# Tornado: Quadratic DoS via Repeated Header Coalescing

Advisory record · `GHSA-c98p-7wgm-6p64`

- **Identifiers:** `GHSA-c98p-7wgm-6p64` · `CVE-2025-67725` · `PYSEC-2025-266`
- **State:** Confirmed
- **Severity:** CVSS 7.5 High
- **Critical:** No
- **Weakness (CWE):** [CWE-400](https://cwe.mitre.org/data/definitions/400.html)
- **Tags:** [Denial of service](https://agentprime.io/advisories/tag/dos.md)
- **Published:** 20 Jul 2026
- **Updated:** 20 Jul 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 20 Jul 2026, 18:57 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `tornado` (PyPI): affected `<6.5.3`; fixed in `6.5.3`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> Tornado versions before 6.5.3 are affected. The HTTPHeaders.add method joins repeated values of the same header name by string concatenation, which becomes quadratic in time as the number of repeats grows. An attacker who can send HTTP requests to the server can send one crafted request that blocks the event loop for an extended time, causing denial of service. The severity depends on max_header_size: it is rated high if that limit has been raised above its 64KB default, and low at the default. Upgrade to Tornado 6.5.3 or later.

Written 5 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 7.5 High, CVSS 3.1: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`, from CNA (GitHub_M)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0060, the 49th percentile, on 15 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 14:42 UTC: First seen by agentprime
- 31 Oct 2026, 14:42 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-c98p-7wgm-6p64)

## Sources

- [GitHub Advisory Database: GHSA-c98p-7wgm-6p64](https://github.com/advisories/GHSA-c98p-7wgm-6p64)
- [OSV: GHSA-c98p-7wgm-6p64](https://osv.dev/vulnerability/GHSA-c98p-7wgm-6p64)
- <https://github.com/tornadoweb/tornado/security/advisories/GHSA-c98p-7wgm-6p64>
- <https://nvd.nist.gov/vuln/detail/CVE-2025-67725>
- <https://github.com/tornadoweb/tornado/commit/771472cfdaeebc0d89a9cc46e249f8891a6b29cd>
- <https://github.com/pypa/advisory-database/tree/main/vulns/tornado/PYSEC-2025-266.yaml>
- <https://github.com/tornadoweb/tornado>
- <https://github.com/tornadoweb/tornado/releases/tag/v6.5.3>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-c98p-7wgm-6p64>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-c98p-7wgm-6p64/>. Everything for agents: <https://agentprime.io/llms.txt>.