Auth bypass in Caddyserver Caddy
CVE-2026-52845
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.
Vulnerability class: Broken Authentication
EPSS: 0.003 (21.9th percentile) — read the EPSS interpretation.
CVSS v3 metric
CVSS v3 base score 8.1 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N.
Affected products
- Caddyserver Caddy — versions < 2.11.4
Weakness classification (CWE)
References
- security-advisories@github.com (x_refsource_CONFIRM, Exploit, Third Party Advisory)
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
Frequently asked questions
- What is CVE-2026-52845?
- CVE-2026-52845 is a high-severity vulnerability in Caddyserver Caddy, classified under Improper Authentication. CVSS score: 8.1/10. Published 2026-06-23.
- How severe is CVE-2026-52845?
- High severity. CVSS v3 base score is 8.1 out of 10.