---
id: CVE-2026-116578
aliases:
  - GHSA-pw9p-v7r6-gx4p
title: 'Tornado: Open redirect in RedirectHandler via protocol-relative URLs'
summary: 'Tornado: Open redirect in RedirectHandler via protocol-relative URLs'
severity: low
cvss: 2.1
cvssVector: 'CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:N/VI:N/VA:N/SC:N/SI:L/SA:N'
vendor: tornado
product: tornado
ecosystem: pip
affected:
  - tornado < 6.6
patched:
  - tornado 6.6
published: '2026-10-28'
updated: '2026-10-28'
sourceUpdated: '2026-10-28T23:00:04.066537287Z'
source: OSV
sourceUrl: 'https://osv.dev/vulnerability/GHSA-pw9p-v7r6-gx4p'
references:
  - url: >-
      https://github.com/tornadoweb/tornado/security/advisories/GHSA-pw9p-v7r6-gx4p
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-116578'
  - url: 'https://github.com/tornadoweb/tornado/pull/3786'
  - url: >-
      https://github.com/tornadoweb/tornado/commit/940ea725bdd9bc68231d68a105eef17ea6e11233
  - url: >-
      https://github.com/tornadoweb/tornado/commit/cc75ab0eb3af459c714895a49f682ad354f50117
  - url: >-
      https://github.com/tornadoweb/tornado/commit/ccb6a2e5482f17997efd4a74b8200d5b4babc035
  - url: 'https://github.com/tornadoweb/tornado'
  - url: 'https://github.com/tornadoweb/tornado/releases/tag/v6.6.0'
  - url: 'https://github.com/advisories/GHSA-pw9p-v7r6-gx4p'
tags:
  - osv
  - pip
  - ghsa
  - cve.org
epss: 0.00459
epssPercentile: 0.39709
cvssSource: cna
cwe:
  - CWE-601
ingestedAt: '2026-10-26T20:39:42.616Z'
---

## Overview

### Summary

`tornado.web.RedirectHandler` builds its target URL by substituting parts of the request path into a template. Depending on where those substitutions appear in the template, an attacker can craft a request that redirects to a host of their choosing. This creates an open redirect on the application's domain, which can be used in phishing.

### Impact

Applications are affected if they use `RedirectHandler` with a URL template where a regex capture group can influence the scheme or host of the redirect. This includes:

- **A substitution at the start of the path**, for example `(r"/(.*)/target", RedirectHandler, {"url": "/{0}/destination"})`. A request to `https://example.com//evil.com/target` redirects to `//evil.com/destination`, a protocol-relative URL pointing to evil.com. Variants using backslashes or encoded characters have the same effect, because browsers treat them as equivalent to `//`.
- **A substitution directly after the host**, for example `(r"/old(.*)", RedirectHandler, {"url": "https://new.example.com{0}"})`. A request to `/old.evil.com/` or `/old@evil.com/` redirects to evil.com.
- **A template that begins with a substitution** (such as `"{0}"`), which can be given an absolute URL.

Applications are not affected if their `RedirectHandler` templates use only static URLs, or if every substitution comes after a fixed host and a `/` (for example `"/static/{0}"` or `"https://example.com/{0}"`). Other uses of `RequestHandler.redirect()` are not affected by this advisory, although applications that pass user-controlled input to `redirect()` should validate it themselves.

### Patches

Fixed in Tornado 6.6. `RedirectHandler` now returns a 400 error if the substituted values would change the scheme or host of the redirect from what the template specifies. As a result, substitutions can no longer be used in the scheme or host part of a template. The similar check in `StaticFileHandler` has also been hardened against backslash and whitespace variants.

### Workarounds

- Change `RedirectHandler` templates so every substitution comes after a fixed prefix ending in `/`, or write capture-group regexes that cannot match characters such as `/`, `\`, `@`, `.` or `:` where they could affect the host.
- Alternatively, replace `RedirectHandler` with a custom handler that validates the target before calling `self.redirect()`.

### Credit

Reported by Longwei Wang (@gn00295120).

## Affected packages

- `tornado < 6.6`

## Remediation

Upgrade to a patched release:

- `tornado 6.6`