# CVE-2025-67726: Tornado is Vulnerable to Quadratic DoS via Crafted Multipart Parameters


- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 12 Dec 2025
- **Updated:** 18 Dec 2025
- **Also known as:** BELL-CVE-2025-67726, DEBIAN-CVE-2025-67726, ECHO-43db-f673-83aa, GHSA-jhmp-mqwm-3gq8, MINI-f2p2-gf63-phgv, MINI-vrpm-vq8p-c7rh, OESA-2025-2881, OESA-2026-1018, PYSEC-2025-267, UBUNTU-CVE-2025-67726

## What is CVE-2025-67726?

CVE-2025-67726 is a high-severity vulnerability in tornadoweb tornado, published on 12 Dec 2025. 

### Official description

Tornado is a Python web framework and asynchronous networking library. Versions 6.5.2 and below use an inefficient algorithm when parsing parameters for HTTP header values, potentially causing a DoS. The _parseparam function in httputil.py is used to parse specific HTTP header values, such as those in multipart/form-data and repeatedly calls string.count() within a nested loop while processing quoted semicolons. If an attacker sends a request with a large number of maliciously crafted parameters in a Content-Disposition header, the server's CPU usage increases quadratically (O(n²)) during parsing. Due to Tornado's single event loop architecture, a single malicious request can cause the entire server to become unresponsive for an extended period. This issue is fixed in version 6.5.3.

Weakness: CWE-834 (Excessive Iteration), CWE-400 (Uncontrolled Resource Consumption)

## Which versions are affected by CVE-2025-67726?

3 open source packages ship an affected version, including tornado. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `tornado` | PyPI | All versions | 6.5.3 |
| `tornado` | PyPI | All versions | 6.5.3 |
| `tornado` | PyPI | All versions | 6.5.3 |

Products: Tornadoweb Tornado.

## How do I fix CVE-2025-67726?

Upgrade tornado to 6.5.3 or later.

## Is CVE-2025-67726 being exploited?

There is no known exploitation of CVE-2025-67726 in the wild and no public exploit code.

## Other vulnerabilities in tornadoweb tornado

- [CVE-2026-31958](https://insights.sbomify.com/CVE-2026-31958/): Tornado has a DoS due to too many multipart parts (high 8.7)
- [CVE-2025-67725](https://insights.sbomify.com/CVE-2025-67725/): Tornado is Vulnerable to Quadratic DoS via Repeated Header Coalescing (high 7.5)
- [CVE-2024-52804](https://insights.sbomify.com/CVE-2024-52804/): Tornado has HTTP cookie parsing DoS vulnerability (high 7.5)
- [CVE-2026-35536](https://insights.sbomify.com/CVE-2026-35536/): Tornado has incomplete validation of cookie attributes (high 7.2)
- [CVE-2014-9720](https://insights.sbomify.com/CVE-2014-9720/): Tornado XSRF cookie allows side-channel attack against TLS (BREACH attack) (medium 6.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2025-67726 and fix it if it is affected.

What it is: CVE-2025-67726 is a high-severity vulnerability in tornadoweb tornado, published on 12 Dec 2025. 
Exploitation: There is no known exploitation of CVE-2025-67726 in the wild and no public exploit code.

Affected packages and fixed versions:
- tornado (PyPI): affected all versions; fixed in 6.5.3
- tornado (PyPI): affected all versions; fixed in 6.5.3
- tornado (PyPI): affected all versions; fixed in 6.5.3

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2025-67726/ (Markdown: https://insights.sbomify.com/CVE-2025-67726/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2025-67726
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2025-67726
- OSV: https://osv.dev/vulnerability/CVE-2025-67726

---
From sbomify Insights: https://insights.sbomify.com/CVE-2025-67726/
Summaries are generated and may contain mistakes; the official description above is the record.