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CVE-2026-52845— Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`

Quick assessment

Affected
caddyserver caddy
Exploitation
Public or AI PoC available; prioritize validation
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

caddyserver caddy是caddyserver团队开源的一款Web服务器软件。 caddyserver caddy 2.11.4之前版本存在安全漏洞，该漏洞源于forward_auth copy_headers在复制认证网关的可信值前删除客户端提供的身份标头，但请求经过php_fastcgi时Caddy会将HTTP标头转换为CGI变量，把-替换为_，从而允许客户端发送下划线别名绕过删除步骤，使得远程客户端能注入或覆盖PHP/FastCGI应用程序信任的身份或组标头。

CVSS 8.1 · High EPSS 0.29% · P22

Affected Version Matrix 1

VendorProduct Version RangeStatus
caddyserver caddy < 2.11.4 affected
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I. Basic Information for CVE-2026-52845

Vulnerability Information
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Vulnerability Title
Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`
Source: CVE Program / CVE List V5
Vulnerability Description
Caddy is an extensible server platform that uses TLS by default. Prior to 2.11.4, forward_auth copy_headers deletes the exact client-supplied identity header before copying the trusted value from the auth gateway. But when the request later goes through php_fastcgi, Caddy normalizes HTTP headers into CGI variables by replacing - with _. This lets a client send an underscore alias that survives the forward_auth delete step but becomes the same PHP/FastCGI variable. Result: a remote client can inject or sometimes override identity/group headers trusted by PHP/FastCGI applications behind Caddy. This vulnerability is fixed in 2.11.4.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
认证机制不恰当
Source: CVE Program / CVE List V5
Vulnerability Title
caddyserver caddy 授权问题漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
caddyserver caddy是caddyserver团队开源的一款Web服务器软件。 caddyserver caddy 2.11.4之前版本存在安全漏洞，该漏洞源于forward_auth copy_headers在复制认证网关的可信值前删除客户端提供的身份标头，但请求经过php_fastcgi时Caddy会将HTTP标头转换为CGI变量，把-替换为_，从而允许客户端发送下划线别名绕过删除步骤，使得远程客户端能注入或覆盖PHP/FastCGI应用程序信任的身份或组标头。
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 < 2.11.4 -

II. Public POCs for CVE-2026-52845

# POC Description Source Link Shenlong Link
AI-Generated POC Premium
Qwen3.6-35B-A3B · 11314 chars
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III. Intelligence Information for CVE-2026-52845

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Vendor Advisories for CVE-2026-52845 (1)

Same Patch Batch · caddyserver · 2026-06-23 · 5 CVEs total

CVE-2026-45135 8.1 HIGH Caddy: Unsafe Unicode Handling in FastCGI splitPos Allows Execution of Non-PHP Files
CVE-2026-52844 7.5 HIGH Caddy: Windows `file_server` path authorization bypass via encoded backslash
CVE-2026-45692 5.4 MEDIUM Caddy: Remote Admin Authorization Bypass in `/config` API via Array Index Normalization
CVE-2026-52846 4.2 MEDIUM Caddy: stripHTML template function bypass

IV. Related Vulnerabilities

Same product: caddy
Same vendor: caddyserver
Same weakness: CWE-287

V. Comments for CVE-2026-52845

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