GHSA-wwv5-g3v4-889x: Tornado Incomplete Fix For CVE-2026-35536: Cookie Vulnerability

LOW
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CWE-74
Published
Advisory Summary

GHSA-wwv5-g3v4-889x is a LOW incomplete fix for cve-2026-35536: cookie vulnerability (CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')) affecting Tornado libraries in the PyPI ecosystem. The issue involves incomplete fix for cve-2026-35536: cookie attribute injection re-opened via the legacy case-insensitive `**kwargs` path in `set_cookie`. Remediation is available by upgrading affected packages to version 6.5.8.

GHSA-wwv5-g3v4-889x at a Glance

CVE Identifier N/A
GHSA Identifier GHSA-wwv5-g3v4-889x
Severity LOW
CVSS v3.1 Base Score N/A
CWE Classification CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
Affected Product Tornado
Package Ecosystem PyPI
Published Date
Last Updated

How to Fix GHSA-wwv5-g3v4-889x

To remediate this vulnerability, upgrade all affected Tornado dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

tornado fixed: 6.5.8 
pip install tornado>=6.5.8
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

pip show tornado

How to Check if You Are Affected

To determine whether your Python environment is affected by GHSA-wwv5-g3v4-889x, inspect your installed packages using pip:

pip show tornado

Analyze the terminal output to locate any version of Tornado in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 6.5.8), your application is affected.

Note: The pip show tornado command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection'), where the product constructs all or part of a command, data structure, or record using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify how it is parsed or interpreted when it is sent to a downstream component. In Tornado, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

## 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. ```python 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).

GHSA-wwv5-g3v4-889x Affected Tornado Versions

1 component(s)
Fixed Versions:
6.5.8
Product / Package Ecosystem Affected Versions Fixed Version
tornado
pypi/tornado
PyPI
≥ 6.5.5
< 6.5.8
6.5.8

References

GitHub Advisory / Wiki ADVISORY
https://github.com/tornadoweb/tornado/security/advisories/GHSA-wwv5-g3v4-889x
Tornado Source Repository REPOSITORY
https://github.com/tornadoweb/tornado/pull/3704
Tornado Source Repository REPOSITORY
https://github.com/tornadoweb/tornado/pull/3706
Security Fix Commit FIX COMMIT
https://github.com/tornadoweb/tornado/commit/6ef836e43e1278530041376adb32504daa977b91
Security Fix Commit FIX COMMIT
https://github.com/tornadoweb/tornado/commit/da284767eae8e1f0484f123b8c3225f6465b09c7
Tornado Source Repository REPOSITORY
https://github.com/tornadoweb/tornado
Tornado Release RELEASE
https://github.com/tornadoweb/tornado/releases/tag/v6.5.8

Frequently Asked Questions

What is GHSA-wwv5-g3v4-889x?
GHSA-wwv5-g3v4-889x is a security vulnerability in Tornado involving Incomplete fix for CVE-2026-35536: cookie attribute injection re-opened via the legacy case-insensitive `**kwargs` path in `set_cookie` under CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection').
Which Tornado versions are affected?
Affected versions include releases prior to 6.5.8 across affected packages: tornado.
What version fixes GHSA-wwv5-g3v4-889x?
Upgrading to 6.5.8 (or the latest patched release for your respective branch) resolves the vulnerability.
Is tornado affected?
Yes, tornado is affected in releases prior to 6.5.8. Upgrade your dependency to 6.5.8 or later.
What CWE is associated with GHSA-wwv5-g3v4-889x?
GHSA-wwv5-g3v4-889x is classified under CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection') (The product constructs all or part of a command, data structure, or record using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify how it is parsed or interpreted when it is sent to a downstream component.).
How can I check PyPI dependencies for GHSA-wwv5-g3v4-889x?
Run the dependency tree command: 'pip show tornado'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-wwv5-g3v4-889x",
  "modified" : "2026-09-01T20:17:23Z",
  "published" : "2026-09-01T20:17:23Z",
  "aliases" : [ ],
  "summary" : "Tornado: Incomplete fix for CVE-2026-35536: cookie attribute injection re-opened via the legacy case-insensitive `**kwargs` path in `set_cookie`",
  "details" : "## Summary\nThe CVE-2026-35536 fix added a validation loop that rejects `[\\x00-\\x20\\x3b\\x7f]`, but only for the\nhardcoded **lowercase** keys `name`/`domain`/`path`/`samesite`. The still-live deprecated `**kwargs` path\nwrites attacker-supplied attribute values straight into the `Morsel` with no validation, and because\n`Morsel.__setitem__` is case-insensitive, a capitalized kwarg (`Domain=`, `Path=`, `SameSite=`, `Max-Age=`)\nroutes to the same reserved attribute while bypassing the loop — re-opening `;`-delimited attribute injection.\n\n```python\nself.set_cookie(\"sid\", \"abc\", Domain=\"evil.com; Secure; SameSite=None\")\n#  -> Set-Cookie: sid=abc; Domain=evil.com; Secure; SameSite=None; Path=/\n# Sanity (the canonical lowercase named arg IS blocked):\nself.set_cookie(\"sid\", \"abc\", domain=\"evil.com; Secure\")   # -> http.cookies.CookieError\n```\n\nThe patch's regression test (`SetCookieForbiddenCharHandler`) only exercises the four named params, never the\n`**kwargs` path, so the gap is not regression-covered.\n\n## Affected code\n- `tornado/web.py` → `RequestHandler.set_cookie`: the validation loop covers only the lowercase named args;\n  the trailing `if kwargs:` loop does `morsel[k] = v` with no character validation.\n\n## Steps to reproduce\n`GET /upper` (uses `Domain=` kwarg) emits `Set-Cookie: c_upper=v; Domain=evil.com; Secure; SameSite=None; Path=/`; `GET /lower` (uses lowercase\n`domain=`) returns a `CookieError`.\n\n## Impact\nInjection of independent cookie attributes (force/drop `Secure`/`HttpOnly`/`SameSite`, rebind `Domain`/`Path`)\n— the same impact CVE-2026-35536 closed, via the sibling path the patch missed. Conditional on the app using\na capitalized/legacy keyword.\n\n## Suggested remediation\nApply the same `[\\x00-\\x20\\x3b\\x7f]` validation to every entry in the `**kwargs` loop (after normalizing the\nkey case), or remove the deprecated kwargs path; add a regression test for capitalized kwargs.\n\n## Credit\nReported as part of an incomplete-patch measurement study (responsible disclosure).",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:P/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
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      "name" : "tornado"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "6.5.5"
      }, {
        "fixed" : "6.5.8"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/security/advisories/GHSA-wwv5-g3v4-889x"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/pull/3704"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/pull/3706"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/commit/6ef836e43e1278530041376adb32504daa977b91"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/commit/da284767eae8e1f0484f123b8c3225f6465b09c7"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/tornadoweb/tornado"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/tornadoweb/tornado/releases/tag/v6.5.8"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-74" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-01T20:17:23Z",
    "nvd_published_at" : null
  }
}