denial of service for all services hosted on that instance of Apache Tomcat, not just the specific application being targeted via AJP.
The operational impact of this vulnerability is severe due to its unauthenticated nature and ease of exploitation. Since it does not require any form of authentication or valid credentials, even external-facing servers with exposed AJP ports are at immediate risk. The attack vector allows for a sustained denial-of-service condition that can take down entire server environments if the thread pool size is relatively small or under heavy load conditions where resources are already constrained. This impacts availability, one of the three pillars of the CIA triad in information security. Furthermore, because AJP is often used behind reverse proxies like Apache HTTP Server, administrators might mistakenly assume their infrastructure is secure due to front-end protections. However, if the proxy configuration allows direct access or if the proxy itself is compromised, this vulnerability becomes a viable path for disruption. The fact that older EOL versions are also affected highlights the importance of maintaining up-to-date software stacks, as legacy systems often lack these modern concurrency safeguards entirely.
Mitigation strategies focus primarily on upgrading to patched versions where the thread allocation logic has been corrected to include proper limits and throttling mechanisms. Users should upgrade Apache Tomcat to version 11.0.26, 10.1.60, or 9.0.122 immediately if they are running any of the affected ranges including those from earlier major releases like 8.5.x and 7.0.x which remain in use despite being end-of-life. In addition to software updates, defensive architectural changes can significantly reduce exposure risk. Administrators should ensure that the AJP connector is not exposed directly to untrusted networks or the public internet. Instead, it should be bound only to localhost interfaces if accessed by a local reverse proxy, thereby preventing external attackers from reaching port 8009 at all. If remote access via AJP is absolutely necessary, implementing network-level firewalls and intrusion detection systems can help monitor for anomalous connection patterns indicative of thread-pinning attacks. Regular security audits and adherence to industry standards such as those outlined in the MITRE ATT&CK framework under resource hijacking techniques are recommended to maintain robust defense-in-depth posture against similar exhaustion vulnerabilities.