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CVE-2026-27590— Caddy: Unicode case-folding length expansion causes incorrect split_path index (SCRIPT_NAME/PATH_INFO confusion) in FastCGI transport

Quick assessment

Affected
caddyserver caddy
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Caddy是Caddy公司的一款开源、跨平台的HTTP/Web服务器。 Caddy 2.11.1之前版本存在安全漏洞，该漏洞源于FastCGI路径分割逻辑在处理Unicode时存在缺陷，可能导致路径混淆和潜在的非.php文件执行。

AI Predicted 9.0 Difficulty: Easy EPSS 0.96% · P61

Possible ATT&CK Techniques 1 AI

T1190 · Exploit Public-Facing Application
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I. Basic Information for CVE-2026-27590

Vulnerability Information
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Vulnerability Title
Caddy: Unicode case-folding length expansion causes incorrect split_path index (SCRIPT_NAME/PATH_INFO confusion) in FastCGI transport
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
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
Source: CVE Program / CVE List V5
Vulnerability Type
输入验证不恰当
Source: CVE Program / CVE List V5
Vulnerability Title
Caddy 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Caddy是Caddy公司的一款开源、跨平台的HTTP/Web服务器。 Caddy 2.11.1之前版本存在安全漏洞，该漏洞源于FastCGI路径分割逻辑在处理Unicode时存在缺陷，可能导致路径混淆和潜在的非.php文件执行。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
caddyserver caddy < 2.11.1 -

II. Public POCs for CVE-2026-27590

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III. Intelligence Information for CVE-2026-27590

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Vendor Advisories for CVE-2026-27590 (1)

Vendor Pages for CVE-2026-27590 (1)

Same Patch Batch · caddyserver · 2026-02-24 · 6 CVEs total

CVE-2026-27585 Caddy's improper sanitization of glob characters in file matcher may lead to bypassing sec
CVE-2026-27586 Caddy's mTLS client authentication silently fails open when CA certificate file is missing
CVE-2026-27587 Caddy: MatchPath %xx (escaped-path) branch skips case normalization, enabling path-based r
CVE-2026-27588 Caddy: MatchHost becomes case-sensitive for large host lists (>100), enabling host-based r
CVE-2026-27589 Caddy vulnerable to cross-origin config application via local admin API /load (caddy)

IV. Related Vulnerabilities

Same product: caddy
Same vendor: caddyserver
Same weakness: CWE-20