GHSA-wwv5-g3v4-889x
ADVISORY - githubSummary
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 trailingif kwargs:loop doesmorsel[k] = vwith 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).
Common Weakness Enumeration (CWE)
Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
GitHub
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CVSS SCORE
2.3low| Package | Type | OS Name | OS Version | Affected Ranges | Fix Versions |
|---|---|---|---|---|---|
| tornado | pypi | - | - | >=6.5.5,<6.5.8 | 6.5.8 |
CVSS:4 Severity and metrics
The CVSS metrics represent different qualitative aspects of a vulnerability that impact the overall score, as defined by the CVSS Specification.
The vulnerable component is bound to the network stack, but the attack is limited at the protocol level to a logically adjacent topology. This can mean an attack must be launched from the same shared physical (e.g., Bluetooth or IEEE 802.11) or logical (e.g., local IP subnet) network, or from within a secure or otherwise limited administrative domain (e.g., MPLS, secure VPN to an administrative network zone). One example of an Adjacent attack would be an ARP (IPv4) or neighbor discovery (IPv6) flood leading to a denial of service on the local LAN segment (e.g., CVE-2013-6014).
Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success when attacking the vulnerable component.
The successful attack depends on the presence of specific deployment and execution conditions of the vulnerable system that enable the attack. These include: A race condition must be won to successfully exploit the vulnerability. The successfulness of the attack is conditioned on execution conditions that are not under full control of the attacker. The attack may need to be launched multiple times against a single target before being successful. Network injection. The attacker must inject themselves into the logical network path between the target and the resource requested by the victim (e.g. vulnerabilities requiring an on-path attacker).
The attacker is unauthenticated prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
Successful exploitation of this vulnerability requires limited interaction by the targeted user with the vulnerable system and the attacker's payload. These interactions would be considered involuntary and do not require that the user actively subvert protections built into the vulnerable system. Examples include: utilizing a website that has been modified to display malicious content when the page is rendered (most stored XSS or CSRF) running an application that calls a malicious binary that has been planted on the system using an application which generates traffic over an untrusted or compromised network (vulnerabilities requiring an on-path attacker).
There is some loss of confidentiality. Access to some restricted information is obtained, but the attacker does not have control over what information is obtained, or the amount or kind of loss is limited. The information disclosure does not cause a direct, serious loss to the Vulnerable System.
There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact to the Vulnerable System.
There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
There is no impact to availability within the Vulnerable System.
There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.