Resource exhaustion in Tornadoweb Tornado
CVE-2025-67725
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.
Vulnerability class: DoS (Denial of Service)
EPSS: 0.006 (44.4th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
Affected products
- Tornadoweb Tornado — versions < 6.5.3
Weakness classification (CWE)
References
- security-advisories@github.com (x_refsource_CONFIRM, Vendor Advisory)
- security-advisories@github.com (Patch, x_refsource_MISC)
- security-advisories@github.com (x_refsource_MISC, Release Notes)
Frequently asked questions
- What is CVE-2025-67725?
- CVE-2025-67725 is a high-severity vulnerability in Tornadoweb Tornado, classified under Uncontrolled Resource Consumption. CVSS score: 7.5/10. Published 2025-12-12.
- How severe is CVE-2025-67725?
- High severity. CVSS v3 base score is 7.5 out of 10.