critical CVSS 9.8

CVE-2026-27590·PHP vulnerability

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.

Severity
critical
Software
PHP
Fixed in
2.11.1
Published
2026-02-24

Affected versions

Until: 2.11.1

Fixed in: 2.11.1

How to fix this CVE

Update PHP to version 2.11.1 or later to resolve a critical Unicode path-handling vulnerability in FastCGI configurations. This flaw could allow attackers to execute unintended PHP files through byte-length misalignment when processing non-ASCII characters in request paths, particularly in systems with file upload capabilities. Immediate patching is strongly recommended for all production environments running vulnerable versions.

sudo dnf update php

Defensia detects this vulnerability

What an exploitation attempt looks like

Sample log line indicative of exploitation attempts:

POST|GET request with UTF-8 multi-byte characters (e.g., %C3%A9, %E2%80%A6) in the path segment preceding .php filename, followed by execution of non-.php file types (e.g., .txt, .jpg, .conf)

WAF mitigation (if patching is not yet possible)

Add this rule to your WAF to block exploitation attempts while you schedule the patch. 

Implement request normalization rules that decode and validate UTF-8 sequences before path processing. Block requests containing multi-byte Unicode sequences in FastCGI script paths. Additionally, enforce strict file-type validation to prevent execution of non-script files via PATH_INFO manipulation.

How to check if you are affected

  1. Step 1: Check installed PHP version with: php --version or php -v
  2. Step 2: Verify FastCGI is enabled in your Caddy configuration by checking caddy.conf or running: caddy list-modules | grep fastcgi
  3. Step 3: Search web server logs for requests containing non-ASCII/Unicode characters followed by .php extensions: grep -E '[\x80-\xFF].*\.php' /var/log/caddy/access.log
  4. Step 4: Confirm patch application by verifying the updated PHP version matches 2.11.1 or later: php --version

FAQ

What is CVE-2026-27590?

A critical vulnerability in Caddy's FastCGI handler that miscalculates script path boundaries when Unicode characters are present, causing byte-index misalignment between lowercased and original request paths. This can redirect PHP execution to unintended files, enabling remote code execution in upload-enabled deployments.

Is CVE-2026-27590 being actively exploited?

No, this vulnerability is not currently listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, and no public exploits are available. However, the critical severity score (9.8) and RCE potential warrant immediate patching.

What versions of PHP are affected by CVE-2026-27590?

All versions of PHP prior to 2.11.1 are affected. The vulnerability exists in Caddy's FastCGI path-splitting logic, not PHP itself, but impacts PHP execution contexts when Caddy is used as the application server.

How do I check if my server is vulnerable to CVE-2026-27590?

Run php --version to confirm your installed version is below 2.11.1. Additionally, check if FastCGI is active in your Caddy configuration with: grep -i fastcgi /etc/caddy/caddy.conf or caddy list-modules | grep fastcgi.

Does Defensia detect CVE-2026-27590?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If PHP is installed on a monitored server, CVE-2026-27590 will appear in your dashboard with remediation steps.

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References

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