Multiple Apache Tomcat Vulnerabilities Let Attackers Trigger DoS Attacks

Apache Tomcat has addressed three critical denial-of-service (DoS) vulnerabilities that could allow malicious actors to disrupt web applications and services. 

These security flaws, tracked as CVE-2025-52434, CVE-2025-52520, and CVE-2025-53506, affect all Apache Tomcat versions from 9.0.0.M1 to 9.0.106. 

The vulnerabilities exploit different attack vectors, including HTTP/2 protocol weaknesses, file upload mechanisms, and stream handling capabilities. 

Key Takeaways
1. CVE-2025-52434, CVE-2025-52520, and CVE-2025-53506 affect Apache Tomcat 9.0.0.M1 to 9.0.106, allowing remote denial-of-service attacks.
2. Exploits target the HTTP/2 protocol with APR/Native, file upload integer overflow, and excessive HTTP/2 stream creation.
3. All vulnerabilities were patched through specific commits implementing proper validation and resource limits.
4. Immediate upgrade to version 9.0.107 is required as exploits need no authentication.

Organizations running affected versions should immediately upgrade to Apache Tomcat 9.0.107 to mitigate these security risks and prevent potential service disruptions.

HTTP/2 and APR/Native Flaw (CVE-2025-52434)

The first vulnerability, CVE-2025-52434, represents a critical flaw in Apache Tomcat’s HTTP/2 implementation when used with the APR/Native connector. 

This vulnerability enables attackers to trigger denial-of-service conditions by exploiting weaknesses in the HTTP/2 protocol handling mechanisms. 

The APR/Native connector, which provides enhanced performance through native library integration, becomes susceptible to resource exhaustion attacks when processing malformed or excessive HTTP/2 requests.

The security team addressed this issue through commit 8a83c3c4, which implements proper validation and resource management for HTTP/2 connections. 

System administrators utilizing APR/Native connectors with HTTP/2 enabled should prioritize this update, as the vulnerability can be exploited remotely without authentication. 

The fix introduces stricter boundary checks and connection lifecycle management to prevent resource depletion scenarios.

Integer Overflow in File Uploads (CVE-2025-52520)

CVE-2025-52520 exploits integer overflow conditions in Apache Tomcat’s file upload processing mechanism. 

Attackers can craft malicious multipart/form-data requests with specially crafted Content-Length headers that trigger integer overflow vulnerabilities, potentially bypassing file size restrictions and causing memory exhaustion. 

This vulnerability affects applications that handle file uploads through servlet containers. The remediation, implemented in commit 927d66fb, introduces robust input validation and proper integer bounds checking for file upload operations. 

The fix ensures that maxRequestSize and maxFileSize parameters are properly validated before processing, preventing overflow conditions that could lead to unlimited memory allocation. 

Web applications with file upload functionality should implement additional validation layers at the application level as a defense-in-depth strategy.

Excessive HTTP/2 Streams (CVE-2025-53506)

The third vulnerability, CVE-2025-53506, allows attackers to overwhelm Apache Tomcat servers by creating excessive HTTP/2 streams within a single connection. 

This attack vector exploits the HTTP/2 multiplexing feature, where multiple streams can be processed simultaneously over a single TCP connection. Malicious clients can rapidly create numerous streams, exhausting server memory and processing resources.

Commit 43477293 addresses this vulnerability by implementing proper stream count limitations and resource management policies. 

The fix introduces configurable parameters for maximum concurrent streams per connection and implements graceful degradation mechanisms when limits are approached. 

Network administrators should configure appropriate values for maxConcurrentStreams and monitor HTTP/2 connection patterns to detect potential abuse.

Organizations running affected Apache Tomcat versions should immediately upgrade to 9.0.107 and review their security configurations to mitigate these critical vulnerabilities.

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