Vulnerability · Published Feb 24, 2026 · Assigned by GitHub_M
CVE-2026-27590Caddy: Unicode case-folding length expansion causes incorrect split_path index (SCRIPT_NAME/PATH_INFO confusion) in FastCGI transport
High 8.9
In plain English
A high-severity improper input validation in caddy from caddyserver. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 50% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Alpine:v3.23 | caddy | 2.11.2-r0 | ALPINE-CVE-2026-27590 |
| Alpine:v3.24 | caddy | 2.11.1-r0 | ALPINE-CVE-2026-27590 |
| Debian:12 | caddy | 2.6.2-5+deb12u1 | DEBIAN-CVE-2026-27590 |
| Debian:13 | caddy | 2.6.2-12+deb13u1 | DEBIAN-CVE-2026-27590 |
| Debian:14 | caddy | 2.11.2-1 | DEBIAN-CVE-2026-27590 |
| Go | github.com/caddyserver/caddy/v2 | 2.11.1 | GO-2026-4536 |
| Go | github.com/caddyserver/caddy/v2 | 2.11.1 | GHSA-5r3v-vc8m-m96g |
| Ubuntu:25.10 | caddy | no fix listed | UBUNTU-CVE-2026-27590 |
| Ubuntu:26.10 | caddy | no fix listed | UBUNTU-CVE-2026-27590 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| caddyserver | caddy | < 2.11.1 |
Weakness: CWE-20, CWE-180 · Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:P
References
- https://github.com/caddyserver/caddy/security/advisories/GHSA-5r3v-vc8m-m96g x_refsource_CONFIRM
- https://github.com/php/frankenphp/security/advisories/GHSA-g966-83w7-6w38 x_refsource_MISC
- https://github.com/caddyserver/caddy/releases/tag/v2.11.1 x_refsource_MISC