Authentication Bypass by Spoofing in Caddy - CVE-2026-52845
Published: November 9, 2026
Vulnerability identifier: #VU163182
CSH Severity: Medium
CVSS v4: 8.6 [CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N]
CVE-ID: CVE-2026-52845
CWE-ID: CWE-290
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a low-privileged remote attacker to inject trusted identity or group values into PHP/FastCGI applications behind Caddy.
The vulnerability exists due to inconsistent header normalization between Caddy's forward_auth copy_headers filtering and subsequent FastCGI processing in deployments whose applications trust the resulting identity variables. A low-privileged remote attacker can send underscore-containing header aliases that survive filtering and become trusted CGI variables, including injecting a group value omitted by the authentication gateway.
Affected software
Caddy
How to mitigate CVE-2026-52845
Install security update from vendor's website.
Caddy - update to 2.11.4