CVE-2026-91991

Tornado before 6.5.8 contains an incomplete fix for cookie attribute injection that allows attackers to inject arbitrary cookie attributes by passing capitalized or legacy keyword arguments to set_cookie. Attackers can embed semicolon-delimited data in capitalized parameters like Domain, Path, or SameSite to bypass validation and modify cookie security attributes.

• https://github.com/tornadoweb/tornado/security/advisories/GHSA-wwv5-g3v4-889x
• https://www.vulncheck.com/advisories/tornado-before-6.5.8-cookie-attribute-injection-via-capitalized-kwargs
• https://github.com/tornadoweb/tornado/security/advisories/GHSA-wwv5-g3v4-889x