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

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2025-12-12 · updated 2026-10-07
• Weakness: CWE-400

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.

Affected products

• tornadoweb tornado: before 6.5.3 — fixed in 6.5.3
• Pypi Tornado: before 6.5.3; 0.2, 1.0, 1.1, 1.1.1, 1.2, 1.2.1 (+72) — fixed in 6.5.3

How to fix

Upgrade to a fixed version: tornadoweb tornado 6.5.3; Pypi Tornado 6.5.3. Check the vendor advisory for mitigations.

References

• https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd
• https://github.com/tornadoweb/tornado/releases/tag/v6.5.3
• https://github.com/tornadoweb/tornado/security/advisories/GHSA-c98p-7wgm-6p64

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