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 literal and not derived from user input. Also, the reason argument is rarely required (reason phrases are not used at all in HTTP/2) and can generally be omitted.

For a general-purpose error message in HTTPError, consider using the log_message argument instead of reason.