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CVE-2026-91991VulnerabilityPublished Updated

nvchecker · Tornado: Incomplete fix for CVE-2026-35536: cookie attribute injection re-opened via the legacy case-insensitive

Affects nvcheckerSource GCVE Database

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Mentioned vulnerabilities

Authoritative record:CVE-2026-91991· Medium 6.3

Not known to be exploited · CVSS 6.3 Medium : upgrade to 2.22 when convenient.

Data as of 30 Oct 2026, 07:30 UTC

CVSS 4.0 base score
6.3Medium
How severe the flaw is on the 0–10 CVSS scale.
Patch
Available – fixed in 2.22
CVSS:4.0/AV:N/AC:H/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:L/SI:L/SA:N

Exploitation evidence

  1. Disclosed: evidence found.

    CVE published 15 Sep 2026

  2. Detectable: no evidence on record.

    No public nuclei template indexed

  3. Weaponized: no evidence on record.

    No public exploit code indexed in Metasploit or Exploit-DB

  4. Exploited in the wild: no evidence on record.

    Not in CISA KEV and no source reports exploitation

  5. Observed by OffSeq Mirage: no evidence on record.

    No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

What to do

Upgrade nvchecker to 2.22.

Affected software

ProductAffectedFixed in
Homebrewnvcheckerpkg:brew/nvcheckerOther threatsAffected
  • >=2.20_1 <2.22
Fixed in
  • 2.22

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

## 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).

References

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