Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`

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Summary

CVECVE-2026-52845
StatePUBLISHED
AssignerGitHub_M
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-06-23 18:18:05 UTC
Updated2026-06-30 03:20:49 UTC
DescriptionCaddy 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.

Risk And Classification

Primary CVSS: v3.1 8.1 HIGH from ADP

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

EPSS: 0.002460000 probability, percentile 0.158240000 (date 2026-07-04)

Problem Types: CWE-287 | CWE-290 | CWE-444 | CWE-287 CWE-287: Improper Authentication | CWE-290 CWE-290: Authentication Bypass by Spoofing | CWE-444 CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') | CWE-444 Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

VersionSourceTypeScoreSeverityVector
3.1ADPCVSS8.1HIGHCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
3.1[email protected]Secondary8.1HIGHCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
3.10b0ca135-0b70-47e7-9f44-1890c2a1c46cSecondary8.1HIGHCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N
3.1CNADECLARED8.1HIGHCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

CVSS v3.1 Breakdown

Attack Vector
Network
Attack Complexity
Low
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:N

NVD Known Affected Configurations (CPE 2.3)

TypeVendorProductVersionUpdateEditionLanguage
Application Caddyserver Caddy All All All All

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Caddyserver Caddy affected < 2.11.4 Not specified
ADP Red Hat Red Hat Hardened Images Not specified Not specified

References

ReferenceSourceLinkTags
bugzilla.redhat.com/show_bug.cgi 0b0ca135-0b70-47e7-9f44-1890c2a1c46c bugzilla.redhat.com
github.com/caddyserver/caddy/security/advisories/GHSA-f59h-q822-g45g 134c704f-9b21-4f2e-91b3-4a467353bcc0 github.com Exploit, Third Party Advisory
access.redhat.com/security/cve/CVE-2026-52845 0b0ca135-0b70-47e7-9f44-1890c2a1c46c access.redhat.com
security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-52845.json 0b0ca135-0b70-47e7-9f44-1890c2a1c46c security.access.redhat.com
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Additional Advisory Data

SourceTimeEvent
ADP2026-06-23T19:02:21.968ZReported to Red Hat.
ADP2026-06-23T17:52:01.871ZMade public.
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