CVE-2026-27590: caddyserver caddy Improper Input Validation

Vulnerability summary

CVE-2026-27590 is a critical (CVSS 9.8) Improper Input Validation vulnerability in caddyserver caddy. EPSS estimates a 0.88% chance of exploitation in the next 30 days, higher than 58.4% of scored vulnerabilities. 1 public exploit is recorded.

Timeline

1. 24-02-2026
   PublishedVulnerability first disclosed.
2. 27-02-2026
   Last modifiedVulnerability information updated.

Description

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.

• CWE-20Improper Input ValidationThe 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.
• CWE-180Incorrect Behavior Order: Validate Before CanonicalizeThe product validates input before it is canonicalized, which prevents the product from detecting data that becomes invalid after the canonicalization step.