CVE-2026-40560

High

Published: 29 April 2026

Published

29 April 2026

Modified

06 May 2026

KEV Added

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Patch

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CVSS Score 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

EPSS Score 0.0004 12.4th percentile

Risk Priority 15 60% EPSS · 20% KEV · 20% CVSS

Summary

CVE-2026-40560 is a high-severity HTTP Request/Response Smuggling (CWE-444) vulnerability in Miyagawa Starman. Its CVSS base score is 7.5 (High).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 12.4th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.

The strongest mitigations our analysis identified are NIST 800-53 SI-10 (Information Input Validation) and SI-2 (Flaw Remediation).

Threat & Defense at a Glance

What attackers do: exploitation maps to Exploit Public-Facing Application (T1190). What defenders deploy: see the NIST 800-53 controls recommended below.

Threat & Defense Details

Mitigating Controls (NIST 800-53 r5)AI

SI-2 Flaw Remediation good match

prevent

Directly mitigates the HTTP request smuggling vulnerability by requiring timely identification, reporting, and patching of the header precedence flaw to Starman version 0.4018 or later.

SI-10 Information Input Validation good match

prevent

Ensures validation of HTTP request inputs, including correct precedence of Transfer-Encoding over Content-Length per RFC 7230, preventing smuggling exploits.

SC-7 Boundary Protection partial match

preventdetect

Implements boundary protection at external interfaces, such as reverse proxies or WAFs, to monitor, normalize, or block malformed HTTP requests exploiting the smuggling flaw.

MITRE ATT&CK Enterprise TechniquesAI

T1190 Exploit Public-Facing Application Initial Access

Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.

attack.mitre.org →

Why these techniques?

The vulnerability is an HTTP request smuggling flaw in a public-facing Perl PSGI web server (Starman), directly enabling remote exploitation via crafted requests through a reverse proxy, matching T1190 Exploit Public-Facing Application for initial access and data disclosure.

Confidence: HIGH · MITRE ATT&CK Enterprise v18.1

NVD Description

Starman versions before 0.4018 for Perl allows HTTP Request Smuggling via Improper Header Precedence. Starman 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.

Deeper analysisAI

CVE-2026-40560 is an HTTP request smuggling vulnerability affecting Starman, a Perl PSGI server, in versions prior to 0.4018. The flaw stems from Starman's incorrect prioritization of the "Content-Length" header over "Transfer-Encoding: chunked" when both are present in an HTTP request, violating RFC 7230 Section 3.3.3, which mandates that Transfer-Encoding takes precedence. This misinterpretation enables request smuggling attacks and is classified under CWE-444 with a CVSS v3.1 base score of 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N).

Remote attackers can exploit this vulnerability by sending crafted HTTP requests through a front-end reverse proxy to the vulnerable Starman instance. No privileges or user interaction are required, allowing unauthenticated network adversaries to smuggle malicious requests that the proxy interprets differently from the backend server, potentially leading to high confidentiality impacts such as unauthorized data disclosure.

Mitigation involves upgrading to Starman version 0.4018 or later, where the issue is addressed via a specific commit that corrects header precedence handling. Relevant advisories, including the OSS-security mailing list announcement from April 29, 2026, and the project's changelog on MetaCPAN, confirm the patch details and release notes.