Tornado is a Python web framework and asynchronous networking library. In versions 6.5.2 and below, a single maliciously crafted HTTP request can block the server's event loop for an extended period, caused by the HTTPHeaders.add method. The function accumulates values using string concatenation when the same header name is repeated, causing a Denial of Service (DoS). Due to Python string immutability, each concatenation copies the entire string, resulting in O(n²) time complexity. The severity can vary from high if max_header_size has been increased from its default, to low if it has its default value of 64KB. This issue is fixed in version 6.5.3.
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Sat, 13 Dec 2025 00:15:00 +0000
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Fri, 12 Dec 2025 06:00:00 +0000
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| Description | Tornado is a Python web framework and asynchronous networking library. In versions 6.5.2 and below, a single maliciously crafted HTTP request can block the server's event loop for an extended period, caused by the HTTPHeaders.add method. The function accumulates values using string concatenation when the same header name is repeated, causing a Denial of Service (DoS). Due to Python string immutability, each concatenation copies the entire string, resulting in O(n²) time complexity. The severity can vary from high if max_header_size has been increased from its default, to low if it has its default value of 64KB. This issue is fixed in version 6.5.3. | |
| Title | Tornado is Vulnerable to Quadratic DoS via Repeated Header Coalescing | |
| Weaknesses | CWE-400 | |
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Status: PUBLISHED
Assigner: GitHub_M
Published: 2025-12-12T05:49:41.523Z
Updated: 2025-12-12T05:49:41.523Z
Reserved: 2025-12-10T19:25:20.819Z
Link: CVE-2025-67725
No data.
Status : Undergoing Analysis
Published: 2025-12-12T06:15:41.380
Modified: 2025-12-12T15:17:31.973
Link: CVE-2025-67725