CVE-2026-52845CWE-287CWE-290CWE-444

Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`

High · published June 23, 2026

CVSS v3.1
8.1
EPSS
0%
Percentile
37.7
In the wild
Unconfirmed
What it is

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.

The record
Technical detail
CVSS v3.1
8.1 · HIGH
Vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
CVSS v4.0
Not supplied
EPSS
0.00445 · 37.7th percentile
Weaknesses
CWE-287 · Improper Authentication; CWE-290 · Authentication Bypass by Spoofing; CWE-444 · Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
Published
2026-06-23T17:52Z
EPSS history
Timeline
  • 23 JUN 17:52Z
    Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`
    cvelistv5