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Apache Tomcat is a popular, open-source web server and servlet container maintained by the Apache Software Foundation. It provides a reliable and scalable environment for executing Java Servlets and serving web pages built using Java Server Pages (JSP). Frequently deployed in both development and production environments, Tomcat plays a crucial role in delivering dynamic Java-based web applications across various enterprise use cases.

Recently, a critical security vulnerability identified as CVE-2025-24813 was discovered in Apache Tomcat. This vulnerability exploits a flaw in the handling of partial file uploads and session file persistence, potentially allowing attackers to achieve remote code execution (RCE) under certain conditions. The issue arises from how Tomcat’s default servlet manages write operations combined with deserialization logic for persisted session files.

CVE-2025-24813

Initially published in early March with a CVSS score of 5.5, the severity of CVE-2025-24813 was later reassessed and upgraded to 9.8 (High). Recognizing the potential impact of this flaw, the Cybersecurity and Infrastructure Security Agency (CISA) added it to its Known Exploited Vulnerabilities (KEV) catalogue, underscoring the urgency for affected organizations to patch their systems.

CVE-2025-24813 is a critical vulnerability in Apache Tomcat that can lead to remote code execution (RCE) when specific server configurations are in place. The issue arises from how Tomcat handles partial PUT requests in conjunction with file-based session persistence.

This issue becomes exploitable when the default servlet is explicitly configured with ‘readonly’ parameter is set to false — a setting that enables write operations such as HTTP PUT. By default, Tomcat sets ‘readonly’ to true, which restricts write access and helps mitigate risk. This parameter is defined in the web.xml configuration file, typically located in the conf/ directory of the Tomcat installation.

When partial PUT support is also enabled (enabled by default), an attacker can exploit this behaviour to upload a crafted serialized payload, targeting a session file. If Tomcat is configured to persist session data to disk, the uploaded file may later be automatically deserialized by the server, resulting in attacker-controlled code execution.

The vulnerability affects the following versions of Apache Tomcat:

• 11.0.0‑M1 through 11.0.2
• 10.1.0‑M1 through 10.1.34
• 9.0.0‑M1 through 9.0.98

Exploitation Prerequisites for CVE-2025-24813

To exploit CVE-2025-24813, several server-side conditions must be in place. These prerequisites enable an attacker to craft a malicious PUT request that results in the deserialization of attacker-controlled data, potentially leading to remote code execution (RCE).

The following conditions must be met:

• The default servlet’s readonly attribute is set to false, permitting write access via HTTP PUT requests
• Partial PUT functionality is enabled — i.e., Tomcat accepts the Content-Range header (enabled by default)
• The application is configured to use Tomcat’s file-based session persistence mechanism

Exploitation Flow

The exploitation of CVE-2025-24813 involves a sequence of carefully crafted steps that take advantage of Tomcat’s handling of partial file uploads and session deserialization. The following outlines a typical attack chain under vulnerable conditions:

Environment Setup: The target server must have ‘readonly’ parameter set to false for the default servlet, partial PUT support enabled, and file-based session persistence configured.

Payload Generation: The attacker generates a malicious serialized object — typically using a tool like ysoserial — embedding a command that will execute upon deserialization.

Payload Upload: The crafted payload is uploaded to the server via an HTTP PUT request with a Content-Range header. This simulates a partial upload and results in the creation of a session file on disk.

Triggering Deserialization: A follow-up request is made to the application with the JSESSIONID set to the uploaded session file’s name. This causes Tomcat to deserialize the file, assuming it to be a legitimate session object.

Code Execution: If a suitable deserialization gadget exists on the classpath, the payload is executed, leading to remote code execution under the privileges of the Tomcat process.

Mitigation

The recommended and most effective mitigation for CVE-2025-24813 is to upgrade Apache Tomcat to a version where the vulnerability has been addressed. This flaw is fully patched in the following Tomcat releases:

These versions include enhancements to the handling of temporary files created via partial PUT requests, ensuring such files are not mistakenly deserialized as session objects — thereby preventing remote code execution.

For environments where immediate upgrades are not possible, the following temporary mitigations can help reduce risk:

• Keep the default servlet’s readonly parameter set to true, which prevents write operations via PUT requests. This is the default and recommended setting.
• Disable support for partial PUT requests, especially if not used by the application. This can be achieved at the connector level or via upstream web server rules (e.g., Nginx or Apache HTTPD).
• Avoid using file-based session persistence, particularly when writable paths overlap with session storage locations.
• Review and sanitize the server classpath to remove unnecessary libraries such as commons-collections, which may introduce exploitable deserialization gadgets.

    Seqrite Endpoint Protection

All Seqrite Customers are protected from this vulnerability by following signatures:

• HTTP/CVE-2025-24813!VS.49414

Authors:

Vinay Kumar

Vineet Sarote