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High severity7.5NVD Advisory· Published Dec 12, 2025· Updated Jun 17, 2026

CVE-2025-67725

CVE-2025-67725

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.

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Affected packages

Versions sourced from the GitHub Security Advisory.

PackageAffected versionsPatched versions
tornadoPyPI
< 6.5.36.5.3

Affected products

333

Patches

Vulnerability mechanics

References

6

News mentions

0

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