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CVE-2025-67725 — Tornado is Vulnerable to Quadratic DoS via Repeated Header Coalescing

Affected: tornadoweb tornado

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.

GHSA-c98p-7wgm-6p64 — Tornado: Quadratic DoS via Repeated Header Coalescing

Affected: PyPI: tornado

## Summary

The `HTTPHeaders.add` method in Tornado accumulates values using string concatenation when the same header name is repeated. Due to Python string immutability, each concatenation copies the entire string, resulting in O(n²) time complexity.

Given Tornado's single event loop architecture, a single maliciously crafted HTTP request can block the server's event loop for an extended period, causing a Denial of Service (DoS).

**Severity:** **High** if `max_header_size` has been increased from its default, **low** if it has its default value of 64KB.

PYSEC-2025-266

Affected: PyPI: tornado