Input validation error in Caddy - CVE-2026-27590
Published: April 27, 2026
Vulnerability identifier: #VU128033
CSH Severity: Medium
CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:U/U:Green
CVE-ID: CVE-2026-27590
CWE-ID: CWE-20
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vendor: Caddy
Affected software:
Caddy
Caddy
Detailed vulnerability description
The vulnerability allows a remote attacker to execute unintended PHP code.
The vulnerability exists due to incorrect calculation of a path split index in fastcgi.Transport.splitPos() and buildEnv() in the FastCGI transport when processing crafted request paths containing Unicode characters whose lowercase form changes UTF-8 byte length. A remote attacker can send a specially crafted request path to execute unintended PHP code.
Exploitation depends on deployment conditions that allow attacker-controlled file contents to be resolved as SCRIPT_FILENAME, such as upload features or writable directories.
How to mitigate CVE-2026-27590
Install security update from vendor's website.