# CVE-2026-126347: Tornado: Open redirect via triple-slash redirect paths

> Attackers can craft triple-slash redirect destinations in Tornado apps using URL templates to send users to attacker-controlled hosts.

- **Severity:** Low 3.1 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Harmful if only installed:** No
- **Needs a specific call:** Yes (`tornado.web.RedirectHandler`)
- **Fix available:** No
- **Published:** 12 Nov 2026
- **Updated:** 12 Nov 2026
- **Also known as:** GHSA-5cjw-f85j-cv6r

## What is CVE-2026-126347?

CVE-2026-126347 is a low-severity open redirect vulnerability in tornadoweb tornado, published on 12 Nov 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

Tornado is a Python web framework and asynchronous networking library. In versions through 6.6.0, applications using  tornado.web.RedirectHandler  with URL templates that allow attacker-controlled substitutions to influence the destination can be vulnerable to an open redirect using three or more leading slashes. Differences between Python’s URL parsing and browser URL interpretation allow a crafted destination to redirect a user to an attacker-controlled host. The remediation for CVE-2026-116578 in version 6.6.0 did not fully address this case. This issue is fixed in version 6.6.1.

Weakness: CWE-601 (URL Redirection to Untrusted Site)

## Which versions are affected by CVE-2026-126347?

An affected version is only exploitable if your software calls tornado.web.RedirectHandler. Many projects that include it are not exposed.


## How do I fix CVE-2026-126347?

No fix has been published for CVE-2026-126347 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-126347 being exploited?

There is no known exploitation of CVE-2026-126347 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-126347 and fix it if it is affected.

What it is: CVE-2026-126347 is a low-severity open redirect vulnerability in tornadoweb tornado, published on 12 Nov 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2026-126347 in the wild and no public exploit code.
Exposure: An affected version is only exploitable if your software calls tornado.web.RedirectHandler. Many projects that include it are not exposed.

Affected product: tornadoweb tornado. No package-level version data is available, so check the vendor advisory linked from the reference page.

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.
   If an affected version is present, also search the code for calls to tornado.web.RedirectHandler and report whether any exist.
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-2026-126347/ (Markdown: https://insights.sbomify.com/CVE-2026-126347/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-126347
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-126347
- OSV: https://osv.dev/vulnerability/CVE-2026-126347

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-126347/
Summaries are generated and may contain mistakes; the official description above is the record.