Tornado: Quadratic DoS via Repeated Header Coalescing
## 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.
describes (1)
Related CVEs.
| ID | severity | exploitable | ||
|---|---|---|---|---|
| CVE-2025-67725US | — | — | ||
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… | ||||
references
External citations — patches, advisories, analysis.
- https://github.com/pypa/advisory-database/tree/main/vulns/tornado/PYSEC-2025-266.yamlWEB
- https://github.com/tornadoweb/tornadoPACKAGE
- https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cdWEB
- https://github.com/tornadoweb/tornado/releases/tag/v6.5.3WEB
- https://github.com/tornadoweb/tornado/security/advisories/GHSA-c98p-7wgm-6p64WEB
- https://nvd.nist.gov/vuln/detail/CVE-2025-67725ADVISORY
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