Vulnerability Database

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Tornado: Incomplete fix for CVE-2026-35536: cookie attribute injection re-opened via the legacy case-insensitive `**kwargs` path in `set_cookie` — tornado

Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')

Summary

The CVE-2026-35536 fix added a validation loop that rejects [\x00-\x20\x3b\x7f], but only for the hardcoded lowercase keys name/domain/path/samesite. The still-live deprecated **kwargs path writes attacker-supplied attribute values straight into the Morsel with no validation, and because Morsel.__setitem__ is case-insensitive, a capitalized kwarg (Domain=, Path=, SameSite=, Max-Age=) routes to the same reserved attribute while bypassing the loop — re-opening ;-delimited attribute injection.

self.set_cookie("sid", "abc", Domain="evil.com; Secure; SameSite=None") # -> Set-Cookie: sid=abc; Domain=evil.com; Secure; SameSite=None; Path=/ # Sanity (the canonical lowercase named arg IS blocked): self.set_cookie("sid", "abc", domain="evil.com; Secure") # -> http.cookies.CookieError

The patch's regression test (SetCookieForbiddenCharHandler) only exercises the four named params, never the **kwargs path, so the gap is not regression-covered.

Affected code

  • tornado/web.py → RequestHandler.set_cookie: the validation loop covers only the lowercase named args; the trailing if kwargs: loop does morsel[k] = v with no character validation.

Steps to reproduce

GET /upper (uses Domain= kwarg) emits Set-Cookie: c_upper=v; Domain=evil.com; Secure; SameSite=None; Path=/; GET /lower (uses lowercase domain=) returns a CookieError.

Impact

Injection of independent cookie attributes (force/drop Secure/HttpOnly/SameSite, rebind Domain/Path) — the same impact CVE-2026-35536 closed, via the sibling path the patch missed. Conditional on the app using a capitalized/legacy keyword.

Suggested remediation

Apply the same [\x00-\x20\x3b\x7f] validation to every entry in the **kwargs loop (after normalizing the key case), or remove the deprecated kwargs path; add a regression test for capitalized kwargs.

Credit

Reported as part of an incomplete-patch measurement study (responsible disclosure).

No technical information available.

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A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.

CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.

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