CVE-2026-40561 Starlet versions through 0.31 for Perl allows HTTP Request Smuggling via Improper Header Precedence
Attackers can smuggle HTTP requests by exploiting improper header precedence in Starlet, bypassing reverse proxy security controls.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-40561?
CVE-2026-40561 is a medium-severity vulnerability in KAZUHO Starlet, published on 3 May 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official 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.
Weakness: CWE-444
Which versions are affected by CVE-2026-40561?
It affects 1 product.
Products include Kazuho Starlet.
How do I fix CVE-2026-40561?
Upgrade to a fixed version.
Is CVE-2026-40561 being exploited?
There is no known exploitation of CVE-2026-40561 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-40561, GHSA-2xqx-75wg-w86v, MGASA-2026-0120, UBUNTU-CVE-2026-40561.