CVE-2026-27590
CRITICAL
GHSA-5r3v-vc8m-m96g
CVSS Vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Lifecycle Timeline
4Description
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.
Analysis
FastCGI path splitting vulnerability in Caddy before 2.11.1 allows request smuggling or path confusion when proxying to FastCGI backends (PHP-FPM). EPSS 0.19% with PoC available.
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Remediation
Within 24 hours: Identify all Caddy instances in production and their versions; disable FastCGI functionality if not essential or restrict access via network segmentation. Within 7 days: Deploy compensating controls (WAF rules blocking malicious path patterns, API gateway filtering); establish monitoring for exploitation attempts. …
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