CVE-2026-40561: Starlet versions through 0.31 for Perl allows HTTP Request Smuggling via Improper Header Precedence. Starlet

Q: How do I fix CVE-2026-40561?

starlet: upgrade to newer than 0.31. Exact commands for each OS are on this page.

Q: Is CVE-2026-40561 being exploited?

CVE-2026-40561 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2026-40561 affect?

CVE-2026-40561 affects starlet. Severity is medium (CVSS 5.3). 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 R

Q: How urgent is CVE-2026-40561?

Priority 31/100. Remotely exploitable, no login, no user action

Routine

MEDIUM 5.3REMOTEUPGRADE

CVE-2026-40561 is a medium-severity flaw (CVSS 5.3) affecting starlet. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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.

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.3 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	None
User interaction	None
EPSS exploit chance	0.4% (top 70%)
CISA exploited list	No
Weakness	CWE-444
Published	2026-05-03
Last modified	2026-10-26

Publisher (CNA): CPANSec · Vendor: kazuho

Last updated 2026-10-26 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2026-05-03 01:15 UTCPublished CVE record published
2026-10-26 20:10 UTCUpdated Record last modified

kazuho starlet UPGRADE

Affected: <= 0.31

Fixed in: newer than 0.31

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy starlet || rpm -q starlet || starlet --version

2. UPGRADE to newer than 0.31 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade starlet

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade starlet

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2026-40561?

starlet: upgrade to newer than 0.31. Exact commands for each OS are on this page.

Is CVE-2026-40561 being exploited?

CVE-2026-40561 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2026-40561 affect?

CVE-2026-40561 affects starlet. Severity is medium (CVSS 5.3). 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 R

How urgent is CVE-2026-40561?

Priority 31/100. Remotely exploitable, no login, no user action

Software pages

Perl vulnerabilities

References

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