Affected: tornadoweb tornado

Tornado is a Python web framework and asynchronous networking library. In versions 6.5.2 and below, the supplied reason phrase is used unescaped in HTTP headers (where it could be used for header injection) or in HTML in the default error page (where it could be used for XSS) and can be exploited by passing untrusted or malicious data into the reason argument. Used by both RequestHandler.set_status and tornado.web.HTTPError, the argument is designed to allow applications to pass custom "reason" phrases (the "Not Found" in HTTP/1.1 404 Not Found) to the HTTP status line (mainly for non-standard status codes). This issue is fixed in version 6.5.3.

Affected: PyPI: tornado

# Header injection and XSS via `reason` argument

## Summary

The `reason` argument (used by both `RequestHandler.set_status` and `tornado.web.HTTPError` is designed to allow applications to pass custom "reason" phrases (the "Not Found" in `HTTP/1.1 404 Not Found`) to the HTTP status line (mainly for non-standard status codes). Vulnerabilities exist in Tornado versions prior to 6.5.3 if untrusted data is passed as the `reason` argument.

## Details

In vulnerable versions, the supplied `reason` phrase is used unescaped in HTTP headers (where it could be used for header injection) or in HTML in the default error page (where it could be used for XSS).

## Impact

* **Type:** Reflected Cross-Site Scripting (CWE-79) in default error page, or header injection (CWE-644) * **Actors:** Remote attacker who can cause the application to raise `HTTPError`/`set_status` with an attacker-controlled `reason` (e.g., via query parameter used by developer). * **Effect:** Execution of arbitrary JavaScript in victims' browsers when they view the error page — possible session token theft, CSRF escalation, UI spoofing, or other client-side attacks depending on context. * **Scope:** Only applications that explicitly reflect untrusted input into `reason` are affected.

## Mitigation

Aside from upgrading to Tornado 6.5.3 or newer, the vulnerability can be mitigated by not using untrusted data for the `reason` argument. In the intended use case the `reason` argument would generally be a string litera…

Affected: PyPI: tornado

Tornado is a Python web framework and asynchronous networking library. In versions 6.5.2 and below, the supplied reason phrase is used unescaped in HTTP headers (where it could be used for header injection) or in HTML in the default error page (where it could be used for XSS) and can be exploited by passing untrusted or malicious data into the reason argument. Used by both RequestHandler.set_status and tornado.web.HTTPError, the argument is designed to allow applications to pass custom "reason" phrases (the "Not Found" in HTTP/1.1 404 Not Found) to the HTTP status line (mainly for non-standard status codes). This issue is fixed in version 6.5.3.