Summary
Caddy: Unicode case-folding length expansion causes incorrect split_path index in FastCGI transport
Impact
Security boundary bypass/path confusion in script resolution.
In typical deployments, .php extension boundaries are relied on to decide what is executed by PHP. This bug can cause Caddy/FPM to execute a different file than intended by confusing SCRIPT_NAME/SCRIPT_FILENAME. If an attacker can place attacker-controlled content into a file that can be resolved as SCRIPT_FILENAME (common in web apps with uploads or writable directories), this can lead to unintended PHP execution of non-.php files and potentially remote code execution. Severity depends on deployment and presence of attacker-controlled file writes, but the primitive itself is remotely triggerable via crafted URLs.
This vulnerability was initially reported to FrankenPHP (https://github.com/php/frankenphp/security/advisories/GHSA-g966-83w7-6w38) by @AbdrrahimDahmani. The affected code has been copied/adapted from Caddy, which, according to research, is also affected.
The patch is a port of the FrankenPHP patch.
The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-27590? CVE-2026-27590 is a high-severity improper input validation vulnerability in github.com/caddyserver/caddy/v2 (go), affecting versions < 2.11.1. It is fixed in 2.11.1. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
- Which versions of github.com/caddyserver/caddy/v2 are affected by CVE-2026-27590? github.com/caddyserver/caddy/v2 (go) versions < 2.11.1 is affected.
- Is there a fix for CVE-2026-27590? Yes. CVE-2026-27590 is fixed in 2.11.1. Upgrade to this version or later.
- Is CVE-2026-27590 exploitable, and should I be worried? Whether CVE-2026-27590 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-27590 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-27590? Upgrade
github.com/caddyserver/caddy/v2to 2.11.1 or later.