CVE-2026-52845

Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`

Published Jun 23, 2026 CVSS 3.1 · 8.1 High EPSS < 1% KEV · Not listed
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.

Remediation

No solution or workaround provided in the CVE record

Tracking

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Advisories

Source ID Title
Debian DLA Debian DLA DLA-4857-1 caddy security update
Debian DSA Debian DSA DSA-6429-1 caddy security update
Github GHSA Github GHSA GHSA-f59h-q822-g45g Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`
History
Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'poc', 'Technical Impact': 'total'}, 'version': '2.0.3'}

Type Values Removed Values Added
First Time appeared Caddyserver
Caddyserver caddy
Vendors & Products Caddyserver
Caddyserver caddy
Type Values Removed Values Added
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.
Title Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`
Weaknesses CWE-287
CWE-290
CWE-444
References
Metrics cvssV3_1

{'score': 8.1, 'vector': 'CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N'}

Weaknesses

Sources

Status
PUBLISHED
Assigner
GitHub_M
Published
Updated
Jul 15, 2026, 12:46 AM
Reserved
Jun 8, 2026, 6:41 PM
Status
Modified
Published
Jun 23, 2026, 6:18 PM
Modified
Oct 25, 2026, 11:10 PM
Severity
Important
Public date
Jun 23, 2026, 5:52 PM