How severe is CVE-2026-40561?

CVE-2026-40561 has a CVSS 3.x base score of 5.3, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity low, and availability none.

Is CVE-2026-40561 being actively exploited?

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

What products are affected by CVE-2026-40561?

CVE-2026-40561 affects Kazuho Starlet. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-40561?

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

Does CVE-2026-40561 have an EU (EUVD) identifier?

Yes. CVE-2026-40561 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-26806.

When was CVE-2026-40561 published?

CVE-2026-40561 was published on 2026-05-03 and last updated on 2026-06-17.

CVE-2026-40561

CVE-2026-40561 is a medium-severity vulnerability in Kazuho Starlet with a CVSS 3.x base score of 5.3. 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-444.

Key facts

Severity: Medium (CVSS 3.x base score 5.3)
EPSS exploit prediction: 0% (30th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-26806
Weakness: CWE-444
Affected product: Kazuho Starlet
Published: 2026-05-03
Last modified: 2026-06-17

Description

Starlet versions through 0.31 for Perl allows HTTP Request Smuggling via Improper Header Precedence. Starlet incorrectly prioritizes "Content-Length" over "Transfer-Encoding: chunked" when both headers are present in an HTTP request. Per RFC 7230 3.3.3, Transfer-Encoding must take precedence. An attacker could exploit this to smuggle malicious HTTP requests via a front-end reverse proxy.

Frequently asked questions

What is CVE-2026-40561?: Starlet versions through 0.31 for Perl allows HTTP Request Smuggling via Improper Header Precedence. Starlet incorrectly prioritizes "Content-Length" over "Transfer-Encoding: chunked" when both headers are present in an HTTP request. Per RFC 7230 3.3.3, Transfer-Encoding must take precedence. An attacker could exploit this to smuggle malicious HTTP requests via a front-end reverse proxy.
How severe is CVE-2026-40561?: CVE-2026-40561 has a CVSS 3.x base score of 5.3, rated medium severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity low, and availability none.
Is CVE-2026-40561 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (30th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-40561?: CVE-2026-40561 affects Kazuho Starlet. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-40561?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
Does CVE-2026-40561 have an EU (EUVD) identifier?: Yes. CVE-2026-40561 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-26806.
When was CVE-2026-40561 published?: CVE-2026-40561 was published on 2026-05-03 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:kazuho:starlet:*:*:*:*:*:perl:*:*

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