How severe is CVE-2026-91991?

CVE-2026-91991 has a CVSS 3.x base score of 5.4, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is low, integrity low, and availability none.

Is CVE-2026-91991 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (15th percentile), an estimate of the probability of exploitation in the next 30 days.

How do I fix CVE-2026-91991?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

When was CVE-2026-91991 published?

CVE-2026-91991 was published on 2026-09-15 and last updated on 2026-09-21.

CVE-2026-91991

CVE-2026-91991 is a medium-severity vulnerability with a CVSS 3.x base score of 5.4. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-113.

Key facts

Severity: Medium (CVSS 3.x base score 5.4)
CVSS v4: 6.3
EPSS exploit prediction: 0% (15th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-113
Published: 2026-09-15
Last modified: 2026-09-21

Description

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.

Frequently asked questions

What is 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.
How severe is CVE-2026-91991?: CVE-2026-91991 has a CVSS 3.x base score of 5.4, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is low, integrity low, and availability none.
Is CVE-2026-91991 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (15th percentile), an estimate of the probability of exploitation in the next 30 days.
How do I fix CVE-2026-91991?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
When was CVE-2026-91991 published?: CVE-2026-91991 was published on 2026-09-15 and last updated on 2026-09-21.

References

Other CWE-113 (HTTP Response Splitting) vulnerabilities

CVE-2026-67289 — Critical (CVSS 9.8): FreeRDP before 3.29.0 (affected versions <= 3.28.0) does not validate CRLF and control characters in the…
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CVE-2026-105324 — Critical (CVSS 9.2): An HTTP header injection vulnerability in start-page-loader.cgi of ADM allows an unauthenticated remote attacker to…
CVE-2026-34520 — Critical (CVSS 9.1): AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. Prior to version 3.13.4, the C parser…
CVE-2025-53007 — High (CVSS 8.9): arduino-esp32 provides an Arduino core for the ESP32. Versions prior to 3.3.0-RC1 and 3.2.1 contain a HTTP Response…
CVE-2026-75419 — High (CVSS 8.8): go-wind-cms (GoWind) before 1.0.0 has a missing authorization vulnerability. The NewAuthorizer() function in…

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