caddyserver caddy up to 2.11.0 Request Path strings.ToLower SCRIPT_NAME/SCRIPT_FILENAME/PATH_INFO input validation

Summaryinfo

A vulnerability described as very critical has been identified in caddyserver caddy up to 2.11.0. The impacted element is the function strings.ToLower of the component Request Path Handler. Such manipulation of the argument SCRIPT_NAME/SCRIPT_FILENAME/PATH_INFO leads to input validation. This vulnerability is listed as CVE-2026-27590. The attack may be performed from remote. There is no available exploit. Upgrading the affected component is recommended.

Detailsinfo

A vulnerability, which was classified as very critical, has been found in caddyserver caddy up to 2.11.0. Affected by this issue is the function strings.ToLower of the component Request Path Handler. The manipulation of the argument SCRIPT_NAME/SCRIPT_FILENAME/PATH_INFO with an unknown input leads to a input validation vulnerability. Using CWE to declare the problem leads to CWE-20. The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. Impacted is confidentiality, integrity, and availability. CVE summarizes:

Caddy is an extensible server platform that uses TLS by default. Prior to version 2.11.1, Caddy's FastCGI path splitting logic computes the split index on a lowercased copy of the request path and then uses that byte index to slice the original path. This is unsafe for Unicode because `strings.ToLower()` can change UTF-8 byte length for some characters. As a result, Caddy can derive an incorrect `SCRIPT_NAME`/`SCRIPT_FILENAME` and `PATH_INFO`, potentially causing a request that contains `.php` to execute a different on-disk file than intended (path confusion). In setups where an attacker can control file contents (e.g., upload features), this can lead to unintended PHP execution of non-.php files (potential RCE depending on deployment). Version 2.11.1 fixes the issue.

The advisory is available at github.com. This vulnerability is handled as CVE-2026-27590 since 02/20/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. Technical details are known, but there is no available exploit. The structure of the vulnerability defines a possible price range of USD $0-$5k at the moment (estimation calculated on 02/28/2026).

It is declared as proof-of-concept. The vulnerability scanner Nessus provides a plugin with the ID 300053 (Linux Distros Unpatched Vulnerability : CVE-2026-27590), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 2.11.1 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at Tenable (300053). If you want to get best quality of vulnerability data, you may have to visit VulDB.

Productinfo

Vendor

• caddyserver

Name

• caddy

Version

• 2.0
• 2.1
• 2.2
• 2.3
• 2.4
• 2.5
• 2.6
• 2.7
• 2.8
• 2.9
• 2.10
• 2.11.0

Website

• Product: https://github.com/caddyserver/caddy/

CPE 2.3info

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CPE 2.2info

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If you want to get best quality of vulnerability data, you may have to visit VulDB.

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