High severity · published Jun 23, 2026
CVE-2026-52845
Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`
8.1
CVSS
0.4%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Confidentiality
High
Integrity
High
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.4% · higher than 39% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
CA
caddy · caddyserver
< 2.11.4 · source: vendor, NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-52845
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-52845
Exploit
https://github.com/caddyserver/caddy/security/advisories/GHSA-f59h-q822-g45g
access.redhat.com
https://access.redhat.com/security/cve/CVE-2026-52845
bugzilla.redhat.com
https://bugzilla.redhat.com/show_bug.cgi?id=2491907
security.access.redhat.com
https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-52845.json
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