Caddy: FastCGI header normalization bypass in forward_auth copy_headers

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

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

EPSS

• CVE-2026-52845: 0.45% estimated exploitation activity in the next 30 days; percentile 38.80 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary forwardauth copyheaders deletes the exact client-supplied identity header before copying the trusted value from the auth gateway. But when the request later goes through phpfastcgi, Caddy normalizes HTTP headers into CGI variables by replacing - with . This lets a client send an underscore alias that survives the forwardauth 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. Details forwardauth copyheaders intentionally removes client-controlled headers before setting values from the auth response: - modules/caddyhttp/reverseproxy/forwardauth/caddyfile.go:212 - modules/caddyhttp/reverseproxy/forwardauth/caddyfile.go:222 That delete is exact-field deletion through http.Header.Del(): - modules/caddyhttp/headers/headers.go:255 - modules/caddyhttp/headers/headers.go:281 So deleting Remote-Groups does not delete RemoteGroups. Later, FastCGI exports…

Published by the source

16 Jun 2026, 21:28 UTC

Last modified by the source

24 Jul 2026, 13:26 UTC

Source last checked

26 Oct 2026, 15:27 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-f59h-q822-g45g

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# Patch Next advisory brief: CVE-2026-52845

Advisory: Caddy: FastCGI header normalization bypass in `forward_auth copy_headers`
IDs: CVE-2026-52845, GHSA-f59h-q822-g45g
Record: https://patchnext.com/advisories/cve-2026-52845 (revision 1, source checked 26 Oct 2026, 15:27 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.1)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Partial vendor fix; fixed versions by package: github.com/caddyserver/caddy/v2 2.11.4. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/caddyserver/caddy/v2: affected < 2.11.4; fixed in 2.11.4
- Go github.com/caddyserver/caddy: affected <= 1.0.5

## Suggested inspection (read-only)
- Go github.com/caddyserver/caddy/v2: check the installed version with your package manager.
- Go github.com/caddyserver/caddy: check the installed version with your package manager.

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/caddyserver/caddy/v2, github.com/caddyserver/caddy.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-f59h-q822-g45g (GitHub-reviewed advisory GHSA-f59h-q822-g45g lists affected ranges: Go github.com/caddyserver/caddy/v2 >=0 =0 <=1.0.5)

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