CVE-2025-67725
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.
⚡ Watch CVE-2025-67725
Get an email if CVE-2025-67725 is added to CISA KEV, gains public exploit code, or a new advisory cites it — max one per day, one-click unsubscribe.
Exploitation outlook
- Low exploitation risk0.52% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 41% of all EPSS-scored CVEs.
Advisory coverage (1)
- highGHSA-c98p-7wgm-6p64: Tornado: Quadratic DoS via Repeated Header Coalescingghsa · 2026-07-20
External references
Embed the live status
— this badge updates automatically when the KEV or exploit status changes. How to embed it →
[](https://www.csirts.com/cve/CVE-2025-67725)